blob: 48bbdd8d1b33fef92b63f203087edecafeb9beaf [file] [log] [blame]
//===- RewriteStatepointsForGC.cpp - Make GC relocations explicit ---------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Rewrite call/invoke instructions so as to make potential relocations
// performed by the garbage collector explicit in the IR.
#include "llvm/Transforms/Scalar/RewriteStatepointsForGC.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallingConv.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Statepoint.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <set>
#include <string>
#include <utility>
#include <vector>
#define DEBUG_TYPE "rewrite-statepoints-for-gc"
using namespace llvm;
// Print the liveset found at the insert location
static cl::opt<bool> PrintLiveSet("spp-print-liveset", cl::Hidden,
static cl::opt<bool> PrintLiveSetSize("spp-print-liveset-size", cl::Hidden,
// Print out the base pointers for debugging
static cl::opt<bool> PrintBasePointers("spp-print-base-pointers", cl::Hidden,
// Cost threshold measuring when it is profitable to rematerialize value instead
// of relocating it
static cl::opt<unsigned>
RematerializationThreshold("spp-rematerialization-threshold", cl::Hidden,
static bool ClobberNonLive = true;
static bool ClobberNonLive = false;
static cl::opt<bool, true> ClobberNonLiveOverride("rs4gc-clobber-non-live",
static cl::opt<bool>
cl::Hidden, cl::init(true));
/// The IR fed into RewriteStatepointsForGC may have had attributes and
/// metadata implying dereferenceability that are no longer valid/correct after
/// RewriteStatepointsForGC has run. This is because semantically, after
/// RewriteStatepointsForGC runs, all calls to gc.statepoint "free" the entire
/// heap. stripNonValidData (conservatively) restores
/// correctness by erasing all attributes in the module that externally imply
/// dereferenceability. Similar reasoning also applies to the noalias
/// attributes and metadata. gc.statepoint can touch the entire heap including
/// noalias objects.
/// Apart from attributes and metadata, we also remove instructions that imply
/// constant physical memory: llvm.invariant.start.
static void stripNonValidData(Module &M);
static bool shouldRewriteStatepointsIn(Function &F);
PreservedAnalyses RewriteStatepointsForGC::run(Module &M,
ModuleAnalysisManager &AM) {
bool Changed = false;
auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
for (Function &F : M) {
// Nothing to do for declarations.
if (F.isDeclaration() || F.empty())
// Policy choice says not to rewrite - the most common reason is that we're
// compiling code without a GCStrategy.
if (!shouldRewriteStatepointsIn(F))
auto &DT = FAM.getResult<DominatorTreeAnalysis>(F);
auto &TTI = FAM.getResult<TargetIRAnalysis>(F);
auto &TLI = FAM.getResult<TargetLibraryAnalysis>(F);
Changed |= runOnFunction(F, DT, TTI, TLI);
if (!Changed)
return PreservedAnalyses::all();
// stripNonValidData asserts that shouldRewriteStatepointsIn
// returns true for at least one function in the module. Since at least
// one function changed, we know that the precondition is satisfied.
PreservedAnalyses PA;
return PA;
namespace {
class RewriteStatepointsForGCLegacyPass : public ModulePass {
RewriteStatepointsForGC Impl;
static char ID; // Pass identification, replacement for typeid
RewriteStatepointsForGCLegacyPass() : ModulePass(ID), Impl() {
bool runOnModule(Module &M) override {
bool Changed = false;
for (Function &F : M) {
// Nothing to do for declarations.
if (F.isDeclaration() || F.empty())
// Policy choice says not to rewrite - the most common reason is that
// we're compiling code without a GCStrategy.
if (!shouldRewriteStatepointsIn(F))
TargetTransformInfo &TTI =
const TargetLibraryInfo &TLI =
auto &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
Changed |= Impl.runOnFunction(F, DT, TTI, TLI);
if (!Changed)
return false;
// stripNonValidData asserts that shouldRewriteStatepointsIn
// returns true for at least one function in the module. Since at least
// one function changed, we know that the precondition is satisfied.
return true;
void getAnalysisUsage(AnalysisUsage &AU) const override {
// We add and rewrite a bunch of instructions, but don't really do much
// else. We could in theory preserve a lot more analyses here.
} // end anonymous namespace
char RewriteStatepointsForGCLegacyPass::ID = 0;
ModulePass *llvm::createRewriteStatepointsForGCLegacyPass() {
return new RewriteStatepointsForGCLegacyPass();
"Make relocations explicit at statepoints", false, false)
"Make relocations explicit at statepoints", false, false)
namespace {
struct GCPtrLivenessData {
/// Values defined in this block.
MapVector<BasicBlock *, SetVector<Value *>> KillSet;
/// Values used in this block (and thus live); does not included values
/// killed within this block.
MapVector<BasicBlock *, SetVector<Value *>> LiveSet;
/// Values live into this basic block (i.e. used by any
/// instruction in this basic block or ones reachable from here)
MapVector<BasicBlock *, SetVector<Value *>> LiveIn;
/// Values live out of this basic block (i.e. live into
/// any successor block)
MapVector<BasicBlock *, SetVector<Value *>> LiveOut;
// The type of the internal cache used inside the findBasePointers family
// of functions. From the callers perspective, this is an opaque type and
// should not be inspected.
// In the actual implementation this caches two relations:
// - The base relation itself (i.e. this pointer is based on that one)
// - The base defining value relation (i.e. before base_phi insertion)
// Generally, after the execution of a full findBasePointer call, only the
// base relation will remain. Internally, we add a mixture of the two
// types, then update all the second type to the first type
using DefiningValueMapTy = MapVector<Value *, Value *>;
using StatepointLiveSetTy = SetVector<Value *>;
using RematerializedValueMapTy =
MapVector<AssertingVH<Instruction>, AssertingVH<Value>>;
struct PartiallyConstructedSafepointRecord {
/// The set of values known to be live across this safepoint
StatepointLiveSetTy LiveSet;
/// Mapping from live pointers to a base-defining-value
MapVector<Value *, Value *> PointerToBase;
/// The *new* gc.statepoint instruction itself. This produces the token
/// that normal path gc.relocates and the gc.result are tied to.
Instruction *StatepointToken;
/// Instruction to which exceptional gc relocates are attached
/// Makes it easier to iterate through them during relocationViaAlloca.
Instruction *UnwindToken;
/// Record live values we are rematerialized instead of relocating.
/// They are not included into 'LiveSet' field.
/// Maps rematerialized copy to it's original value.
RematerializedValueMapTy RematerializedValues;
} // end anonymous namespace
static ArrayRef<Use> GetDeoptBundleOperands(const CallBase *Call) {
Optional<OperandBundleUse> DeoptBundle =
if (!DeoptBundle.hasValue()) {
assert(AllowStatepointWithNoDeoptInfo &&
"Found non-leaf call without deopt info!");
return None;
return DeoptBundle.getValue().Inputs;
/// Compute the live-in set for every basic block in the function
static void computeLiveInValues(DominatorTree &DT, Function &F,
GCPtrLivenessData &Data);
/// Given results from the dataflow liveness computation, find the set of live
/// Values at a particular instruction.
static void findLiveSetAtInst(Instruction *inst, GCPtrLivenessData &Data,
StatepointLiveSetTy &out);
// TODO: Once we can get to the GCStrategy, this becomes
// Optional<bool> isGCManagedPointer(const Type *Ty) const override {
static bool isGCPointerType(Type *T) {
if (auto *PT = dyn_cast<PointerType>(T))
// For the sake of this example GC, we arbitrarily pick addrspace(1) as our
// GC managed heap. We know that a pointer into this heap needs to be
// updated and that no other pointer does.
return PT->getAddressSpace() == 1;
return false;
// Return true if this type is one which a) is a gc pointer or contains a GC
// pointer and b) is of a type this code expects to encounter as a live value.
// (The insertion code will assert that a type which matches (a) and not (b)
// is not encountered.)
static bool isHandledGCPointerType(Type *T) {
// We fully support gc pointers
if (isGCPointerType(T))
return true;
// We partially support vectors of gc pointers. The code will assert if it
// can't handle something.
if (auto VT = dyn_cast<VectorType>(T))
if (isGCPointerType(VT->getElementType()))
return true;
return false;
#ifndef NDEBUG
/// Returns true if this type contains a gc pointer whether we know how to
/// handle that type or not.
static bool containsGCPtrType(Type *Ty) {
if (isGCPointerType(Ty))
return true;
if (VectorType *VT = dyn_cast<VectorType>(Ty))
return isGCPointerType(VT->getScalarType());
if (ArrayType *AT = dyn_cast<ArrayType>(Ty))
return containsGCPtrType(AT->getElementType());
if (StructType *ST = dyn_cast<StructType>(Ty))
return llvm::any_of(ST->elements(), containsGCPtrType);
return false;
// Returns true if this is a type which a) is a gc pointer or contains a GC
// pointer and b) is of a type which the code doesn't expect (i.e. first class
// aggregates). Used to trip assertions.
static bool isUnhandledGCPointerType(Type *Ty) {
return containsGCPtrType(Ty) && !isHandledGCPointerType(Ty);
// Return the name of the value suffixed with the provided value, or if the
// value didn't have a name, the default value specified.
static std::string suffixed_name_or(Value *V, StringRef Suffix,
StringRef DefaultName) {
return V->hasName() ? (V->getName() + Suffix).str() : DefaultName.str();
// Conservatively identifies any definitions which might be live at the
// given instruction. The analysis is performed immediately before the
// given instruction. Values defined by that instruction are not considered
// live. Values used by that instruction are considered live.
static void analyzeParsePointLiveness(
DominatorTree &DT, GCPtrLivenessData &OriginalLivenessData, CallBase *Call,
PartiallyConstructedSafepointRecord &Result) {
StatepointLiveSetTy LiveSet;
findLiveSetAtInst(Call, OriginalLivenessData, LiveSet);
if (PrintLiveSet) {
dbgs() << "Live Variables:\n";
for (Value *V : LiveSet)
dbgs() << " " << V->getName() << " " << *V << "\n";
if (PrintLiveSetSize) {
dbgs() << "Safepoint For: " << Call->getCalledValue()->getName() << "\n";
dbgs() << "Number live values: " << LiveSet.size() << "\n";
Result.LiveSet = LiveSet;
static bool isKnownBaseResult(Value *V);
namespace {
/// A single base defining value - An immediate base defining value for an
/// instruction 'Def' is an input to 'Def' whose base is also a base of 'Def'.
/// For instructions which have multiple pointer [vector] inputs or that
/// transition between vector and scalar types, there is no immediate base
/// defining value. The 'base defining value' for 'Def' is the transitive
/// closure of this relation stopping at the first instruction which has no
/// immediate base defining value. The b.d.v. might itself be a base pointer,
/// but it can also be an arbitrary derived pointer.
struct BaseDefiningValueResult {
/// Contains the value which is the base defining value.
Value * const BDV;
/// True if the base defining value is also known to be an actual base
/// pointer.
const bool IsKnownBase;
BaseDefiningValueResult(Value *BDV, bool IsKnownBase)
: BDV(BDV), IsKnownBase(IsKnownBase) {
#ifndef NDEBUG
// Check consistency between new and old means of checking whether a BDV is
// a base.
bool MustBeBase = isKnownBaseResult(BDV);
assert(!MustBeBase || MustBeBase == IsKnownBase);
} // end anonymous namespace
static BaseDefiningValueResult findBaseDefiningValue(Value *I);
/// Return a base defining value for the 'Index' element of the given vector
/// instruction 'I'. If Index is null, returns a BDV for the entire vector
/// 'I'. As an optimization, this method will try to determine when the
/// element is known to already be a base pointer. If this can be established,
/// the second value in the returned pair will be true. Note that either a
/// vector or a pointer typed value can be returned. For the former, the
/// vector returned is a BDV (and possibly a base) of the entire vector 'I'.
/// If the later, the return pointer is a BDV (or possibly a base) for the
/// particular element in 'I'.
static BaseDefiningValueResult
findBaseDefiningValueOfVector(Value *I) {
// Each case parallels findBaseDefiningValue below, see that code for
// detailed motivation.
if (isa<Argument>(I))
// An incoming argument to the function is a base pointer
return BaseDefiningValueResult(I, true);
if (isa<Constant>(I))
// Base of constant vector consists only of constant null pointers.
// For reasoning see similar case inside 'findBaseDefiningValue' function.
return BaseDefiningValueResult(ConstantAggregateZero::get(I->getType()),
if (isa<LoadInst>(I))
return BaseDefiningValueResult(I, true);
if (isa<InsertElementInst>(I))
// We don't know whether this vector contains entirely base pointers or
// not. To be conservatively correct, we treat it as a BDV and will
// duplicate code as needed to construct a parallel vector of bases.
return BaseDefiningValueResult(I, false);
if (isa<ShuffleVectorInst>(I))
// We don't know whether this vector contains entirely base pointers or
// not. To be conservatively correct, we treat it as a BDV and will
// duplicate code as needed to construct a parallel vector of bases.
// TODO: There a number of local optimizations which could be applied here
// for particular sufflevector patterns.
return BaseDefiningValueResult(I, false);
// The behavior of getelementptr instructions is the same for vector and
// non-vector data types.
if (auto *GEP = dyn_cast<GetElementPtrInst>(I))
return findBaseDefiningValue(GEP->getPointerOperand());
// If the pointer comes through a bitcast of a vector of pointers to
// a vector of another type of pointer, then look through the bitcast
if (auto *BC = dyn_cast<BitCastInst>(I))
return findBaseDefiningValue(BC->getOperand(0));
// We assume that functions in the source language only return base
// pointers. This should probably be generalized via attributes to support
// both source language and internal functions.
if (isa<CallInst>(I) || isa<InvokeInst>(I))
return BaseDefiningValueResult(I, true);
// A PHI or Select is a base defining value. The outer findBasePointer
// algorithm is responsible for constructing a base value for this BDV.
assert((isa<SelectInst>(I) || isa<PHINode>(I)) &&
"unknown vector instruction - no base found for vector element");
return BaseDefiningValueResult(I, false);
/// Helper function for findBasePointer - Will return a value which either a)
/// defines the base pointer for the input, b) blocks the simple search
/// (i.e. a PHI or Select of two derived pointers), or c) involves a change
/// from pointer to vector type or back.
static BaseDefiningValueResult findBaseDefiningValue(Value *I) {
assert(I->getType()->isPtrOrPtrVectorTy() &&
"Illegal to ask for the base pointer of a non-pointer type");
if (I->getType()->isVectorTy())
return findBaseDefiningValueOfVector(I);
if (isa<Argument>(I))
// An incoming argument to the function is a base pointer
// We should have never reached here if this argument isn't an gc value
return BaseDefiningValueResult(I, true);
if (isa<Constant>(I)) {
// We assume that objects with a constant base (e.g. a global) can't move
// and don't need to be reported to the collector because they are always
// live. Besides global references, all kinds of constants (e.g. undef,
// constant expressions, null pointers) can be introduced by the inliner or
// the optimizer, especially on dynamically dead paths.
// Here we treat all of them as having single null base. By doing this we
// trying to avoid problems reporting various conflicts in a form of
// "phi (const1, const2)" or "phi (const, regular gc ptr)".
// See constant.ll file for relevant test cases.
return BaseDefiningValueResult(
ConstantPointerNull::get(cast<PointerType>(I->getType())), true);
if (CastInst *CI = dyn_cast<CastInst>(I)) {
Value *Def = CI->stripPointerCasts();
// If stripping pointer casts changes the address space there is an
// addrspacecast in between.
assert(cast<PointerType>(Def->getType())->getAddressSpace() ==
cast<PointerType>(CI->getType())->getAddressSpace() &&
"unsupported addrspacecast");
// If we find a cast instruction here, it means we've found a cast which is
// not simply a pointer cast (i.e. an inttoptr). We don't know how to
// handle int->ptr conversion.
assert(!isa<CastInst>(Def) && "shouldn't find another cast here");
return findBaseDefiningValue(Def);
if (isa<LoadInst>(I))
// The value loaded is an gc base itself
return BaseDefiningValueResult(I, true);
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I))
// The base of this GEP is the base
return findBaseDefiningValue(GEP->getPointerOperand());
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
// fall through to general call handling
case Intrinsic::experimental_gc_statepoint:
llvm_unreachable("statepoints don't produce pointers");
case Intrinsic::experimental_gc_relocate:
// Rerunning safepoint insertion after safepoints are already
// inserted is not supported. It could probably be made to work,
// but why are you doing this? There's no good reason.
llvm_unreachable("repeat safepoint insertion is not supported");
case Intrinsic::gcroot:
// Currently, this mechanism hasn't been extended to work with gcroot.
// There's no reason it couldn't be, but I haven't thought about the
// implications much.
"interaction with the gcroot mechanism is not supported");
// We assume that functions in the source language only return base
// pointers. This should probably be generalized via attributes to support
// both source language and internal functions.
if (isa<CallInst>(I) || isa<InvokeInst>(I))
return BaseDefiningValueResult(I, true);
// TODO: I have absolutely no idea how to implement this part yet. It's not
// necessarily hard, I just haven't really looked at it yet.
assert(!isa<LandingPadInst>(I) && "Landing Pad is unimplemented");
if (isa<AtomicCmpXchgInst>(I))
// A CAS is effectively a atomic store and load combined under a
// predicate. From the perspective of base pointers, we just treat it
// like a load.
return BaseDefiningValueResult(I, true);
assert(!isa<AtomicRMWInst>(I) && "Xchg handled above, all others are "
"binary ops which don't apply to pointers");
// The aggregate ops. Aggregates can either be in the heap or on the
// stack, but in either case, this is simply a field load. As a result,
// this is a defining definition of the base just like a load is.
if (isa<ExtractValueInst>(I))
return BaseDefiningValueResult(I, true);
// We should never see an insert vector since that would require we be
// tracing back a struct value not a pointer value.
assert(!isa<InsertValueInst>(I) &&
"Base pointer for a struct is meaningless");
// An extractelement produces a base result exactly when it's input does.
// We may need to insert a parallel instruction to extract the appropriate
// element out of the base vector corresponding to the input. Given this,
// it's analogous to the phi and select case even though it's not a merge.
if (isa<ExtractElementInst>(I))
// Note: There a lot of obvious peephole cases here. This are deliberately
// handled after the main base pointer inference algorithm to make writing
// test cases to exercise that code easier.
return BaseDefiningValueResult(I, false);
// The last two cases here don't return a base pointer. Instead, they
// return a value which dynamically selects from among several base
// derived pointers (each with it's own base potentially). It's the job of
// the caller to resolve these.
assert((isa<SelectInst>(I) || isa<PHINode>(I)) &&
"missing instruction case in findBaseDefiningValing");
return BaseDefiningValueResult(I, false);
/// Returns the base defining value for this value.
static Value *findBaseDefiningValueCached(Value *I, DefiningValueMapTy &Cache) {
Value *&Cached = Cache[I];
if (!Cached) {
Cached = findBaseDefiningValue(I).BDV;
LLVM_DEBUG(dbgs() << "fBDV-cached: " << I->getName() << " -> "
<< Cached->getName() << "\n");
assert(Cache[I] != nullptr);
return Cached;
/// Return a base pointer for this value if known. Otherwise, return it's
/// base defining value.
static Value *findBaseOrBDV(Value *I, DefiningValueMapTy &Cache) {
Value *Def = findBaseDefiningValueCached(I, Cache);
auto Found = Cache.find(Def);
if (Found != Cache.end()) {
// Either a base-of relation, or a self reference. Caller must check.
return Found->second;
// Only a BDV available
return Def;
/// Given the result of a call to findBaseDefiningValue, or findBaseOrBDV,
/// is it known to be a base pointer? Or do we need to continue searching.
static bool isKnownBaseResult(Value *V) {
if (!isa<PHINode>(V) && !isa<SelectInst>(V) &&
!isa<ExtractElementInst>(V) && !isa<InsertElementInst>(V) &&
!isa<ShuffleVectorInst>(V)) {
// no recursion possible
return true;
if (isa<Instruction>(V) &&
cast<Instruction>(V)->getMetadata("is_base_value")) {
// This is a previously inserted base phi or select. We know
// that this is a base value.
return true;
// We need to keep searching
return false;
namespace {
/// Models the state of a single base defining value in the findBasePointer
/// algorithm for determining where a new instruction is needed to propagate
/// the base of this BDV.
class BDVState {
enum Status { Unknown, Base, Conflict };
BDVState() : BaseValue(nullptr) {}
explicit BDVState(Status Status, Value *BaseValue = nullptr)
: Status(Status), BaseValue(BaseValue) {
assert(Status != Base || BaseValue);
explicit BDVState(Value *BaseValue) : Status(Base), BaseValue(BaseValue) {}
Status getStatus() const { return Status; }
Value *getBaseValue() const { return BaseValue; }
bool isBase() const { return getStatus() == Base; }
bool isUnknown() const { return getStatus() == Unknown; }
bool isConflict() const { return getStatus() == Conflict; }
bool operator==(const BDVState &Other) const {
return BaseValue == Other.BaseValue && Status == Other.Status;
bool operator!=(const BDVState &other) const { return !(*this == other); }
void dump() const {
dbgs() << '\n';
void print(raw_ostream &OS) const {
switch (getStatus()) {
case Unknown:
OS << "U";
case Base:
OS << "B";
case Conflict:
OS << "C";
OS << " (" << getBaseValue() << " - "
<< (getBaseValue() ? getBaseValue()->getName() : "nullptr") << "): ";
Status Status = Unknown;
AssertingVH<Value> BaseValue; // Non-null only if Status == Base.
} // end anonymous namespace
#ifndef NDEBUG
static raw_ostream &operator<<(raw_ostream &OS, const BDVState &State) {
return OS;
static BDVState meetBDVStateImpl(const BDVState &LHS, const BDVState &RHS) {
switch (LHS.getStatus()) {
case BDVState::Unknown:
return RHS;
case BDVState::Base:
assert(LHS.getBaseValue() && "can't be null");
if (RHS.isUnknown())
return LHS;
if (RHS.isBase()) {
if (LHS.getBaseValue() == RHS.getBaseValue()) {
assert(LHS == RHS && "equality broken!");
return LHS;
return BDVState(BDVState::Conflict);
assert(RHS.isConflict() && "only three states!");
return BDVState(BDVState::Conflict);
case BDVState::Conflict:
return LHS;
llvm_unreachable("only three states!");
// Values of type BDVState form a lattice, and this function implements the meet
// operation.
static BDVState meetBDVState(const BDVState &LHS, const BDVState &RHS) {
BDVState Result = meetBDVStateImpl(LHS, RHS);
assert(Result == meetBDVStateImpl(RHS, LHS) &&
"Math is wrong: meet does not commute!");
return Result;
/// For a given value or instruction, figure out what base ptr its derived from.
/// For gc objects, this is simply itself. On success, returns a value which is
/// the base pointer. (This is reliable and can be used for relocation.) On
/// failure, returns nullptr.
static Value *findBasePointer(Value *I, DefiningValueMapTy &Cache) {
Value *Def = findBaseOrBDV(I, Cache);
if (isKnownBaseResult(Def))
return Def;
// Here's the rough algorithm:
// - For every SSA value, construct a mapping to either an actual base
// pointer or a PHI which obscures the base pointer.
// - Construct a mapping from PHI to unknown TOP state. Use an
// optimistic algorithm to propagate base pointer information. Lattice
// looks like:
// b1 b2 b3 b4
// When algorithm terminates, all PHIs will either have a single concrete
// base or be in a conflict state.
// - For every conflict, insert a dummy PHI node without arguments. Add
// these to the base[Instruction] = BasePtr mapping. For every
// non-conflict, add the actual base.
// - For every conflict, add arguments for the base[a] of each input
// arguments.
// Note: A simpler form of this would be to add the conflict form of all
// PHIs without running the optimistic algorithm. This would be
// analogous to pessimistic data flow and would likely lead to an
// overall worse solution.
#ifndef NDEBUG
auto isExpectedBDVType = [](Value *BDV) {
return isa<PHINode>(BDV) || isa<SelectInst>(BDV) ||
isa<ExtractElementInst>(BDV) || isa<InsertElementInst>(BDV) ||
// Once populated, will contain a mapping from each potentially non-base BDV
// to a lattice value (described above) which corresponds to that BDV.
// We use the order of insertion (DFS over the def/use graph) to provide a
// stable deterministic ordering for visiting DenseMaps (which are unordered)
// below. This is important for deterministic compilation.
MapVector<Value *, BDVState> States;
// Recursively fill in all base defining values reachable from the initial
// one for which we don't already know a definite base value for
/* scope */ {
SmallVector<Value*, 16> Worklist;
States.insert({Def, BDVState()});
while (!Worklist.empty()) {
Value *Current = Worklist.pop_back_val();
assert(!isKnownBaseResult(Current) && "why did it get added?");
auto visitIncomingValue = [&](Value *InVal) {
Value *Base = findBaseOrBDV(InVal, Cache);
if (isKnownBaseResult(Base))
// Known bases won't need new instructions introduced and can be
// ignored safely
assert(isExpectedBDVType(Base) && "the only non-base values "
"we see should be base defining values");
if (States.insert(std::make_pair(Base, BDVState())).second)
if (PHINode *PN = dyn_cast<PHINode>(Current)) {
for (Value *InVal : PN->incoming_values())
} else if (SelectInst *SI = dyn_cast<SelectInst>(Current)) {
} else if (auto *EE = dyn_cast<ExtractElementInst>(Current)) {
} else if (auto *IE = dyn_cast<InsertElementInst>(Current)) {
visitIncomingValue(IE->getOperand(0)); // vector operand
visitIncomingValue(IE->getOperand(1)); // scalar operand
} else if (auto *SV = dyn_cast<ShuffleVectorInst>(Current)) {
else {
llvm_unreachable("Unimplemented instruction case");
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << "States after initialization:\n");
for (auto Pair : States) {
LLVM_DEBUG(dbgs() << " " << Pair.second << " for " << *Pair.first << "\n");
// Return a phi state for a base defining value. We'll generate a new
// base state for known bases and expect to find a cached state otherwise.
auto getStateForBDV = [&](Value *baseValue) {
if (isKnownBaseResult(baseValue))
return BDVState(baseValue);
auto I = States.find(baseValue);
assert(I != States.end() && "lookup failed!");
return I->second;
bool Progress = true;
while (Progress) {
#ifndef NDEBUG
const size_t OldSize = States.size();
Progress = false;
// We're only changing values in this loop, thus safe to keep iterators.
// Since this is computing a fixed point, the order of visit does not
// effect the result. TODO: We could use a worklist here and make this run
// much faster.
for (auto Pair : States) {
Value *BDV = Pair.first;
assert(!isKnownBaseResult(BDV) && "why did it get added?");
// Given an input value for the current instruction, return a BDVState
// instance which represents the BDV of that value.
auto getStateForInput = [&](Value *V) mutable {
Value *BDV = findBaseOrBDV(V, Cache);
return getStateForBDV(BDV);
BDVState NewState;
if (SelectInst *SI = dyn_cast<SelectInst>(BDV)) {
NewState = meetBDVState(NewState, getStateForInput(SI->getTrueValue()));
NewState =
meetBDVState(NewState, getStateForInput(SI->getFalseValue()));
} else if (PHINode *PN = dyn_cast<PHINode>(BDV)) {
for (Value *Val : PN->incoming_values())
NewState = meetBDVState(NewState, getStateForInput(Val));
} else if (auto *EE = dyn_cast<ExtractElementInst>(BDV)) {
// The 'meet' for an extractelement is slightly trivial, but it's still
// useful in that it drives us to conflict if our input is.
NewState =
meetBDVState(NewState, getStateForInput(EE->getVectorOperand()));
} else if (auto *IE = dyn_cast<InsertElementInst>(BDV)){
// Given there's a inherent type mismatch between the operands, will
// *always* produce Conflict.
NewState = meetBDVState(NewState, getStateForInput(IE->getOperand(0)));
NewState = meetBDVState(NewState, getStateForInput(IE->getOperand(1)));
} else {
// The only instance this does not return a Conflict is when both the
// vector operands are the same vector.
auto *SV = cast<ShuffleVectorInst>(BDV);
NewState = meetBDVState(NewState, getStateForInput(SV->getOperand(0)));
NewState = meetBDVState(NewState, getStateForInput(SV->getOperand(1)));
BDVState OldState = States[BDV];
if (OldState != NewState) {
Progress = true;
States[BDV] = NewState;
assert(OldSize == States.size() &&
"fixed point shouldn't be adding any new nodes to state");
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << "States after meet iteration:\n");
for (auto Pair : States) {
LLVM_DEBUG(dbgs() << " " << Pair.second << " for " << *Pair.first << "\n");
// Insert Phis for all conflicts
// TODO: adjust naming patterns to avoid this order of iteration dependency
for (auto Pair : States) {
Instruction *I = cast<Instruction>(Pair.first);
BDVState State = Pair.second;
assert(!isKnownBaseResult(I) && "why did it get added?");
assert(!State.isUnknown() && "Optimistic algorithm didn't complete!");
// extractelement instructions are a bit special in that we may need to
// insert an extract even when we know an exact base for the instruction.
// The problem is that we need to convert from a vector base to a scalar
// base for the particular indice we're interested in.
if (State.isBase() && isa<ExtractElementInst>(I) &&
isa<VectorType>(State.getBaseValue()->getType())) {
auto *EE = cast<ExtractElementInst>(I);
// TODO: In many cases, the new instruction is just EE itself. We should
// exploit this, but can't do it here since it would break the invariant
// about the BDV not being known to be a base.
auto *BaseInst = ExtractElementInst::Create(
State.getBaseValue(), EE->getIndexOperand(), "base_ee", EE);
BaseInst->setMetadata("is_base_value", MDNode::get(I->getContext(), {}));
States[I] = BDVState(BDVState::Base, BaseInst);
// Since we're joining a vector and scalar base, they can never be the
// same. As a result, we should always see insert element having reached
// the conflict state.
assert(!isa<InsertElementInst>(I) || State.isConflict());
if (!State.isConflict())
/// Create and insert a new instruction which will represent the base of
/// the given instruction 'I'.
auto MakeBaseInstPlaceholder = [](Instruction *I) -> Instruction* {
if (isa<PHINode>(I)) {
BasicBlock *BB = I->getParent();
int NumPreds = pred_size(BB);
assert(NumPreds > 0 && "how did we reach here");
std::string Name = suffixed_name_or(I, ".base", "base_phi");
return PHINode::Create(I->getType(), NumPreds, Name, I);
} else if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
// The undef will be replaced later
UndefValue *Undef = UndefValue::get(SI->getType());
std::string Name = suffixed_name_or(I, ".base", "base_select");
return SelectInst::Create(SI->getCondition(), Undef, Undef, Name, SI);
} else if (auto *EE = dyn_cast<ExtractElementInst>(I)) {
UndefValue *Undef = UndefValue::get(EE->getVectorOperand()->getType());
std::string Name = suffixed_name_or(I, ".base", "base_ee");
return ExtractElementInst::Create(Undef, EE->getIndexOperand(), Name,
} else if (auto *IE = dyn_cast<InsertElementInst>(I)) {
UndefValue *VecUndef = UndefValue::get(IE->getOperand(0)->getType());
UndefValue *ScalarUndef = UndefValue::get(IE->getOperand(1)->getType());
std::string Name = suffixed_name_or(I, ".base", "base_ie");
return InsertElementInst::Create(VecUndef, ScalarUndef,
IE->getOperand(2), Name, IE);
} else {
auto *SV = cast<ShuffleVectorInst>(I);
UndefValue *VecUndef = UndefValue::get(SV->getOperand(0)->getType());
std::string Name = suffixed_name_or(I, ".base", "base_sv");
return new ShuffleVectorInst(VecUndef, VecUndef, SV->getOperand(2),
Name, SV);
Instruction *BaseInst = MakeBaseInstPlaceholder(I);
// Add metadata marking this as a base value
BaseInst->setMetadata("is_base_value", MDNode::get(I->getContext(), {}));
States[I] = BDVState(BDVState::Conflict, BaseInst);
// Returns a instruction which produces the base pointer for a given
// instruction. The instruction is assumed to be an input to one of the BDVs
// seen in the inference algorithm above. As such, we must either already
// know it's base defining value is a base, or have inserted a new
// instruction to propagate the base of it's BDV and have entered that newly
// introduced instruction into the state table. In either case, we are
// assured to be able to determine an instruction which produces it's base
// pointer.
auto getBaseForInput = [&](Value *Input, Instruction *InsertPt) {
Value *BDV = findBaseOrBDV(Input, Cache);
Value *Base = nullptr;
if (isKnownBaseResult(BDV)) {
Base = BDV;
} else {
// Either conflict or base.
Base = States[BDV].getBaseValue();
assert(Base && "Can't be null");
// The cast is needed since base traversal may strip away bitcasts
if (Base->getType() != Input->getType() && InsertPt)
Base = new BitCastInst(Base, Input->getType(), "cast", InsertPt);
return Base;
// Fixup all the inputs of the new PHIs. Visit order needs to be
// deterministic and predictable because we're naming newly created
// instructions.
for (auto Pair : States) {
Instruction *BDV = cast<Instruction>(Pair.first);
BDVState State = Pair.second;
assert(!isKnownBaseResult(BDV) && "why did it get added?");
assert(!State.isUnknown() && "Optimistic algorithm didn't complete!");
if (!State.isConflict())
if (PHINode *BasePHI = dyn_cast<PHINode>(State.getBaseValue())) {
PHINode *PN = cast<PHINode>(BDV);
unsigned NumPHIValues = PN->getNumIncomingValues();
for (unsigned i = 0; i < NumPHIValues; i++) {
Value *InVal = PN->getIncomingValue(i);
BasicBlock *InBB = PN->getIncomingBlock(i);
// If we've already seen InBB, add the same incoming value
// we added for it earlier. The IR verifier requires phi
// nodes with multiple entries from the same basic block
// to have the same incoming value for each of those
// entries. If we don't do this check here and basephi
// has a different type than base, we'll end up adding two
// bitcasts (and hence two distinct values) as incoming
// values for the same basic block.
int BlockIndex = BasePHI->getBasicBlockIndex(InBB);
if (BlockIndex != -1) {
Value *OldBase = BasePHI->getIncomingValue(BlockIndex);
BasePHI->addIncoming(OldBase, InBB);
#ifndef NDEBUG
Value *Base = getBaseForInput(InVal, nullptr);
// In essence this assert states: the only way two values
// incoming from the same basic block may be different is by
// being different bitcasts of the same value. A cleanup
// that remains TODO is changing findBaseOrBDV to return an
// llvm::Value of the correct type (and still remain pure).
// This will remove the need to add bitcasts.
assert(Base->stripPointerCasts() == OldBase->stripPointerCasts() &&
"Sanity -- findBaseOrBDV should be pure!");
// Find the instruction which produces the base for each input. We may
// need to insert a bitcast in the incoming block.
// TODO: Need to split critical edges if insertion is needed
Value *Base = getBaseForInput(InVal, InBB->getTerminator());
BasePHI->addIncoming(Base, InBB);
assert(BasePHI->getNumIncomingValues() == NumPHIValues);
} else if (SelectInst *BaseSI =
dyn_cast<SelectInst>(State.getBaseValue())) {
SelectInst *SI = cast<SelectInst>(BDV);
// Find the instruction which produces the base for each input.
// We may need to insert a bitcast.
BaseSI->setTrueValue(getBaseForInput(SI->getTrueValue(), BaseSI));
BaseSI->setFalseValue(getBaseForInput(SI->getFalseValue(), BaseSI));
} else if (auto *BaseEE =
dyn_cast<ExtractElementInst>(State.getBaseValue())) {
Value *InVal = cast<ExtractElementInst>(BDV)->getVectorOperand();
// Find the instruction which produces the base for each input. We may
// need to insert a bitcast.
BaseEE->setOperand(0, getBaseForInput(InVal, BaseEE));
} else if (auto *BaseIE = dyn_cast<InsertElementInst>(State.getBaseValue())){
auto *BdvIE = cast<InsertElementInst>(BDV);
auto UpdateOperand = [&](int OperandIdx) {
Value *InVal = BdvIE->getOperand(OperandIdx);
Value *Base = getBaseForInput(InVal, BaseIE);
BaseIE->setOperand(OperandIdx, Base);
UpdateOperand(0); // vector operand
UpdateOperand(1); // scalar operand
} else {
auto *BaseSV = cast<ShuffleVectorInst>(State.getBaseValue());
auto *BdvSV = cast<ShuffleVectorInst>(BDV);
auto UpdateOperand = [&](int OperandIdx) {
Value *InVal = BdvSV->getOperand(OperandIdx);
Value *Base = getBaseForInput(InVal, BaseSV);
BaseSV->setOperand(OperandIdx, Base);
UpdateOperand(0); // vector operand
UpdateOperand(1); // vector operand
// Cache all of our results so we can cheaply reuse them
// NOTE: This is actually two caches: one of the base defining value
// relation and one of the base pointer relation! FIXME
for (auto Pair : States) {
auto *BDV = Pair.first;
Value *Base = Pair.second.getBaseValue();
assert(BDV && Base);
assert(!isKnownBaseResult(BDV) && "why did it get added?");
dbgs() << "Updating base value cache"
<< " for: " << BDV->getName() << " from: "
<< (Cache.count(BDV) ? Cache[BDV]->getName().str() : "none")
<< " to: " << Base->getName() << "\n");
if (Cache.count(BDV)) {
assert(isKnownBaseResult(Base) &&
"must be something we 'know' is a base pointer");
// Once we transition from the BDV relation being store in the Cache to
// the base relation being stored, it must be stable
assert((!isKnownBaseResult(Cache[BDV]) || Cache[BDV] == Base) &&
"base relation should be stable");
Cache[BDV] = Base;
return Cache[Def];
// For a set of live pointers (base and/or derived), identify the base
// pointer of the object which they are derived from. This routine will
// mutate the IR graph as needed to make the 'base' pointer live at the
// definition site of 'derived'. This ensures that any use of 'derived' can
// also use 'base'. This may involve the insertion of a number of
// additional PHI nodes.
// preconditions: live is a set of pointer type Values
// side effects: may insert PHI nodes into the existing CFG, will preserve
// CFG, will not remove or mutate any existing nodes
// post condition: PointerToBase contains one (derived, base) pair for every
// pointer in live. Note that derived can be equal to base if the original
// pointer was a base pointer.
static void
findBasePointers(const StatepointLiveSetTy &live,
MapVector<Value *, Value *> &PointerToBase,
DominatorTree *DT, DefiningValueMapTy &DVCache) {
for (Value *ptr : live) {
Value *base = findBasePointer(ptr, DVCache);
assert(base && "failed to find base pointer");
PointerToBase[ptr] = base;
assert((!isa<Instruction>(base) || !isa<Instruction>(ptr) ||
cast<Instruction>(ptr)->getParent())) &&
"The base we found better dominate the derived pointer");
/// Find the required based pointers (and adjust the live set) for the given
/// parse point.
static void findBasePointers(DominatorTree &DT, DefiningValueMapTy &DVCache,
CallBase *Call,
PartiallyConstructedSafepointRecord &result) {
MapVector<Value *, Value *> PointerToBase;
findBasePointers(result.LiveSet, PointerToBase, &DT, DVCache);
if (PrintBasePointers) {
errs() << "Base Pairs (w/o Relocation):\n";
for (auto &Pair : PointerToBase) {
errs() << " derived ";
Pair.first->printAsOperand(errs(), false);
errs() << " base ";
Pair.second->printAsOperand(errs(), false);
errs() << "\n";;
result.PointerToBase = PointerToBase;
/// Given an updated version of the dataflow liveness results, update the
/// liveset and base pointer maps for the call site CS.
static void recomputeLiveInValues(GCPtrLivenessData &RevisedLivenessData,
CallBase *Call,
PartiallyConstructedSafepointRecord &result);
static void recomputeLiveInValues(
Function &F, DominatorTree &DT, ArrayRef<CallBase *> toUpdate,
MutableArrayRef<struct PartiallyConstructedSafepointRecord> records) {
// TODO-PERF: reuse the original liveness, then simply run the dataflow
// again. The old values are still live and will help it stabilize quickly.
GCPtrLivenessData RevisedLivenessData;
computeLiveInValues(DT, F, RevisedLivenessData);
for (size_t i = 0; i < records.size(); i++) {
struct PartiallyConstructedSafepointRecord &info = records[i];
recomputeLiveInValues(RevisedLivenessData, toUpdate[i], info);
// When inserting gc.relocate and gc.result calls, we need to ensure there are
// no uses of the original value / return value between the gc.statepoint and
// the gc.relocate / gc.result call. One case which can arise is a phi node
// starting one of the successor blocks. We also need to be able to insert the
// gc.relocates only on the path which goes through the statepoint. We might
// need to split an edge to make this possible.
static BasicBlock *
normalizeForInvokeSafepoint(BasicBlock *BB, BasicBlock *InvokeParent,
DominatorTree &DT) {
BasicBlock *Ret = BB;
if (!BB->getUniquePredecessor())
Ret = SplitBlockPredecessors(BB, InvokeParent, "", &DT);
// Now that 'Ret' has unique predecessor we can safely remove all phi nodes
// from it
assert(!isa<PHINode>(Ret->begin()) &&
"All PHI nodes should have been removed!");
// At this point, we can safely insert a gc.relocate or gc.result as the first
// instruction in Ret if needed.
return Ret;
// Create new attribute set containing only attributes which can be transferred
// from original call to the safepoint.
static AttributeList legalizeCallAttributes(AttributeList AL) {
if (AL.isEmpty())
return AL;
// Remove the readonly, readnone, and statepoint function attributes.
AttrBuilder FnAttrs = AL.getFnAttributes();
for (Attribute A : AL.getFnAttributes()) {
if (isStatepointDirectiveAttr(A))
// Just skip parameter and return attributes for now
LLVMContext &Ctx = AL.getContext();
return AttributeList::get(Ctx, AttributeList::FunctionIndex,
AttributeSet::get(Ctx, FnAttrs));
/// Helper function to place all gc relocates necessary for the given
/// statepoint.
/// Inputs:
/// liveVariables - list of variables to be relocated.
/// liveStart - index of the first live variable.
/// basePtrs - base pointers.
/// statepointToken - statepoint instruction to which relocates should be
/// bound.
/// Builder - Llvm IR builder to be used to construct new calls.
static void CreateGCRelocates(ArrayRef<Value *> LiveVariables,
const int LiveStart,
ArrayRef<Value *> BasePtrs,
Instruction *StatepointToken,
IRBuilder<> Builder) {
if (LiveVariables.empty())
auto FindIndex = [](ArrayRef<Value *> LiveVec, Value *Val) {
auto ValIt = llvm::find(LiveVec, Val);
assert(ValIt != LiveVec.end() && "Val not found in LiveVec!");
size_t Index = std::distance(LiveVec.begin(), ValIt);
assert(Index < LiveVec.size() && "Bug in std::find?");
return Index;
Module *M = StatepointToken->getModule();
// All gc_relocate are generated as i8 addrspace(1)* (or a vector type whose
// element type is i8 addrspace(1)*). We originally generated unique
// declarations for each pointer type, but this proved problematic because
// the intrinsic mangling code is incomplete and fragile. Since we're moving
// towards a single unified pointer type anyways, we can just cast everything
// to an i8* of the right address space. A bitcast is added later to convert
// gc_relocate to the actual value's type.
auto getGCRelocateDecl = [&] (Type *Ty) {
auto AS = Ty->getScalarType()->getPointerAddressSpace();
Type *NewTy = Type::getInt8PtrTy(M->getContext(), AS);
if (auto *VT = dyn_cast<VectorType>(Ty))
NewTy = VectorType::get(NewTy, VT->getNumElements());
return Intrinsic::getDeclaration(M, Intrinsic::experimental_gc_relocate,
// Lazily populated map from input types to the canonicalized form mentioned
// in the comment above. This should probably be cached somewhere more
// broadly.
DenseMap<Type *, Function *> TypeToDeclMap;
for (unsigned i = 0; i < LiveVariables.size(); i++) {
// Generate the gc.relocate call and save the result
Value *BaseIdx =
Builder.getInt32(LiveStart + FindIndex(LiveVariables, BasePtrs[i]));
Value *LiveIdx = Builder.getInt32(LiveStart + i);
Type *Ty = LiveVariables[i]->getType();
if (!TypeToDeclMap.count(Ty))
TypeToDeclMap[Ty] = getGCRelocateDecl(Ty);
Function *GCRelocateDecl = TypeToDeclMap[Ty];
// only specify a debug name if we can give a useful one
CallInst *Reloc = Builder.CreateCall(
GCRelocateDecl, {StatepointToken, BaseIdx, LiveIdx},
suffixed_name_or(LiveVariables[i], ".relocated", ""));
// Trick CodeGen into thinking there are lots of free registers at this
// fake call.
namespace {
/// This struct is used to defer RAUWs and `eraseFromParent` s. Using this
/// avoids having to worry about keeping around dangling pointers to Values.
class DeferredReplacement {
AssertingVH<Instruction> Old;
AssertingVH<Instruction> New;
bool IsDeoptimize = false;
DeferredReplacement() = default;
static DeferredReplacement createRAUW(Instruction *Old, Instruction *New) {
assert(Old != New && Old && New &&
"Cannot RAUW equal values or to / from null!");
DeferredReplacement D;
D.Old = Old;
D.New = New;
return D;
static DeferredReplacement createDelete(Instruction *ToErase) {
DeferredReplacement D;
D.Old = ToErase;
return D;
static DeferredReplacement createDeoptimizeReplacement(Instruction *Old) {
#ifndef NDEBUG
auto *F = cast<CallInst>(Old)->getCalledFunction();
assert(F && F->getIntrinsicID() == Intrinsic::experimental_deoptimize &&
"Only way to construct a deoptimize deferred replacement");
DeferredReplacement D;
D.Old = Old;
D.IsDeoptimize = true;
return D;
/// Does the task represented by this instance.
void doReplacement() {
Instruction *OldI = Old;
Instruction *NewI = New;
assert(OldI != NewI && "Disallowed at construction?!");
assert((!IsDeoptimize || !New) &&
"Deoptimize intrinsics are not replaced!");
Old = nullptr;
New = nullptr;
if (NewI)
if (IsDeoptimize) {
// Note: we've inserted instructions, so the call to llvm.deoptimize may
// not necessarily be followed by the matching return.
auto *RI = cast<ReturnInst>(OldI->getParent()->getTerminator());
new UnreachableInst(RI->getContext(), RI);
} // end anonymous namespace
static StringRef getDeoptLowering(CallBase *Call) {
const char *DeoptLowering = "deopt-lowering";
if (Call->hasFnAttr(DeoptLowering)) {
// FIXME: Calls have a *really* confusing interface around attributes
// with values.
const AttributeList &CSAS = Call->getAttributes();
if (CSAS.hasAttribute(AttributeList::FunctionIndex, DeoptLowering))
return CSAS.getAttribute(AttributeList::FunctionIndex, DeoptLowering)
Function *F = Call->getCalledFunction();
assert(F && F->hasFnAttribute(DeoptLowering));
return F->getFnAttribute(DeoptLowering).getValueAsString();
return "live-through";
static void
makeStatepointExplicitImpl(CallBase *Call, /* to replace */
const SmallVectorImpl<Value *> &BasePtrs,
const SmallVectorImpl<Value *> &LiveVariables,
PartiallyConstructedSafepointRecord &Result,
std::vector<DeferredReplacement> &Replacements) {
assert(BasePtrs.size() == LiveVariables.size());
// Then go ahead and use the builder do actually do the inserts. We insert
// immediately before the previous instruction under the assumption that all
// arguments will be available here. We can't insert afterwards since we may
// be replacing a terminator.
IRBuilder<> Builder(Call);
ArrayRef<Value *> GCArgs(LiveVariables);
uint64_t StatepointID = StatepointDirectives::DefaultStatepointID;
uint32_t NumPatchBytes = 0;
uint32_t Flags = uint32_t(StatepointFlags::None);
ArrayRef<Use> CallArgs(Call->arg_begin(), Call->arg_end());
ArrayRef<Use> DeoptArgs = GetDeoptBundleOperands(Call);
ArrayRef<Use> TransitionArgs;
if (auto TransitionBundle =
Call->getOperandBundle(LLVMContext::OB_gc_transition)) {
Flags |= uint32_t(StatepointFlags::GCTransition);
TransitionArgs = TransitionBundle->Inputs;
// Instead of lowering calls to @llvm.experimental.deoptimize as normal calls
// with a return value, we lower then as never returning calls to
// __llvm_deoptimize that are followed by unreachable to get better codegen.
bool IsDeoptimize = false;
StatepointDirectives SD =
if (SD.NumPatchBytes)
NumPatchBytes = *SD.NumPatchBytes;
if (SD.StatepointID)
StatepointID = *SD.StatepointID;
// Pass through the requested lowering if any. The default is live-through.
StringRef DeoptLowering = getDeoptLowering(Call);
if (DeoptLowering.equals("live-in"))
Flags |= uint32_t(StatepointFlags::DeoptLiveIn);
else {
assert(DeoptLowering.equals("live-through") && "Unsupported value!");
Value *CallTarget = Call->getCalledValue();
if (Function *F = dyn_cast<Function>(CallTarget)) {
if (F->getIntrinsicID() == Intrinsic::experimental_deoptimize) {
// Calls to llvm.experimental.deoptimize are lowered to calls to the
// __llvm_deoptimize symbol. We want to resolve this now, since the
// verifier does not allow taking the address of an intrinsic function.
SmallVector<Type *, 8> DomainTy;
for (Value *Arg : CallArgs)
auto *FTy = FunctionType::get(Type::getVoidTy(F->getContext()), DomainTy,
/* isVarArg = */ false);
// Note: CallTarget can be a bitcast instruction of a symbol if there are
// calls to @llvm.experimental.deoptimize with different argument types in
// the same module. This is fine -- we assume the frontend knew what it
// was doing when generating this kind of IR.
CallTarget = F->getParent()
->getOrInsertFunction("__llvm_deoptimize", FTy)
IsDeoptimize = true;
// Create the statepoint given all the arguments
Instruction *Token = nullptr;
if (auto *CI = dyn_cast<CallInst>(Call)) {
CallInst *SPCall = Builder.CreateGCStatepointCall(
StatepointID, NumPatchBytes, CallTarget, Flags, CallArgs,
TransitionArgs, DeoptArgs, GCArgs, "safepoint_token");
// Currently we will fail on parameter attributes and on certain
// function attributes. In case if we can handle this set of attributes -
// set up function attrs directly on statepoint and return attrs later for
// gc_result intrinsic.
Token = SPCall;
// Put the following gc_result and gc_relocate calls immediately after the
// the old call (which we're about to delete)
assert(CI->getNextNode() && "Not a terminator, must have next!");
} else {
auto *II = cast<InvokeInst>(Call);
// Insert the new invoke into the old block. We'll remove the old one in a
// moment at which point this will become the new terminator for the
// original block.
InvokeInst *SPInvoke = Builder.CreateGCStatepointInvoke(
StatepointID, NumPatchBytes, CallTarget, II->getNormalDest(),
II->getUnwindDest(), Flags, CallArgs, TransitionArgs, DeoptArgs, GCArgs,
// Currently we will fail on parameter attributes and on certain
// function attributes. In case if we can handle this set of attributes -
// set up function attrs directly on statepoint and return attrs later for
// gc_result intrinsic.
Token = SPInvoke;
// Generate gc relocates in exceptional path
BasicBlock *UnwindBlock = II->getUnwindDest();
assert(!isa<PHINode>(UnwindBlock->begin()) &&
UnwindBlock->getUniquePredecessor() &&
"can't safely insert in this block!");
// Attach exceptional gc relocates to the landingpad.
Instruction *ExceptionalToken = UnwindBlock->getLandingPadInst();
Result.UnwindToken = ExceptionalToken;
const unsigned LiveStartIdx = Statepoint(Token).gcArgsStartIdx();
CreateGCRelocates(LiveVariables, LiveStartIdx, BasePtrs, ExceptionalToken,
// Generate gc relocates and returns for normal block
BasicBlock *NormalDest = II->getNormalDest();
assert(!isa<PHINode>(NormalDest->begin()) &&
NormalDest->getUniquePredecessor() &&
"can't safely insert in this block!");
// gc relocates will be generated later as if it were regular call
// statepoint
assert(Token && "Should be set in one of the above branches!");
if (IsDeoptimize) {
// If we're wrapping an @llvm.experimental.deoptimize in a statepoint, we
// transform the tail-call like structure to a call to a void function
// followed by unreachable to get better codegen.
} else {
if (!Call->getType()->isVoidTy() && !Call->use_empty()) {
StringRef Name = Call->hasName() ? Call->getName() : "";
CallInst *GCResult = Builder.CreateGCResult(Token, Call->getType(), Name);
AttributeList::get(GCResult->getContext(), AttributeList::ReturnIndex,
// We cannot RAUW or delete CS.getInstruction() because it could be in the
// live set of some other safepoint, in which case that safepoint's
// PartiallyConstructedSafepointRecord will hold a raw pointer to this
// llvm::Instruction. Instead, we defer the replacement and deletion to
// after the live sets have been made explicit in the IR, and we no longer
// have raw pointers to worry about.
DeferredReplacement::createRAUW(Call, GCResult));
} else {
Result.StatepointToken = Token;
// Second, create a gc.relocate for every live variable
const unsigned LiveStartIdx = Statepoint(Token).gcArgsStartIdx();
CreateGCRelocates(LiveVariables, LiveStartIdx, BasePtrs, Token, Builder);
// Replace an existing gc.statepoint with a new one and a set of gc.relocates
// which make the relocations happening at this safepoint explicit.
// WARNING: Does not do any fixup to adjust users of the original live
// values. That's the callers responsibility.
static void
makeStatepointExplicit(DominatorTree &DT, CallBase *Call,
PartiallyConstructedSafepointRecord &Result,
std::vector<DeferredReplacement> &Replacements) {
const auto &LiveSet = Result.LiveSet;
const auto &PointerToBase = Result.PointerToBase;
// Convert to vector for efficient cross referencing.
SmallVector<Value *, 64> BaseVec, LiveVec;
for (Value *L : LiveSet) {
Value *Base = PointerToBase.find(L)->second;
assert(LiveVec.size() == BaseVec.size());
// Do the actual rewriting and delete the old statepoint
makeStatepointExplicitImpl(Call, BaseVec, LiveVec, Result, Replacements);
// Helper function for the relocationViaAlloca.
// It receives iterator to the statepoint gc relocates and emits a store to the
// assigned location (via allocaMap) for the each one of them. It adds the
// visited values into the visitedLiveValues set, which we will later use them
// for sanity checking.
static void
insertRelocationStores(iterator_range<Value::user_iterator> GCRelocs,
DenseMap<Value *, AllocaInst *> &AllocaMap,
DenseSet<Value *> &VisitedLiveValues) {
for (User *U : GCRelocs) {
GCRelocateInst *Relocate = dyn_cast<GCRelocateInst>(U);
if (!Relocate)
Value *OriginalValue = Relocate->getDerivedPtr();
Value *Alloca = AllocaMap[OriginalValue];
// Emit store into the related alloca
// All gc_relocates are i8 addrspace(1)* typed, and it must be bitcasted to
// the correct type according to alloca.
assert(Relocate->getNextNode() &&
"Should always have one since it's not a terminator");
IRBuilder<> Builder(Relocate->getNextNode());
Value *CastedRelocatedValue =
suffixed_name_or(Relocate, ".casted", ""));
StoreInst *Store = new StoreInst(CastedRelocatedValue, Alloca);
#ifndef NDEBUG
// Helper function for the "relocationViaAlloca". Similar to the
// "insertRelocationStores" but works for rematerialized values.
static void insertRematerializationStores(
const RematerializedValueMapTy &RematerializedValues,
DenseMap<Value *, AllocaInst *> &AllocaMap,
DenseSet<Value *> &VisitedLiveValues) {
for (auto RematerializedValuePair: RematerializedValues) {
Instruction *RematerializedValue = RematerializedValuePair.first;
Value *OriginalValue = RematerializedValuePair.second;
assert(AllocaMap.count(OriginalValue) &&
"Can not find alloca for rematerialized value");
Value *Alloca = AllocaMap[OriginalValue];
StoreInst *Store = new StoreInst(RematerializedValue, Alloca);
#ifndef NDEBUG
/// Do all the relocation update via allocas and mem2reg
static void relocationViaAlloca(
Function &F, DominatorTree &DT, ArrayRef<Value *> Live,
ArrayRef<PartiallyConstructedSafepointRecord> Records) {
#ifndef NDEBUG
// record initial number of (static) allocas; we'll check we have the same
// number when we get done.
int InitialAllocaNum = 0;
for (Instruction &I : F.getEntryBlock())
if (isa<AllocaInst>(I))
// TODO-PERF: change data structures, reserve
DenseMap<Value *, AllocaInst *> AllocaMap;
SmallVector<AllocaInst *, 200> PromotableAllocas;
// Used later to chack that we have enough allocas to store all values
std::size_t NumRematerializedValues = 0;
// Emit alloca for "LiveValue" and record it in "allocaMap" and
// "PromotableAllocas"
const DataLayout &DL = F.getParent()->getDataLayout();
auto emitAllocaFor = [&](Value *LiveValue) {
AllocaInst *Alloca = new AllocaInst(LiveValue->getType(),
DL.getAllocaAddrSpace(), "",
AllocaMap[LiveValue] = Alloca;
// Emit alloca for each live gc pointer
for (Value *V : Live)
// Emit allocas for rematerialized values
for (const auto &Info : Records)
for (auto RematerializedValuePair : Info.RematerializedValues) {
Value *OriginalValue = RematerializedValuePair.second;
if (AllocaMap.count(OriginalValue) != 0)
// The next two loops are part of the same conceptual operation. We need to
// insert a store to the alloca after the original def and at each
// redefinition. We need to insert a load before each use. These are split
// into distinct loops for performance reasons.
// Update gc pointer after each statepoint: either store a relocated value or
// null (if no relocated value was found for this gc pointer and it is not a
// gc_result). This must happen before we update the statepoint with load of
// alloca otherwise we lose the link between statepoint and old def.
for (const auto &Info : Records) {
Value *Statepoint = Info.StatepointToken;
// This will be used for consistency check
DenseSet<Value *> VisitedLiveValues;
// Insert stores for normal statepoint gc relocates
insertRelocationStores(Statepoint->users(), AllocaMap, VisitedLiveValues);
// In case if it was invoke statepoint
// we will insert stores for exceptional path gc relocates.
if (isa<InvokeInst>(Statepoint)) {
insertRelocationStores(Info.UnwindToken->users(), AllocaMap,
// Do similar thing with rematerialized values
insertRematerializationStores(Info.RematerializedValues, AllocaMap,
if (ClobberNonLive) {
// As a debugging aid, pretend that an unrelocated pointer becomes null at
// the gc.statepoint. This will turn some subtle GC problems into
// slightly easier to debug SEGVs. Note that on large IR files with
// lots of gc.statepoints this is extremely costly both memory and time
// wise.
SmallVector<AllocaInst *, 64> ToClobber;
for (auto Pair : AllocaMap) {
Value *Def = Pair.first;
AllocaInst *Alloca = Pair.second;
// This value was relocated
if (VisitedLiveValues.count(Def)) {
auto InsertClobbersAt = [&](Instruction *IP) {
for (auto *AI : ToClobber) {
auto PT = cast<PointerType>(AI->getAllocatedType());
Constant *CPN = ConstantPointerNull::get(PT);
StoreInst *Store = new StoreInst(CPN, AI);
// Insert the clobbering stores. These may get intermixed with the
// gc.results and gc.relocates, but that's fine.
if (auto II = dyn_cast<InvokeInst>(Statepoint)) {
} else {
// Update use with load allocas and add store for gc_relocated.
for (auto Pair : AllocaMap) {
Value *Def = Pair.first;
AllocaInst *Alloca = Pair.second;
// We pre-record the uses of allocas so that we dont have to worry about
// later update that changes the user information..
SmallVector<Instruction *, 20> Uses;
// PERF: trade a linear scan for repeated reallocation
for (User *U : Def->users()) {
if (!isa<ConstantExpr>(U)) {
// If the def has a ConstantExpr use, then the def is either a
// ConstantExpr use itself or null. In either case
// (recursively in the first, directly in the second), the oop
// it is ultimately dependent on is null and this particular
// use does not need to be fixed up.
auto Last = std::unique(Uses.begin(), Uses.end());
Uses.erase(Last, Uses.end());
for (Instruction *Use : Uses) {
if (isa<PHINode>(Use)) {
PHINode *Phi = cast<PHINode>(Use);
for (unsigned i = 0; i < Phi->getNumIncomingValues(); i++) {
if (Def == Phi->getIncomingValue(i)) {
LoadInst *Load =
new LoadInst(Alloca->getAllocatedType(), Alloca, "",
Phi->setIncomingValue(i, Load);
} else {
LoadInst *Load =
new LoadInst(Alloca->getAllocatedType(), Alloca, "", Use);
Use->replaceUsesOfWith(Def, Load);
// Emit store for the initial gc value. Store must be inserted after load,
// otherwise store will be in alloca's use list and an extra load will be
// inserted before it.
StoreInst *Store = new StoreInst(Def, Alloca);
if (Instruction *Inst = dyn_cast<Instruction>(Def)) {
if (InvokeInst *Invoke = dyn_cast<InvokeInst>(Inst)) {
// InvokeInst is a terminator so the store need to be inserted into its
// normal destination block.
BasicBlock *NormalDest = Invoke->getNormalDest();
} else {
assert(!Inst->isTerminator() &&
"The only terminator that can produce a value is "
"InvokeInst which is handled above.");
} else {
assert(PromotableAllocas.size() == Live.size() + NumRematerializedValues &&
"we must have the same allocas with lives");
if (!PromotableAllocas.empty()) {
// Apply mem2reg to promote alloca to SSA
PromoteMemToReg(PromotableAllocas, DT);
#ifndef NDEBUG
for (auto &I : F.getEntryBlock())
if (isa<AllocaInst>(I))
assert(InitialAllocaNum == 0 && "We must not introduce any extra allocas");
/// Implement a unique function which doesn't require we sort the input
/// vector. Doing so has the effect of changing the output of a couple of
/// tests in ways which make them less useful in testing fused safepoints.
template <typename T> static void unique_unsorted(SmallVectorImpl<T> &Vec) {
SmallSet<T, 8> Seen;
Vec.erase(remove_if(Vec, [&](const T &V) { return !Seen.insert(V).second; }),
/// Insert holders so that each Value is obviously live through the entire
/// lifetime of the call.
static void insertUseHolderAfter(CallBase *Call, const ArrayRef<Value *> Values,
SmallVectorImpl<CallInst *> &Holders) {
if (Values.empty())
// No values to hold live, might as well not insert the empty holder
Module *M = Call->getModule();
// Use a dummy vararg function to actually hold the values live
FunctionCallee Func = M->getOrInsertFunction(
"__tmp_use", FunctionType::get(Type::getVoidTy(M->getContext()), true));
if (isa<CallInst>(Call)) {
// For call safepoints insert dummy calls right after safepoint
CallInst::Create(Func, Values, "", &*++Call->getIterator()));
// For invoke safepooints insert dummy calls both in normal and
// exceptional destination blocks
auto *II = cast<InvokeInst>(Call);
Func, Values, "", &*II->getNormalDest()->getFirstInsertionPt()));
Func, Values, "", &*II->getUnwindDest()->getFirstInsertionPt()));
static void findLiveReferences(
Function &F, DominatorTree &DT, ArrayRef<CallBase *> toUpdate,
MutableArrayRef<struct PartiallyConstructedSafepointRecord> records) {
GCPtrLivenessData OriginalLivenessData;
computeLiveInValues(DT, F, OriginalLivenessData);
for (size_t i = 0; i < records.size(); i++) {
struct PartiallyConstructedSafepointRecord &info = records[i];
analyzeParsePointLiveness(DT, OriginalLivenessData, toUpdate[i], info);
// Helper function for the "rematerializeLiveValues". It walks use chain
// starting from the "CurrentValue" until it reaches the root of the chain, i.e.
// the base or a value it cannot process. Only "simple" values are processed
// (currently it is GEP's and casts). The returned root is examined by the
// callers of findRematerializableChainToBasePointer. Fills "ChainToBase" array
// with all visited values.
static Value* findRematerializableChainToBasePointer(
SmallVectorImpl<Instruction*> &ChainToBase,
Value *CurrentValue) {
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(CurrentValue)) {
return findRematerializableChainToBasePointer(ChainToBase,
if (CastInst *CI = dyn_cast<CastInst>(CurrentValue)) {
if (!CI->isNoopCast(CI->getModule()->getDataLayout()))
return CI;
return findRematerializableChainToBasePointer(ChainToBase,
// We have reached the root of the chain, which is either equal to the base or
// is the first unsupported value along the use chain.
return CurrentValue;
// Helper function for the "rematerializeLiveValues". Compute cost of the use
// chain we are going to rematerialize.
static unsigned
chainToBasePointerCost(SmallVectorImpl<Instruction*> &Chain,
TargetTransformInfo &TTI) {
unsigned Cost = 0;
for (Instruction *Instr : Chain) {
if (CastInst *CI = dyn_cast<CastInst>(Instr)) {
assert(CI->isNoopCast(CI->getModule()->getDataLayout()) &&
"non noop cast is found during rematerialization");
Type *SrcTy = CI->getOperand(0)->getType();
Cost += TTI.getCastInstrCost(CI->getOpcode(), CI->getType(), SrcTy, CI);
} else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Instr)) {
// Cost of the address calculation
Type *ValTy = GEP->getSourceElementType();
Cost += TTI.getAddressComputationCost(ValTy);
// And cost of the GEP itself
// TODO: Use TTI->getGEPCost here (it exists, but appears to be not
// allowed for the external usage)
if (!GEP->hasAllConstantIndices())
Cost += 2;
} else {
llvm_unreachable("unsupported instruction type during rematerialization");
return Cost;
static bool AreEquivalentPhiNodes(PHINode &OrigRootPhi, PHINode &AlternateRootPhi) {
unsigned PhiNum = OrigRootPhi.getNumIncomingValues();
if (PhiNum != AlternateRootPhi.getNumIncomingValues() ||
OrigRootPhi.getParent() != AlternateRootPhi.getParent())
return false;
// Map of incoming values and their corresponding basic blocks of
// OrigRootPhi.
SmallDenseMap<Value *, BasicBlock *, 8> CurrentIncomingValues;
for (unsigned i = 0; i < PhiNum; i++)
CurrentIncomingValues[OrigRootPhi.getIncomingValue(i)] =
// Both current and base PHIs should have same incoming values and
// the same basic blocks corresponding to the incoming values.
for (unsigned i = 0; i < PhiNum; i++) {
auto CIVI =
if (CIVI == CurrentIncomingValues.end())
return false;
BasicBlock *CurrentIncomingBB = CIVI->second;
if (CurrentIncomingBB != AlternateRootPhi.getIncomingBlock(i))
return false;
return true;
// From the statepoint live set pick values that are cheaper to recompute then
// to relocate. Remove this values from the live set, rematerialize them after
// statepoint and record them in "Info" structure. Note that similar to
// relocated values we don't do any user adjustments here.
static void rematerializeLiveValues(CallBase *Call,
PartiallyConstructedSafepointRecord &Info,
TargetTransformInfo &TTI) {
const unsigned int ChainLengthThreshold = 10;
// Record values we are going to delete from this statepoint live set.
// We can not di this in following loop due to iterator invalidation.
SmallVector<Value *, 32> LiveValuesToBeDeleted;
for (Value *LiveValue: Info.LiveSet) {
// For each live pointer find its defining chain
SmallVector<Instruction *, 3> ChainToBase;
Value *RootOfChain =
// Nothing to do, or chain is too long
if ( ChainToBase.size() == 0 ||
ChainToBase.size() > ChainLengthThreshold)
// Handle the scenario where the RootOfChain is not equal to the
// Base Value, but they are essentially the same phi values.
if (RootOfChain != Info.PointerToBase[LiveValue]) {
PHINode *OrigRootPhi = dyn_cast<PHINode>(RootOfChain);
PHINode *AlternateRootPhi = dyn_cast<PHINode>(Info.PointerToBase[LiveValue]);
if (!OrigRootPhi || !AlternateRootPhi)
// PHI nodes that have the same incoming values, and belonging to the same
// basic blocks are essentially the same SSA value. When the original phi
// has incoming values with different base pointers, the original phi is
// marked as conflict, and an additional `AlternateRootPhi` with the same
// incoming values get generated by the findBasePointer function. We need
// to identify the newly generated AlternateRootPhi (.base version of phi)
// and RootOfChain (the original phi node itself) are the same, so that we
// can rematerialize the gep and casts. This is a workaround for the
// deficiency in the findBasePointer algorithm.
if (!AreEquivalentPhiNodes(*OrigRootPhi, *AlternateRootPhi))
// Now that the phi nodes are proved to be the same, assert that
// findBasePointer's newly generated AlternateRootPhi is present in the
// liveset of the call.
// Compute cost of this chain
unsigned Cost = chainToBasePointerCost(ChainToBase, TTI);
// TODO: We can also account for cases when we will be able to remove some
// of the rematerialized values by later optimization passes. I.e if
// we rematerialized several intersecting chains. Or if original values
// don't have any uses besides this statepoint.
// For invokes we need to rematerialize each chain twice - for normal and
// for unwind basic blocks. Model this by multiplying cost by two.
if (isa<InvokeInst>(Call)) {
Cost *= 2;
// If it's too expensive - skip it
if (Cost >= RematerializationThreshold)
// Remove value from the live set
// Clone instructions and record them inside "Info" structure
// Walk backwards to visit top-most instructions first
std::reverse(ChainToBase.begin(), ChainToBase.end());
// Utility function which clones all instructions from "ChainToBase"
// and inserts them before "InsertBefore". Returns rematerialized value
// which should be used after statepoint.
auto rematerializeChain = [&ChainToBase](
Instruction *InsertBefore, Value *RootOfChain, Value *AlternateLiveBase) {
Instruction *LastClonedValue = nullptr;
Instruction *LastValue = nullptr;
for (Instruction *Instr: ChainToBase) {
// Only GEP's and casts are supported as we need to be careful to not
// introduce any new uses of pointers not in the liveset.
// Note that it's fine to introduce new uses of pointers which were
// otherwise not used after this statepoint.
assert(isa<GetElementPtrInst>(Instr) || isa<CastInst>(Instr));
Instruction *ClonedValue = Instr->clone();
ClonedValue->setName(Instr->getName() + ".remat");
// If it is not first instruction in the chain then it uses previously
// cloned value. We should update it to use cloned value.
if (LastClonedValue) {
ClonedValue->replaceUsesOfWith(LastValue, LastClonedValue);
#ifndef NDEBUG
for (auto OpValue : ClonedValue->operand_values()) {
// Assert that cloned instruction does not use any instructions from
// this chain other than LastClonedValue
assert(!is_contained(ChainToBase, OpValue) &&
"incorrect use in rematerialization chain");
// Assert that the cloned instruction does not use the RootOfChain
// or the AlternateLiveBase.
assert(OpValue != RootOfChain && OpValue != AlternateLiveBase);
} else {
// For the first instruction, replace the use of unrelocated base i.e.
// RootOfChain/OrigRootPhi, with the corresponding PHI present in the
// live set. They have been proved to be the same PHI nodes. Note
// that the *only* use of the RootOfChain in the ChainToBase list is
// the first Value in the list.
if (RootOfChain != AlternateLiveBase)
ClonedValue->replaceUsesOfWith(RootOfChain, AlternateLiveBase);
LastClonedValue = ClonedValue;
LastValue = Instr;
return LastClonedValue;
// Different cases for calls and invokes. For invokes we need to clone
// instructions both on normal and unwind path.
if (isa<CallInst>(Call)) {
Instruction *InsertBefore = Call->getNextNode();
Instruction *RematerializedValue = rematerializeChain(
InsertBefore, RootOfChain, Info.PointerToBase[LiveValue]);
Info.RematerializedValues[RematerializedValue] = LiveValue;
} else {
auto *Invoke = cast<InvokeInst>(Call);
Instruction *NormalInsertBefore =
Instruction *UnwindInsertBefore =
Instruction *NormalRematerializedValue = rematerializeChain(
NormalInsertBefore, RootOfChain, Info.PointerToBase[LiveValue]);
Instruction *UnwindRematerializedValue = rematerializeChain(
UnwindInsertBefore, RootOfChain, Info.PointerToBase[LiveValue]);
Info.RematerializedValues[NormalRematerializedValue] = LiveValue;
Info.RematerializedValues[UnwindRematerializedValue] = LiveValue;
// Remove rematerializaed values from the live set
for (auto LiveValue: LiveValuesToBeDeleted) {
static bool insertParsePoints(Function &F, DominatorTree &DT,
TargetTransformInfo &TTI,
SmallVectorImpl<CallBase *> &ToUpdate) {
#ifndef NDEBUG
// sanity check the input
std::set<CallBase *> Uniqued;
Uniqued.insert(ToUpdate.begin(), ToUpdate.end());
assert(Uniqued.size() == ToUpdate.size() && "no duplicates please!");
for (CallBase *Call : ToUpdate)
assert(Call->getFunction() == &F);
// When inserting gc.relocates for invokes, we need to be able to insert at
// the top of the successor blocks. See the comment on
// normalForInvokeSafepoint on exactly what is needed. Note that this step
// may restructure the CFG.
for (CallBase *Call : ToUpdate) {
auto *II = dyn_cast<InvokeInst>(Call);
if (!II)
normalizeForInvokeSafepoint(II->getNormalDest(), II->getParent(), DT);
normalizeForInvokeSafepoint(II->getUnwindDest(), II->getParent(), DT);
// A list of dummy calls added to the IR to keep various values obviously
// live in the IR. We'll remove all of these when done.
SmallVector<CallInst *, 64> Holders;
// Insert a dummy call with all of the deopt operands we'll need for the
// actual safepoint insertion as arguments. This ensures reference operands
// in the deopt argument list are considered live through the safepoint (and
// thus makes sure they get relocated.)
for (CallBase *Call : ToUpdate) {
SmallVector<Value *, 64> DeoptValues;
for (Value *Arg : GetDeoptBundleOperands(Call)) {
assert(!isUnhandledGCPointerType(Arg->getType()) &&
"support for FCA unimplemented");
if (isHandledGCPointerType(Arg->getType()))
insertUseHolderAfter(Call, DeoptValues, Holders);
SmallVector<PartiallyConstructedSafepointRecord, 64> Records(ToUpdate.size());
// A) Identify all gc pointers which are statically live at the given call
// site.
findLiveReferences(F, DT, ToUpdate, Records);
// B) Find the base pointers for each live pointer
/* scope for caching */ {
// Cache the 'defining value' relation used in the computation and
// insertion of base phis and selects. This ensures that we don't insert
// large numbers of duplicate base_phis.
DefiningValueMapTy DVCache;
for (size_t i = 0; i < Records.size(); i++) {
PartiallyConstructedSafepointRecord &info = Records[i];
findBasePointers(DT, DVCache, ToUpdate[i], info);
} // end of cache scope
// The base phi insertion logic (for any safepoint) may have inserted new
// instructions which are now live at some safepoint. The simplest such
// example is:
// loop:
// phi a <-- will be a new base_phi here
// safepoint 1 <-- that needs to be live here
// gep a + 1
// safepoint 2
// br loop
// We insert some dummy calls after each safepoint to definitely hold live
// the base pointers which were identified for that safepoint. We'll then
// ask liveness for _every_ base inserted to see what is now live. Then we
// remove the dummy calls.
Holders.reserve(Holders.size() + Records.size());
for (size_t i = 0; i < Records.size(); i++) {
PartiallyConstructedSafepointRecord &Info = Records[i];
SmallVector<Value *, 128> Bases;
for (auto Pair : Info.PointerToBase)
insertUseHolderAfter(ToUpdate[i], Bases, Holders);
// By selecting base pointers, we've effectively inserted new uses. Thus, we
// need to rerun liveness. We may *also* have inserted new defs, but that's
// not the key issue.
recomputeLiveInValues(F, DT, ToUpdate, Records);
if (PrintBasePointers) {
for (auto &Info : Records) {
errs() << "Base Pairs: (w/Relocation)\n";
for (auto Pair : Info.PointerToBase) {
errs() << " derived ";
Pair.first->printAsOperand(errs(), false);
errs() << " base ";
Pair.second->printAsOperand(errs(), false);
errs() << "\n";
// It is possible that non-constant live variables have a constant base. For
// example, a GEP with a variable offset from a global. In this case we can
// remove it from the liveset. We already don't add constants to the liveset
// because we assume they won't move at runtime and the GC doesn't need to be
// informed about them. The same reasoning applies if the base is constant.
// Note that the relocation placement code relies on this filtering for
// correctness as it expects the base to be in the liveset, which isn't true
// if the base is constant.
for (auto &Info : Records)
for (auto &BasePair : Info.PointerToBase)
if (isa<Constant>(BasePair.second))
for (CallInst *CI : Holders)
// In order to reduce live set of statepoint we might choose to rematerialize
// some values instead of relocating them. This is purely an optimization and
// does not influence correctness.
for (size_t i = 0; i < Records.size(); i++)
rematerializeLiveValues(ToUpdate[i], Records[i], TTI);
// We need this to safely RAUW and delete call or invoke return values that
// may themselves be live over a statepoint. For details, please see usage in
// makeStatepointExplicitImpl.
std::vector<DeferredReplacement> Replacements;
// Now run through and replace the existing statepoints with new ones with
// the live variables listed. We do not yet update uses of the values being
// relocated. We have references to live variables that need to
// survive to the last iteration of this loop. (By construction, the
// previous statepoint can not be a live variable, thus we can and remove
// the old statepoint calls as we go.)
for (size_t i = 0; i < Records.size(); i++)
makeStatepointExplicit(DT, ToUpdate[i], Records[i], Replacements);
ToUpdate.clear(); // prevent accident use of invalid calls.
for (auto &PR : Replacements)
for (auto &Info : Records) {
// These live sets may contain state Value pointers, since we replaced calls
// with operand bundles with calls wrapped in gc.statepoint, and some of
// those calls may have been def'ing live gc pointers. Clear these out to
// avoid accidentally using them.
// TODO: We should create a separate data structure that does not contain
// these live sets, and migrate to using that data structure from this point
// onward.
// Do all the fixups of the original live variables to their relocated selves
SmallVector<Value *, 128> Live;
for (size_t i = 0; i < Records.size(); i++) {
PartiallyConstructedSafepointRecord &Info = Records[i];
// We can't simply save the live set from the original insertion. One of
// the live values might be the result of a call which needs a safepoint.
// That Value* no longer exists and we need to use the new gc_result.
// Thankfully, the live set is embedded in the statepoint (and updated), so
// we just grab that.
Statepoint Statepoint(Info.StatepointToken);
Live.insert(Live.end(), Statepoint.gc_args_begin(),
#ifndef NDEBUG
// Do some basic sanity checks on our liveness results before performing
// relocation. Relocation can and will turn mistakes in liveness results
// into non-sensical code which is must harder to debug.
// TODO: It would be nice to test consistency as well
assert(DT.isReachableFromEntry(Info.StatepointToken->getParent()) &&
"statepoint must be reachable or liveness is meaningless");
for (Value *V : Statepoint.gc_args()) {
if (!isa<Instruction>(V))
// Non-instruction values trivial dominate all possible uses
auto *LiveInst = cast<Instruction>(V);
assert(DT.isReachableFromEntry(LiveInst->getParent()) &&
"unreachable values should never be live");
assert(DT.dominates(LiveInst, Info.StatepointToken) &&
"basic SSA liveness expectation violated by liveness analysis");
#ifndef NDEBUG
// sanity check
for (auto *Ptr : Live)
assert(isHandledGCPointerType(Ptr->getType()) &&
"must be a gc pointer type");
relocationViaAlloca(F, DT, Live, Records);
return !Records.empty();
// Handles both return values and arguments for Functions and calls.
template <typename AttrHolder>
static void RemoveNonValidAttrAtIndex(LLVMContext &Ctx, AttrHolder &AH,
unsigned Index) {
AttrBuilder R;
if (AH.getDereferenceableBytes(Index))
R.addAttribute(Attribute::get(Ctx, Attribute::Dereferenceable,
if (AH.getDereferenceableOrNullBytes(Index))
R.addAttribute(Attribute::get(Ctx, Attribute::DereferenceableOrNull,
if (AH.getAttributes().hasAttribute(Index, Attribute::NoAlias))
if (!R.empty())
AH.setAttributes(AH.getAttributes().removeAttributes(Ctx, Index, R));
static void stripNonValidAttributesFromPrototype(Function &F) {
LLVMContext &Ctx = F.getContext();
for (Argument &A : F.args())
if (isa<PointerType>(A.getType()))
RemoveNonValidAttrAtIndex(Ctx, F,
A.getArgNo() + AttributeList::FirstArgIndex);
if (isa<PointerType>(F.getReturnType()))
RemoveNonValidAttrAtIndex(Ctx, F, AttributeList::ReturnIndex);
/// Certain metadata on instructions are invalid after running RS4GC.
/// Optimizations that run after RS4GC can incorrectly use this metadata to
/// optimize functions. We drop such metadata on the instruction.
static void stripInvalidMetadataFromInstruction(Instruction &I) {
if (!isa<LoadInst>(I) && !isa<StoreInst>(I))
// These are the attributes that are still valid on loads and stores after
// RS4GC.
// The metadata implying dereferenceability and noalias are (conservatively)
// dropped. This is because semantically, after RewriteStatepointsForGC runs,
// all calls to gc.statepoint "free" the entire heap. Also, gc.statepoint can
// touch the entire heap including noalias objects. Note: The reasoning is
// same as stripping the dereferenceability and noalias attributes that are
// analogous to the metadata counterparts.
// We also drop the invariant.load metadata on the load because that metadata
// implies the address operand to the load points to memory that is never
// changed once it became dereferenceable. This is no longer true after RS4GC.
// Similar reasoning applies to metadata, which applies to
// loads within a group.
unsigned ValidMetadataAfterRS4GC[] = {LLVMContext::MD_tbaa,
// Drops all metadata on the instruction other than ValidMetadataAfterRS4GC.
static void stripNonValidDataFromBody(Function &F) {
if (F.empty())
LLVMContext &Ctx = F.getContext();
MDBuilder Builder(Ctx);
// Set of invariantstart instructions that we need to remove.
// Use this to avoid invalidating the instruction iterator.
SmallVector<IntrinsicInst*, 12> InvariantStartInstructions;
for (Instruction &I : instructions(F)) {
// invariant.start on memory location implies that the referenced memory
// location is constant and unchanging. This is no longer true after
// RewriteStatepointsForGC runs because there can be calls to gc.statepoint
// which frees the entire heap and the presence of invariant.start allows
// the optimizer to sink the load of a memory location past a statepoint,
// which is incorrect.
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::invariant_start) {
if (MDNode *Tag = I.getMetadata(LLVMContext::MD_tbaa)) {
MDNode *MutableTBAA = Builder.createMutableTBAAAccessTag(Tag);
I.setMetadata(LLVMContext::MD_tbaa, MutableTBAA);
if (auto *Call = dyn_cast<CallBase>(&I)) {
for (int i = 0, e = Call->arg_size(); i != e; i++)
if (isa<PointerType>(Call->getArgOperand(i)->getType()))
RemoveNonValidAttrAtIndex(Ctx, *Call,
i + AttributeList::FirstArgIndex);
if (isa<PointerType>(Call->getType()))
RemoveNonValidAttrAtIndex(Ctx, *Call, AttributeList::ReturnIndex);
// Delete the invariant.start instructions and RAUW undef.
for (auto *II : InvariantStartInstructions) {
/// Returns true if this function should be rewritten by this pass. The main
/// point of this function is as an extension point for custom logic.
static bool shouldRewriteStatepointsIn(Function &F) {
// TODO: This should check the GCStrategy
if (F.hasGC()) {
const auto &FunctionGCName = F.getGC();
const StringRef StatepointExampleName("statepoint-example");
const StringRef CoreCLRName("coreclr");
return (StatepointExampleName == FunctionGCName) ||
(CoreCLRName == FunctionGCName);
} else
return false;
static void stripNonValidData(Module &M) {
#ifndef NDEBUG
assert(llvm::any_of(M, shouldRewriteStatepointsIn) && "precondition!");
for (Function &F : M)
for (Function &F : M)
bool RewriteStatepointsForGC::runOnFunction(Function &F, DominatorTree &DT,
TargetTransformInfo &TTI,
const TargetLibraryInfo &TLI) {
assert(!F.isDeclaration() && !F.empty() &&
"need function body to rewrite statepoints in");
assert(shouldRewriteStatepointsIn(F) && "mismatch in rewrite decision");
auto NeedsRewrite = [&TLI](Instruction &I) {
if (const auto *Call = dyn_cast<CallBase>(&I))
return !callsGCLeafFunction(Call, TLI) && !isStatepoint(Call);
return false;
// Delete any unreachable statepoints so that we don't have unrewritten
// statepoints surviving this pass. This makes testing easier and the
// resulting IR less confusing to human readers.
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
bool MadeChange = removeUnreachableBlocks(F, &DTU);
// Flush the Dominator Tree.
// Gather all the statepoints which need rewritten. Be careful to only
// consider those in reachable code since we need to ask dominance queries
// when rewriting. We'll delete the unreachable ones in a moment.
SmallVector<CallBase *, 64> ParsePointNeeded;
for (Instruction &I : instructions(F)) {
// TODO: only the ones with the flag set!
if (NeedsRewrite(I)) {
// NOTE removeUnreachableBlocks() is stronger than
// DominatorTree::isReachableFromEntry(). In other words
// removeUnreachableBlocks can remove some blocks for which
// isReachableFromEntry() returns true.
assert(DT.isReachableFromEntry(I.getParent()) &&
"no unreachable blocks expected");
// Return early if no work to do.
if (ParsePointNeeded.empty())
return MadeChange;
// As a prepass, go ahead and aggressively destroy single entry phi nodes.
// These are created by LCSSA. They have the effect of increasing the size
// of liveness sets for no good reason. It may be harder to do this post
// insertion since relocations and base phis can confuse things.
for (BasicBlock &BB : F)
if (BB.getUniquePredecessor()) {
MadeChange = true;
// Before we start introducing relocations, we want to tweak the IR a bit to
// avoid unfortunate code generation effects. The main example is that we
// want to try to make sure the comparison feeding a branch is after any
// safepoints. Otherwise, we end up with a comparison of pre-relocation
// values feeding a branch after relocation. This is semantically correct,
// but results in extra register pressure since both the pre-relocation and
// post-relocation copies must be available in registers. For code without
// relocations this is handled elsewhere, but teaching the scheduler to
// reverse the transform we're about to do would be slightly complex.
// Note: This may extend the live range of the inputs to the icmp and thus
// increase the liveset of any statepoint we move over. This is profitable
// as long as all statepoints are in rare blocks. If we had in-register
// lowering for live values this would be a much safer transform.
auto getConditionInst = [](Instruction *TI) -> Instruction * {
if (auto *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional())
return dyn_cast<Instruction>(BI->getCondition());
// TODO: Extend this to handle switches
return nullptr;
for (BasicBlock &BB : F) {
Instruction *TI = BB.getTerminator();
if (auto *Cond = getConditionInst(TI))
// TODO: Handle more than just ICmps here. We should be able to move
// most instructions without side effects or memory access.
if (isa<ICmpInst>(Cond) && Cond->hasOneUse()) {
MadeChange = true;
// Nasty workaround - The base computation code in the main algorithm doesn't
// consider the fact that a GEP can be used to convert a scalar to a vector.
// The right fix for this is to integrate GEPs into the base rewriting
// algorithm properly, this is just a short term workaround to prevent
// crashes by canonicalizing such GEPs into fully vector GEPs.
for (Instruction &I : instructions(F)) {
if (!isa<GetElementPtrInst>(I))
unsigned VF = 0;
for (unsigned i = 0; i < I.getNumOperands(); i++)
if (I.getOperand(i)->getType()->isVectorTy()) {
assert(VF == 0 ||
VF == I.getOperand(i)->getType()->getVectorNumElements());
VF = I.getOperand(i)->getType()->getVectorNumElements();
// It's the vector to scalar traversal through the pointer operand which
// confuses base pointer rewriting, so limit ourselves to that case.
if (!I.getOperand(0)->getType()->isVectorTy() && VF != 0) {
IRBuilder<> B(&I);
auto *Splat = B.CreateVectorSplat(VF, I.getOperand(0));
I.setOperand(0, Splat);
MadeChange = true;
MadeChange |= insertParsePoints(F, DT, TTI, ParsePointNeeded);
return MadeChange;
// liveness computation via standard dataflow
// -------------------------------------------------------------------
// TODO: Consider using bitvectors for liveness, the set of potentially
// interesting values should be small and easy to pre-compute.
/// Compute the live-in set for the location rbegin starting from
/// the live-out set of the basic block
static void computeLiveInValues(BasicBlock::reverse_iterator Begin,
BasicBlock::reverse_iterator End,
SetVector<Value *> &LiveTmp) {
for (auto &I : make_range(Begin, End)) {
// KILL/Def - Remove this definition from LiveIn
// Don't consider *uses* in PHI nodes, we handle their contribution to
// predecessor blocks when we seed the LiveOut sets
if (isa<PHINode>(I))
// USE - Add to the LiveIn set for this instruction
for (Value *V : I.operands()) {
assert(!isUnhandledGCPointerType(V->getType()) &&
"support for FCA unimplemented");
if (isHandledGCPointerType(V->getType()) && !isa<Constant>(V)) {
// The choice to exclude all things constant here is slightly subtle.
// There are two independent reasons:
// - We assume that things which are constant (from LLVM's definition)
// do not move at runtime. For example, the address of a global
// variable is fixed, even though it's contents may not be.
// - Second, we can't disallow arbitrary inttoptr constants even
// if the language frontend does. Optimization passes are free to
// locally exploit facts without respect to global reachability. This
// can create sections of code which are dynamically unreachable and
// contain just about anything. (see constants.ll in tests)
static void computeLiveOutSeed(BasicBlock *BB, SetVector<Value *> &LiveTmp) {
for (BasicBlock *Succ : successors(BB)) {
for (auto &I : *Succ) {
PHINode *PN = dyn_cast<PHINode>(&I);
if (!PN)
Value *V = PN->getIncomingValueForBlock(BB);
assert(!isUnhandledGCPointerType(V->getType()) &&
"support for FCA unimplemented");
if (isHandledGCPointerType(V->getType()) && !isa<Constant>(V))
static SetVector<Value *> computeKillSet(BasicBlock *BB) {
SetVector<Value *> KillSet;
for (Instruction &I : *BB)
if (isHandledGCPointerType(I.getType()))
return KillSet;
#ifndef NDEBUG
/// Check that the items in 'Live' dominate 'TI'. This is used as a basic
/// sanity check for the liveness computation.
static void checkBasicSSA(DominatorTree &DT, SetVector<Value *> &Live,
Instruction *TI, bool TermOkay = false) {
for (Value *V : Live) {
if (auto *I = dyn_cast<Instruction>(V)) {
// The terminator can be a member of the LiveOut set. LLVM's definition
// of instruction dominance states that V does not dominate itself. As
// such, we need to special case this to allow it.
if (TermOkay && TI == I)
assert(DT.dominates(I, TI) &&
"basic SSA liveness expectation violated by liveness analysis");
/// Check that all the liveness sets used during the computation of liveness
/// obey basic SSA properties. This is useful for finding cases where we miss
/// a def.
static void checkBasicSSA(DominatorTree &DT, GCPtrLivenessData &Data,
BasicBlock &BB) {
checkBasicSSA(DT, Data.LiveSet[&BB], BB.getTerminator());
checkBasicSSA(DT, Data.LiveOut[&BB], BB.getTerminator(), true);
checkBasicSSA(DT, Data.LiveIn[&BB], BB.getTerminator());
static void computeLiveInValues(DominatorTree &DT, Function &F,
GCPtrLivenessData &Data) {
SmallSetVector<BasicBlock *, 32> Worklist;
// Seed the liveness for each individual block
for (BasicBlock &BB : F) {
Data.KillSet[&BB] = computeKillSet(&BB);
computeLiveInValues(BB.rbegin(), BB.rend(), Data.LiveSet[&BB]);
#ifndef NDEBUG
for (Value *Kill : Data.KillSet[&BB])
assert(!Data.LiveSet[&BB].count(Kill) && "live set contains kill");
Data.LiveOut[&BB] = SetVector<Value *>();
computeLiveOutSeed(&BB, Data.LiveOut[&BB]);
Data.LiveIn[&BB] = Data.LiveSet[&BB];
if (!Data.LiveIn[&BB].empty())
Worklist.insert(pred_begin(&BB), pred_end(&BB));
// Propagate that liveness until stable
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
// Compute our new liveout set, then exit early if it hasn't changed despite
// the contribution of our successor.
SetVector<Value *> LiveOut = Data.LiveOut[BB];
const auto OldLiveOutSize = LiveOut.size();
for (BasicBlock *Succ : successors(BB)) {
// assert OutLiveOut is a subset of LiveOut
if (OldLiveOutSize == LiveOut.size()) {
// If the sets are the same size, then we didn't actually add anything
// when unioning our successors LiveIn. Thus, the LiveIn of this block
// hasn't changed.
Data.LiveOut[BB] = LiveOut;
// Apply the effects of this basic block
SetVector<Value *> LiveTmp = LiveOut;