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//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
// 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
// This file implements the Jump Threading pass.
#include "llvm/Transforms/Scalar/JumpThreading.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyValueInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <memory>
#include <utility>
using namespace llvm;
using namespace jumpthreading;
#define DEBUG_TYPE "jump-threading"
STATISTIC(NumThreads, "Number of jumps threaded");
STATISTIC(NumFolds, "Number of terminators folded");
STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
static cl::opt<unsigned>
cl::desc("Max block size to duplicate for jump threading"),
cl::init(6), cl::Hidden);
static cl::opt<unsigned>
cl::desc("The number of predecessors to search for a stronger "
"condition to use to thread over a weaker condition"),
cl::init(3), cl::Hidden);
static cl::opt<bool> PrintLVIAfterJumpThreading(
cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false),
namespace {
/// This pass performs 'jump threading', which looks at blocks that have
/// multiple predecessors and multiple successors. If one or more of the
/// predecessors of the block can be proven to always jump to one of the
/// successors, we forward the edge from the predecessor to the successor by
/// duplicating the contents of this block.
/// An example of when this can occur is code like this:
/// if () { ...
/// X = 4;
/// }
/// if (X < 3) {
/// In this case, the unconditional branch at the end of the first if can be
/// revectored to the false side of the second if.
class JumpThreading : public FunctionPass {
JumpThreadingPass Impl;
static char ID; // Pass identification
JumpThreading(int T = -1) : FunctionPass(ID), Impl(T) {
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
void releaseMemory() override { Impl.releaseMemory(); }
} // end anonymous namespace
char JumpThreading::ID = 0;
INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
"Jump Threading", false, false)
INITIALIZE_PASS_END(JumpThreading, "jump-threading",
"Jump Threading", false, false)
// Public interface to the Jump Threading pass
FunctionPass *llvm::createJumpThreadingPass(int Threshold) {
return new JumpThreading(Threshold);
JumpThreadingPass::JumpThreadingPass(int T) {
BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
// Update branch probability information according to conditional
// branch probability. This is usually made possible for cloned branches
// in inline instances by the context specific profile in the caller.
// For instance,
// [Block PredBB]
// [Branch PredBr]
// if (t) {
// Block A;
// } else {
// Block B;
// }
// [Block BB]
// cond = PN([true, %A], [..., %B]); // PHI node
// [Branch CondBr]
// if (cond) {
// ... // P(cond == true) = 1%
// }
// Here we know that when block A is taken, cond must be true, which means
// P(cond == true | A) = 1
// Given that P(cond == true) = P(cond == true | A) * P(A) +
// P(cond == true | B) * P(B)
// we get:
// P(cond == true ) = P(A) + P(cond == true | B) * P(B)
// which gives us:
// P(A) is less than P(cond == true), i.e.
// P(t == true) <= P(cond == true)
// In other words, if we know P(cond == true) is unlikely, we know
// that P(t == true) is also unlikely.
static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
if (!CondBr)
BranchProbability BP;
uint64_t TrueWeight, FalseWeight;
if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight))
// Returns the outgoing edge of the dominating predecessor block
// that leads to the PhiNode's incoming block:
auto GetPredOutEdge =
[](BasicBlock *IncomingBB,
BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
auto *PredBB = IncomingBB;
auto *SuccBB = PhiBB;
while (true) {
BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
if (PredBr && PredBr->isConditional())
return {PredBB, SuccBB};
auto *SinglePredBB = PredBB->getSinglePredecessor();
if (!SinglePredBB)
return {nullptr, nullptr};
SuccBB = PredBB;
PredBB = SinglePredBB;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *PhiOpnd = PN->getIncomingValue(i);
ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
if (!CI || !CI->getType()->isIntegerTy(1))
BP = (CI->isOne() ? BranchProbability::getBranchProbability(
TrueWeight, TrueWeight + FalseWeight)
: BranchProbability::getBranchProbability(
FalseWeight, TrueWeight + FalseWeight));
auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
if (!PredOutEdge.first)
BasicBlock *PredBB = PredOutEdge.first;
BranchInst *PredBr = cast<BranchInst>(PredBB->getTerminator());
uint64_t PredTrueWeight, PredFalseWeight;
// FIXME: We currently only set the profile data when it is missing.
// With PGO, this can be used to refine even existing profile data with
// context information. This needs to be done after more performance
// testing.
if (PredBr->extractProfMetadata(PredTrueWeight, PredFalseWeight))
// We can not infer anything useful when BP >= 50%, because BP is the
// upper bound probability value.
if (BP >= BranchProbability(50, 100))
SmallVector<uint32_t, 2> Weights;
if (PredBr->getSuccessor(0) == PredOutEdge.second) {
} else {
/// runOnFunction - Toplevel algorithm.
bool JumpThreading::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
auto TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
// Get DT analysis before LVI. When LVI is initialized it conditionally adds
// DT if it's available.
auto DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
auto AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Lazy);
std::unique_ptr<BlockFrequencyInfo> BFI;
std::unique_ptr<BranchProbabilityInfo> BPI;
bool HasProfileData = F.hasProfileData();
if (HasProfileData) {
LoopInfo LI{DominatorTree(F)};
BPI.reset(new BranchProbabilityInfo(F, LI, TLI));
BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
bool Changed = Impl.runImpl(F, TLI, LVI, AA, &DTU, HasProfileData,
std::move(BFI), std::move(BPI));
if (PrintLVIAfterJumpThreading) {
dbgs() << "LVI for function '" << F.getName() << "':\n";
LVI->printLVI(F, *DT, dbgs());
return Changed;
PreservedAnalyses JumpThreadingPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
// Get DT analysis before LVI. When LVI is initialized it conditionally adds
// DT if it's available.
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &LVI = AM.getResult<LazyValueAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
std::unique_ptr<BlockFrequencyInfo> BFI;
std::unique_ptr<BranchProbabilityInfo> BPI;
if (F.hasProfileData()) {
LoopInfo LI{DominatorTree(F)};
BPI.reset(new BranchProbabilityInfo(F, LI, &TLI));
BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
bool Changed = runImpl(F, &TLI, &LVI, &AA, &DTU, HasProfileData,
std::move(BFI), std::move(BPI));
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
LazyValueInfo *LVI_, AliasAnalysis *AA_,
DomTreeUpdater *DTU_, bool HasProfileData_,
std::unique_ptr<BlockFrequencyInfo> BFI_,
std::unique_ptr<BranchProbabilityInfo> BPI_) {
LLVM_DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
AA = AA_;
// When profile data is available, we need to update edge weights after
// successful jump threading, which requires both BPI and BFI being available.
HasProfileData = HasProfileData_;
auto *GuardDecl = F.getParent()->getFunction(
HasGuards = GuardDecl && !GuardDecl->use_empty();
if (HasProfileData) {
BPI = std::move(BPI_);
BFI = std::move(BFI_);
// JumpThreading must not processes blocks unreachable from entry. It's a
// waste of compute time and can potentially lead to hangs.
SmallPtrSet<BasicBlock *, 16> Unreachable;
assert(DTU && "DTU isn't passed into JumpThreading before using it.");
assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
DominatorTree &DT = DTU->getDomTree();
for (auto &BB : F)
if (!DT.isReachableFromEntry(&BB))
bool EverChanged = false;
bool Changed;
do {
Changed = false;
for (auto &BB : F) {
if (Unreachable.count(&BB))
while (ProcessBlock(&BB)) // Thread all of the branches we can over BB.
Changed = true;
// Stop processing BB if it's the entry or is now deleted. The following
// routines attempt to eliminate BB and locating a suitable replacement
// for the entry is non-trivial.
if (&BB == &F.getEntryBlock() || DTU->isBBPendingDeletion(&BB))
if (pred_empty(&BB)) {
// When ProcessBlock makes BB unreachable it doesn't bother to fix up
// the instructions in it. We must remove BB to prevent invalid IR.
LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB.getName()
<< "' with terminator: " << *BB.getTerminator()
<< '\n');
DeleteDeadBlock(&BB, DTU);
Changed = true;
// ProcessBlock doesn't thread BBs with unconditional TIs. However, if BB
// is "almost empty", we attempt to merge BB with its sole successor.
auto *BI = dyn_cast<BranchInst>(BB.getTerminator());
if (BI && BI->isUnconditional() &&
// The terminator must be the only non-phi instruction in BB.
BB.getFirstNonPHIOrDbg()->isTerminator() &&
// Don't alter Loop headers and latches to ensure another pass can
// detect and transform nested loops later.
!LoopHeaders.count(&BB) && !LoopHeaders.count(BI->getSuccessor(0)) &&
TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU)) {
// BB is valid for cleanup here because we passed in DTU. F remains
// BB's parent until a DTU->getDomTree() event.
Changed = true;
EverChanged |= Changed;
} while (Changed);
// Flush only the Dominator Tree.
return EverChanged;
// Replace uses of Cond with ToVal when safe to do so. If all uses are
// replaced, we can remove Cond. We cannot blindly replace all uses of Cond
// because we may incorrectly replace uses when guards/assumes are uses of
// of `Cond` and we used the guards/assume to reason about the `Cond` value
// at the end of block. RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
static void ReplaceFoldableUses(Instruction *Cond, Value *ToVal) {
assert(Cond->getType() == ToVal->getType());
auto *BB = Cond->getParent();
// We can unconditionally replace all uses in non-local blocks (i.e. uses
// strictly dominated by BB), since LVI information is true from the
// terminator of BB.
replaceNonLocalUsesWith(Cond, ToVal);
for (Instruction &I : reverse(*BB)) {
// Reached the Cond whose uses we are trying to replace, so there are no
// more uses.
if (&I == Cond)
// We only replace uses in instructions that are guaranteed to reach the end
// of BB, where we know Cond is ToVal.
if (!isGuaranteedToTransferExecutionToSuccessor(&I))
I.replaceUsesOfWith(Cond, ToVal);
if (Cond->use_empty() && !Cond->mayHaveSideEffects())
/// Return the cost of duplicating a piece of this block from first non-phi
/// and before StopAt instruction to thread across it. Stop scanning the block
/// when exceeding the threshold. If duplication is impossible, returns ~0U.
static unsigned getJumpThreadDuplicationCost(BasicBlock *BB,
Instruction *StopAt,
unsigned Threshold) {
assert(StopAt->getParent() == BB && "Not an instruction from proper BB?");
/// Ignore PHI nodes, these will be flattened when duplication happens.
BasicBlock::const_iterator I(BB->getFirstNonPHI());
// FIXME: THREADING will delete values that are just used to compute the
// branch, so they shouldn't count against the duplication cost.
unsigned Bonus = 0;
if (BB->getTerminator() == StopAt) {
// Threading through a switch statement is particularly profitable. If this
// block ends in a switch, decrease its cost to make it more likely to
// happen.
if (isa<SwitchInst>(StopAt))
Bonus = 6;
// The same holds for indirect branches, but slightly more so.
if (isa<IndirectBrInst>(StopAt))
Bonus = 8;
// Bump the threshold up so the early exit from the loop doesn't skip the
// terminator-based Size adjustment at the end.
Threshold += Bonus;
// Sum up the cost of each instruction until we get to the terminator. Don't
// include the terminator because the copy won't include it.
unsigned Size = 0;
for (; &*I != StopAt; ++I) {
// Stop scanning the block if we've reached the threshold.
if (Size > Threshold)
return Size;
// Debugger intrinsics don't incur code size.
if (isa<DbgInfoIntrinsic>(I)) continue;
// If this is a pointer->pointer bitcast, it is free.
if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
// Bail out if this instruction gives back a token type, it is not possible
// to duplicate it if it is used outside this BB.
if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
return ~0U;
// All other instructions count for at least one unit.
// Calls are more expensive. If they are non-intrinsic calls, we model them
// as having cost of 4. If they are a non-vector intrinsic, we model them
// as having cost of 2 total, and if they are a vector intrinsic, we model
// them as having cost 1.
if (const CallInst *CI = dyn_cast<CallInst>(I)) {
if (CI->cannotDuplicate() || CI->isConvergent())
// Blocks with NoDuplicate are modelled as having infinite cost, so they
// are never duplicated.
return ~0U;
else if (!isa<IntrinsicInst>(CI))
Size += 3;
else if (!CI->getType()->isVectorTy())
Size += 1;
return Size > Bonus ? Size - Bonus : 0;
/// FindLoopHeaders - We do not want jump threading to turn proper loop
/// structures into irreducible loops. Doing this breaks up the loop nesting
/// hierarchy and pessimizes later transformations. To prevent this from
/// happening, we first have to find the loop headers. Here we approximate this
/// by finding targets of backedges in the CFG.
/// Note that there definitely are cases when we want to allow threading of
/// edges across a loop header. For example, threading a jump from outside the
/// loop (the preheader) to an exit block of the loop is definitely profitable.
/// It is also almost always profitable to thread backedges from within the loop
/// to exit blocks, and is often profitable to thread backedges to other blocks
/// within the loop (forming a nested loop). This simple analysis is not rich
/// enough to track all of these properties and keep it up-to-date as the CFG
/// mutates, so we don't allow any of these transformations.
void JumpThreadingPass::FindLoopHeaders(Function &F) {
SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
FindFunctionBackedges(F, Edges);
for (const auto &Edge : Edges)
/// getKnownConstant - Helper method to determine if we can thread over a
/// terminator with the given value as its condition, and if so what value to
/// use for that. What kind of value this is depends on whether we want an
/// integer or a block address, but an undef is always accepted.
/// Returns null if Val is null or not an appropriate constant.
static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
if (!Val)
return nullptr;
// Undef is "known" enough.
if (UndefValue *U = dyn_cast<UndefValue>(Val))
return U;
if (Preference == WantBlockAddress)
return dyn_cast<BlockAddress>(Val->stripPointerCasts());
return dyn_cast<ConstantInt>(Val);
/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
/// if we can infer that the value is a known ConstantInt/BlockAddress or undef
/// in any of our predecessors. If so, return the known list of value and pred
/// BB in the result vector.
/// This returns true if there were any known values.
bool JumpThreadingPass::ComputeValueKnownInPredecessorsImpl(
Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference,
DenseSet<std::pair<Value *, BasicBlock *>> &RecursionSet,
Instruction *CxtI) {
// This method walks up use-def chains recursively. Because of this, we could
// get into an infinite loop going around loops in the use-def chain. To
// prevent this, keep track of what (value, block) pairs we've already visited
// and terminate the search if we loop back to them
if (!RecursionSet.insert(std::make_pair(V, BB)).second)
return false;
// If V is a constant, then it is known in all predecessors.
if (Constant *KC = getKnownConstant(V, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
Result.push_back(std::make_pair(KC, Pred));
return !Result.empty();
// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (!I || I->getParent() != BB) {
// Okay, if this is a live-in value, see if it has a known value at the end
// of any of our predecessors.
// FIXME: This should be an edge property, not a block end property.
/// TODO: Per PR2563, we could infer value range information about a
/// predecessor based on its terminator.
// FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
// "I" is a non-local compare-with-a-constant instruction. This would be
// able to handle value inequalities better, for example if the compare is
// "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
// Perhaps getConstantOnEdge should be smart enough to do this?
if (DTU->hasPendingDomTreeUpdates())
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
if (Constant *KC = getKnownConstant(PredCst, Preference))
Result.push_back(std::make_pair(KC, P));
return !Result.empty();
/// If I is a PHI node, then we know the incoming values for any constants.
if (PHINode *PN = dyn_cast<PHINode>(I)) {
if (DTU->hasPendingDomTreeUpdates())
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *InVal = PN->getIncomingValue(i);
if (Constant *KC = getKnownConstant(InVal, Preference)) {
Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
} else {
Constant *CI = LVI->getConstantOnEdge(InVal,
BB, CxtI);
if (Constant *KC = getKnownConstant(CI, Preference))
Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
return !Result.empty();
// Handle Cast instructions. Only see through Cast when the source operand is
// PHI or Cmp to save the compilation time.
if (CastInst *CI = dyn_cast<CastInst>(I)) {
Value *Source = CI->getOperand(0);
if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
return false;
ComputeValueKnownInPredecessorsImpl(Source, BB, Result, Preference,
RecursionSet, CxtI);
if (Result.empty())
return false;
// Convert the known values.
for (auto &R : Result)
R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
return true;
// Handle some boolean conditions.
if (I->getType()->getPrimitiveSizeInBits() == 1) {
assert(Preference == WantInteger && "One-bit non-integer type?");
// X | true -> true
// X & false -> false
if (I->getOpcode() == Instruction::Or ||
I->getOpcode() == Instruction::And) {
PredValueInfoTy LHSVals, RHSVals;
ComputeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals,
WantInteger, RecursionSet, CxtI);
ComputeValueKnownInPredecessorsImpl(I->getOperand(1), BB, RHSVals,
WantInteger, RecursionSet, CxtI);
if (LHSVals.empty() && RHSVals.empty())
return false;
ConstantInt *InterestingVal;
if (I->getOpcode() == Instruction::Or)
InterestingVal = ConstantInt::getTrue(I->getContext());
InterestingVal = ConstantInt::getFalse(I->getContext());
SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
// Scan for the sentinel. If we find an undef, force it to the
// interesting value: x|undef -> true and x&undef -> false.
for (const auto &LHSVal : LHSVals)
if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) {
Result.emplace_back(InterestingVal, LHSVal.second);
for (const auto &RHSVal : RHSVals)
if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) {
// If we already inferred a value for this block on the LHS, don't
// re-add it.
if (!LHSKnownBBs.count(RHSVal.second))
Result.emplace_back(InterestingVal, RHSVal.second);
return !Result.empty();
// Handle the NOT form of XOR.
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
ComputeValueKnownInPredecessorsImpl(I->getOperand(0), BB, Result,
WantInteger, RecursionSet, CxtI);
if (Result.empty())
return false;
// Invert the known values.
for (auto &R : Result)
R.first = ConstantExpr::getNot(R.first);
return true;
// Try to simplify some other binary operator values.
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
assert(Preference != WantBlockAddress
&& "A binary operator creating a block address?");
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessorsImpl(BO->getOperand(0), BB, LHSVals,
WantInteger, RecursionSet, CxtI);
// Try to use constant folding to simplify the binary operator.
for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVal.second));
return !Result.empty();
// Handle compare with phi operand, where the PHI is defined in this block.
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
assert(Preference == WantInteger && "Compares only produce integers");
Type *CmpType = Cmp->getType();
Value *CmpLHS = Cmp->getOperand(0);
Value *CmpRHS = Cmp->getOperand(1);
CmpInst::Predicate Pred = Cmp->getPredicate();
PHINode *PN = dyn_cast<PHINode>(CmpLHS);
if (!PN)
PN = dyn_cast<PHINode>(CmpRHS);
if (PN && PN->getParent() == BB) {
const DataLayout &DL = PN->getModule()->getDataLayout();
// We can do this simplification if any comparisons fold to true or false.
// See if any do.
if (DTU->hasPendingDomTreeUpdates())
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
Value *LHS, *RHS;
if (PN == CmpLHS) {
LHS = PN->getIncomingValue(i);
RHS = CmpRHS->DoPHITranslation(BB, PredBB);
} else {
LHS = CmpLHS->DoPHITranslation(BB, PredBB);
RHS = PN->getIncomingValue(i);
Value *Res = SimplifyCmpInst(Pred, LHS, RHS, {DL});
if (!Res) {
if (!isa<Constant>(RHS))
// getPredicateOnEdge call will make no sense if LHS is defined in BB.
auto LHSInst = dyn_cast<Instruction>(LHS);
if (LHSInst && LHSInst->getParent() == BB)
ResT = LVI->getPredicateOnEdge(Pred, LHS,
cast<Constant>(RHS), PredBB, BB,
CxtI ? CxtI : Cmp);
if (ResT == LazyValueInfo::Unknown)
Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
if (Constant *KC = getKnownConstant(Res, WantInteger))
Result.push_back(std::make_pair(KC, PredBB));
return !Result.empty();
// If comparing a live-in value against a constant, see if we know the
// live-in value on any predecessors.
if (isa<Constant>(CmpRHS) && !CmpType->isVectorTy()) {
Constant *CmpConst = cast<Constant>(CmpRHS);
if (!isa<Instruction>(CmpLHS) ||
cast<Instruction>(CmpLHS)->getParent() != BB) {
if (DTU->hasPendingDomTreeUpdates())
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
LazyValueInfo::Tristate Res =
LVI->getPredicateOnEdge(Pred, CmpLHS,
CmpConst, P, BB, CxtI ? CxtI : Cmp);
if (Res == LazyValueInfo::Unknown)
Constant *ResC = ConstantInt::get(CmpType, Res);
Result.push_back(std::make_pair(ResC, P));
return !Result.empty();
// InstCombine can fold some forms of constant range checks into
// (icmp (add (x, C1)), C2). See if we have we have such a thing with
// x as a live-in.
using namespace PatternMatch;
Value *AddLHS;
ConstantInt *AddConst;
if (isa<ConstantInt>(CmpConst) &&
match(CmpLHS, m_Add(m_Value(AddLHS), m_ConstantInt(AddConst)))) {
if (!isa<Instruction>(AddLHS) ||
cast<Instruction>(AddLHS)->getParent() != BB) {
if (DTU->hasPendingDomTreeUpdates())
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a ConstantRange in
// a predecessor, use that information to try to thread this
// block.
ConstantRange CR = LVI->getConstantRangeOnEdge(
AddLHS, P, BB, CxtI ? CxtI : cast<Instruction>(CmpLHS));
// Propagate the range through the addition.
CR = CR.add(AddConst->getValue());
// Get the range where the compare returns true.
ConstantRange CmpRange = ConstantRange::makeExactICmpRegion(
Pred, cast<ConstantInt>(CmpConst)->getValue());
Constant *ResC;
if (CmpRange.contains(CR))
ResC = ConstantInt::getTrue(CmpType);
else if (CmpRange.inverse().contains(CR))
ResC = ConstantInt::getFalse(CmpType);
Result.push_back(std::make_pair(ResC, P));
return !Result.empty();
// Try to find a constant value for the LHS of a comparison,
// and evaluate it statically if we can.
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals,
WantInteger, RecursionSet, CxtI);
for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
Constant *Folded = ConstantExpr::getCompare(Pred, V, CmpConst);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVal.second));
return !Result.empty();
if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
// Handle select instructions where at least one operand is a known constant
// and we can figure out the condition value for any predecessor block.
Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
PredValueInfoTy Conds;
if ((TrueVal || FalseVal) &&
ComputeValueKnownInPredecessorsImpl(SI->getCondition(), BB, Conds,
WantInteger, RecursionSet, CxtI)) {
for (auto &C : Conds) {
Constant *Cond = C.first;
// Figure out what value to use for the condition.
bool KnownCond;
if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
// A known boolean.
KnownCond = CI->isOne();
} else {
assert(isa<UndefValue>(Cond) && "Unexpected condition value");
// Either operand will do, so be sure to pick the one that's a known
// constant.
// FIXME: Do this more cleverly if both values are known constants?
KnownCond = (TrueVal != nullptr);
// See if the select has a known constant value for this predecessor.
if (Constant *Val = KnownCond ? TrueVal : FalseVal)
Result.push_back(std::make_pair(Val, C.second));
return !Result.empty();
// If all else fails, see if LVI can figure out a constant value for us.
if (DTU->hasPendingDomTreeUpdates())
Constant *CI = LVI->getConstant(V, BB, CxtI);
if (Constant *KC = getKnownConstant(CI, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
Result.push_back(std::make_pair(KC, Pred));
return !Result.empty();
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
/// in an undefined jump, decide which block is best to revector to.
/// Since we can pick an arbitrary destination, we pick the successor with the
/// fewest predecessors. This should reduce the in-degree of the others.
static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
Instruction *BBTerm = BB->getTerminator();
unsigned MinSucc = 0;
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
// Compute the successor with the minimum number of predecessors.
unsigned MinNumPreds = pred_size(TestBB);
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
TestBB = BBTerm->getSuccessor(i);
unsigned NumPreds = pred_size(TestBB);
if (NumPreds < MinNumPreds) {
MinSucc = i;
MinNumPreds = NumPreds;
return MinSucc;
static bool hasAddressTakenAndUsed(BasicBlock *BB) {
if (!BB->hasAddressTaken()) return false;
// If the block has its address taken, it may be a tree of dead constants
// hanging off of it. These shouldn't keep the block alive.
BlockAddress *BA = BlockAddress::get(BB);
return !BA->use_empty();
/// ProcessBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
// If the block is trivially dead, just return and let the caller nuke it.
// This simplifies other transformations.
if (DTU->isBBPendingDeletion(BB) ||
(pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()))
return false;
// If this block has a single predecessor, and if that pred has a single
// successor, merge the blocks. This encourages recursive jump threading
// because now the condition in this block can be threaded through
// predecessors of our predecessor block.
if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
const Instruction *TI = SinglePred->getTerminator();
if (!TI->isExceptionalTerminator() && TI->getNumSuccessors() == 1 &&
SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
// If SinglePred was a loop header, BB becomes one.
if (LoopHeaders.erase(SinglePred))
MergeBasicBlockIntoOnlyPred(BB, DTU);
// Now that BB is merged into SinglePred (i.e. SinglePred Code followed by
// BB code within one basic block `BB`), we need to invalidate the LVI
// information associated with BB, because the LVI information need not be
// true for all of BB after the merge. For example,
// Before the merge, LVI info and code is as follows:
// SinglePred: <LVI info1 for %p val>
// %y = use of %p
// call @exit() // need not transfer execution to successor.
// assume(%p) // from this point on %p is true
// br label %BB
// BB: <LVI info2 for %p val, i.e. %p is true>
// %x = use of %p
// br label exit
// Note that this LVI info for blocks BB and SinglPred is correct for %p
// (info2 and info1 respectively). After the merge and the deletion of the
// LVI info1 for SinglePred. We have the following code:
// BB: <LVI info2 for %p val>
// %y = use of %p
// call @exit()
// assume(%p)
// %x = use of %p <-- LVI info2 is correct from here onwards.
// br label exit
// LVI info2 for BB is incorrect at the beginning of BB.
// Invalidate LVI information for BB if the LVI is not provably true for
// all of BB.
if (!isGuaranteedToTransferExecutionToSuccessor(BB))
return true;
if (TryToUnfoldSelectInCurrBB(BB))
return true;
// Look if we can propagate guards to predecessors.
if (HasGuards && ProcessGuards(BB))
return true;
// What kind of constant we're looking for.
ConstantPreference Preference = WantInteger;
// Look to see if the terminator is a conditional branch, switch or indirect
// branch, if not we can't thread it.
Value *Condition;
Instruction *Terminator = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
// Can't thread an unconditional jump.
if (BI->isUnconditional()) return false;
Condition = BI->getCondition();
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
Condition = SI->getCondition();
} else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
// Can't thread indirect branch with no successors.
if (IB->getNumSuccessors() == 0) return false;
Condition = IB->getAddress()->stripPointerCasts();
Preference = WantBlockAddress;
} else {
return false; // Must be an invoke or callbr.
// Run constant folding to see if we can reduce the condition to a simple
// constant.
if (Instruction *I = dyn_cast<Instruction>(Condition)) {
Value *SimpleVal =
ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
if (SimpleVal) {
if (isInstructionTriviallyDead(I, TLI))
Condition = SimpleVal;
// If the terminator is branching on an undef, we can pick any of the
// successors to branch to. Let GetBestDestForJumpOnUndef decide.
if (isa<UndefValue>(Condition)) {
unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
std::vector<DominatorTree::UpdateType> Updates;
// Fold the branch/switch.
Instruction *BBTerm = BB->getTerminator();
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
if (i == BestSucc) continue;
BasicBlock *Succ = BBTerm->getSuccessor(i);
Succ->removePredecessor(BB, true);
Updates.push_back({DominatorTree::Delete, BB, Succ});
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm << '\n');
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
return true;
// If the terminator of this block is branching on a constant, simplify the
// terminator to an unconditional branch. This can occur due to threading in
// other blocks.
if (getKnownConstant(Condition, Preference)) {
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator()
<< '\n');
ConstantFoldTerminator(BB, true, nullptr, DTU);
return true;
Instruction *CondInst = dyn_cast<Instruction>(Condition);
// All the rest of our checks depend on the condition being an instruction.
if (!CondInst) {
// FIXME: Unify this with code below.
if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
return true;
return false;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
// If we're branching on a conditional, LVI might be able to determine
// it's value at the branch instruction. We only handle comparisons
// against a constant at this time.
// TODO: This should be extended to handle switches as well.
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
if (CondBr && CondConst) {
// We should have returned as soon as we turn a conditional branch to
// unconditional. Because its no longer interesting as far as jump
// threading is concerned.
assert(CondBr->isConditional() && "Threading on unconditional terminator");
if (DTU->hasPendingDomTreeUpdates())
LazyValueInfo::Tristate Ret =
LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
CondConst, CondBr);
if (Ret != LazyValueInfo::Unknown) {
unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
BasicBlock *ToRemoveSucc = CondBr->getSuccessor(ToRemove);
ToRemoveSucc->removePredecessor(BB, true);
BranchInst *UncondBr =
BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
if (CondCmp->use_empty())
// We can safely replace *some* uses of the CondInst if it has
// exactly one value as returned by LVI. RAUW is incorrect in the
// presence of guards and assumes, that have the `Cond` as the use. This
// is because we use the guards/assume to reason about the `Cond` value
// at the end of block, but RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
else if (CondCmp->getParent() == BB) {
auto *CI = Ret == LazyValueInfo::True ?
ConstantInt::getTrue(CondCmp->getType()) :
ReplaceFoldableUses(CondCmp, CI);
{{DominatorTree::Delete, BB, ToRemoveSucc}});
return true;
// We did not manage to simplify this branch, try to see whether
// CondCmp depends on a known phi-select pattern.
if (TryToUnfoldSelect(CondCmp, BB))
return true;
if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
if (TryToUnfoldSelect(SI, BB))
return true;
// Check for some cases that are worth simplifying. Right now we want to look
// for loads that are used by a switch or by the condition for the branch. If
// we see one, check to see if it's partially redundant. If so, insert a PHI
// which can then be used to thread the values.
Value *SimplifyValue = CondInst;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
if (isa<Constant>(CondCmp->getOperand(1)))
SimplifyValue = CondCmp->getOperand(0);
// TODO: There are other places where load PRE would be profitable, such as
// more complex comparisons.
if (LoadInst *LoadI = dyn_cast<LoadInst>(SimplifyValue))
if (SimplifyPartiallyRedundantLoad(LoadI))
return true;
// Before threading, try to propagate profile data backwards:
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
updatePredecessorProfileMetadata(PN, BB);
// Handle a variety of cases where we are branching on something derived from
// a PHI node in the current block. If we can prove that any predecessors
// compute a predictable value based on a PHI node, thread those predecessors.
if (ProcessThreadableEdges(CondInst, BB, Preference, Terminator))
return true;
// If this is an otherwise-unfoldable branch on a phi node in the current
// block, see if we can simplify.
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnPHI(PN);
// If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
if (CondInst->getOpcode() == Instruction::Xor &&
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
// Search for a stronger dominating condition that can be used to simplify a
// conditional branch leaving BB.
if (ProcessImpliedCondition(BB))
return true;
return false;
bool JumpThreadingPass::ProcessImpliedCondition(BasicBlock *BB) {
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
Value *Cond = BI->getCondition();
BasicBlock *CurrentBB = BB;
BasicBlock *CurrentPred = BB->getSinglePredecessor();
unsigned Iter = 0;
auto &DL = BB->getModule()->getDataLayout();
while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
if (!PBI || !PBI->isConditional())
return false;
if (PBI->getSuccessor(0) != CurrentBB && PBI->getSuccessor(1) != CurrentBB)
return false;
bool CondIsTrue = PBI->getSuccessor(0) == CurrentBB;
Optional<bool> Implication =
isImpliedCondition(PBI->getCondition(), Cond, DL, CondIsTrue);
if (Implication) {
BasicBlock *KeepSucc = BI->getSuccessor(*Implication ? 0 : 1);
BasicBlock *RemoveSucc = BI->getSuccessor(*Implication ? 1 : 0);
BranchInst *UncondBI = BranchInst::Create(KeepSucc, BI);
DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}});
return true;
CurrentBB = CurrentPred;
CurrentPred = CurrentBB->getSinglePredecessor();
return false;
/// Return true if Op is an instruction defined in the given block.
static bool isOpDefinedInBlock(Value *Op, BasicBlock *BB) {
if (Instruction *OpInst = dyn_cast<Instruction>(Op))
if (OpInst->getParent() == BB)
return true;
return false;
/// SimplifyPartiallyRedundantLoad - If LoadI is an obviously partially
/// redundant load instruction, eliminate it by replacing it with a PHI node.
/// This is an important optimization that encourages jump threading, and needs
/// to be run interlaced with other jump threading tasks.
bool JumpThreadingPass::SimplifyPartiallyRedundantLoad(LoadInst *LoadI) {
// Don't hack volatile and ordered loads.
if (!LoadI->isUnordered()) return false;
// If the load is defined in a block with exactly one predecessor, it can't be
// partially redundant.
BasicBlock *LoadBB = LoadI->getParent();
if (LoadBB->getSinglePredecessor())
return false;
// If the load is defined in an EH pad, it can't be partially redundant,
// because the edges between the invoke and the EH pad cannot have other
// instructions between them.
if (LoadBB->isEHPad())
return false;
Value *LoadedPtr = LoadI->getOperand(0);
// If the loaded operand is defined in the LoadBB and its not a phi,
// it can't be available in predecessors.
if (isOpDefinedInBlock(LoadedPtr, LoadBB) && !isa<PHINode>(LoadedPtr))
return false;
// Scan a few instructions up from the load, to see if it is obviously live at
// the entry to its block.
BasicBlock::iterator BBIt(LoadI);
bool IsLoadCSE;
if (Value *AvailableVal = FindAvailableLoadedValue(
LoadI, LoadBB, BBIt, DefMaxInstsToScan, AA, &IsLoadCSE)) {
// If the value of the load is locally available within the block, just use
// it. This frequently occurs for reg2mem'd allocas.
if (IsLoadCSE) {
LoadInst *NLoadI = cast<LoadInst>(AvailableVal);
combineMetadataForCSE(NLoadI, LoadI, false);
// If the returned value is the load itself, replace with an undef. This can
// only happen in dead loops.
if (AvailableVal == LoadI)
AvailableVal = UndefValue::get(LoadI->getType());
if (AvailableVal->getType() != LoadI->getType())
AvailableVal = CastInst::CreateBitOrPointerCast(
AvailableVal, LoadI->getType(), "", LoadI);
return true;
// Otherwise, if we scanned the whole block and got to the top of the block,
// we know the block is locally transparent to the load. If not, something
// might clobber its value.
if (BBIt != LoadBB->begin())
return false;
// If all of the loads and stores that feed the value have the same AA tags,
// then we can propagate them onto any newly inserted loads.
AAMDNodes AATags;
SmallPtrSet<BasicBlock*, 8> PredsScanned;
using AvailablePredsTy = SmallVector<std::pair<BasicBlock *, Value *>, 8>;
AvailablePredsTy AvailablePreds;
BasicBlock *OneUnavailablePred = nullptr;
SmallVector<LoadInst*, 8> CSELoads;
// If we got here, the loaded value is transparent through to the start of the
// block. Check to see if it is available in any of the predecessor blocks.
for (BasicBlock *PredBB : predecessors(LoadBB)) {
// If we already scanned this predecessor, skip it.
if (!PredsScanned.insert(PredBB).second)
BBIt = PredBB->end();
unsigned NumScanedInst = 0;
Value *PredAvailable = nullptr;
// NOTE: We don't CSE load that is volatile or anything stronger than
// unordered, that should have been checked when we entered the function.
assert(LoadI->isUnordered() &&
"Attempting to CSE volatile or atomic loads");
// If this is a load on a phi pointer, phi-translate it and search
// for available load/store to the pointer in predecessors.
Value *Ptr = LoadedPtr->DoPHITranslation(LoadBB, PredBB);
PredAvailable = FindAvailablePtrLoadStore(
Ptr, LoadI->getType(), LoadI->isAtomic(), PredBB, BBIt,
DefMaxInstsToScan, AA, &IsLoadCSE, &NumScanedInst);
// If PredBB has a single predecessor, continue scanning through the
// single predecessor.
BasicBlock *SinglePredBB = PredBB;
while (!PredAvailable && SinglePredBB && BBIt == SinglePredBB->begin() &&
NumScanedInst < DefMaxInstsToScan) {
SinglePredBB = SinglePredBB->getSinglePredecessor();
if (SinglePredBB) {
BBIt = SinglePredBB->end();
PredAvailable = FindAvailablePtrLoadStore(
Ptr, LoadI->getType(), LoadI->isAtomic(), SinglePredBB, BBIt,
(DefMaxInstsToScan - NumScanedInst), AA, &IsLoadCSE,
if (!PredAvailable) {
OneUnavailablePred = PredBB;
if (IsLoadCSE)
// If so, this load is partially redundant. Remember this info so that we
// can create a PHI node.
AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
// If the loaded value isn't available in any predecessor, it isn't partially
// redundant.
if (AvailablePreds.empty()) return false;
// Okay, the loaded value is available in at least one (and maybe all!)
// predecessors. If the value is unavailable in more than one unique
// predecessor, we want to insert a merge block for those common predecessors.
// This ensures that we only have to insert one reload, thus not increasing
// code size.
BasicBlock *UnavailablePred = nullptr;
// If the value is unavailable in one of predecessors, we will end up
// inserting a new instruction into them. It is only valid if all the
// instructions before LoadI are guaranteed to pass execution to its
// successor, or if LoadI is safe to speculate.
// TODO: If this logic becomes more complex, and we will perform PRE insertion
// farther than to a predecessor, we need to reuse the code from GVN's PRE.
// It requires domination tree analysis, so for this simple case it is an
// overkill.
if (PredsScanned.size() != AvailablePreds.size() &&
for (auto I = LoadBB->begin(); &*I != LoadI; ++I)
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
return false;
// If there is exactly one predecessor where the value is unavailable, the
// already computed 'OneUnavailablePred' block is it. If it ends in an
// unconditional branch, we know that it isn't a critical edge.
if (PredsScanned.size() == AvailablePreds.size()+1 &&
OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
UnavailablePred = OneUnavailablePred;
} else if (PredsScanned.size() != AvailablePreds.size()) {
// Otherwise, we had multiple unavailable predecessors or we had a critical
// edge from the one.
SmallVector<BasicBlock*, 8> PredsToSplit;
SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
for (const auto &AvailablePred : AvailablePreds)
// Add all the unavailable predecessors to the PredsToSplit list.
for (BasicBlock *P : predecessors(LoadBB)) {
// If the predecessor is an indirect goto, we can't split the edge.
// Same for CallBr.
if (isa<IndirectBrInst>(P->getTerminator()) ||
return false;
if (!AvailablePredSet.count(P))
// Split them out to their own block.
UnavailablePred = SplitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
// If the value isn't available in all predecessors, then there will be
// exactly one where it isn't available. Insert a load on that edge and add
// it to the AvailablePreds list.
if (UnavailablePred) {
assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
"Can't handle critical edge here!");
LoadInst *NewVal = new LoadInst(
LoadI->getType(), LoadedPtr->DoPHITranslation(LoadBB, UnavailablePred),
LoadI->getName() + ".pr", false, LoadI->getAlignment(),
LoadI->getOrdering(), LoadI->getSyncScopeID(),
if (AATags)
AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
// Now we know that each predecessor of this block has a value in
// AvailablePreds, sort them for efficient access as we're walking the preds.
array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
// Create a PHI node at the start of the block for the PRE'd load value.
pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
PHINode *PN = PHINode::Create(LoadI->getType(), std::distance(PB, PE), "",
// Insert new entries into the PHI for each predecessor. A single block may
// have multiple entries here.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
AvailablePredsTy::iterator I =
std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
std::make_pair(P, (Value*)nullptr));
assert(I != AvailablePreds.end() && I->first == P &&
"Didn't find entry for predecessor!");
// If we have an available predecessor but it requires casting, insert the
// cast in the predecessor and use the cast. Note that we have to update the
// AvailablePreds vector as we go so that all of the PHI entries for this
// predecessor use the same bitcast.
Value *&PredV = I->second;
if (PredV->getType() != LoadI->getType())
PredV = CastInst::CreateBitOrPointerCast(PredV, LoadI->getType(), "",
PN->addIncoming(PredV, I->first);
for (LoadInst *PredLoadI : CSELoads) {
combineMetadataForCSE(PredLoadI, LoadI, true);
return true;
/// FindMostPopularDest - The specified list contains multiple possible
/// threadable destinations. Pick the one that occurs the most frequently in
/// the list.
static BasicBlock *
FindMostPopularDest(BasicBlock *BB,
const SmallVectorImpl<std::pair<BasicBlock *,
BasicBlock *>> &PredToDestList) {
// Determine popularity. If there are multiple possible destinations, we
// explicitly choose to ignore 'undef' destinations. We prefer to thread
// blocks with known and real destinations to threading undef. We'll handle
// them later if interesting.
DenseMap<BasicBlock*, unsigned> DestPopularity;
for (const auto &PredToDest : PredToDestList)
if (PredToDest.second)
if (DestPopularity.empty())
return nullptr;
// Find the most popular dest.
DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
BasicBlock *MostPopularDest = DPI->first;
unsigned Popularity = DPI->second;
SmallVector<BasicBlock*, 4> SamePopularity;
for (++DPI; DPI != DestPopularity.end(); ++DPI) {
// If the popularity of this entry isn't higher than the popularity we've
// seen so far, ignore it.
if (DPI->second < Popularity)
; // ignore.
else if (DPI->second == Popularity) {
// If it is the same as what we've seen so far, keep track of it.
} else {
// If it is more popular, remember it.
MostPopularDest = DPI->first;
Popularity = DPI->second;
// Okay, now we know the most popular destination. If there is more than one
// destination, we need to determine one. This is arbitrary, but we need
// to make a deterministic decision. Pick the first one that appears in the
// successor list.
if (!SamePopularity.empty()) {
Instruction *TI = BB->getTerminator();
for (unsigned i = 0; ; ++i) {
assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
if (!is_contained(SamePopularity, TI->getSuccessor(i)))
MostPopularDest = TI->getSuccessor(i);
// Okay, we have finally picked the most popular destination.
return MostPopularDest;
bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference,
Instruction *CxtI) {
// If threading this would thread across a loop header, don't even try to
// thread the edge.
if (LoopHeaders.count(BB))
return false;
PredValueInfoTy PredValues;
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
return false;
assert(!PredValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
LLVM_DEBUG(dbgs() << "IN BB: " << *BB;
for (const auto &PredValue : PredValues) {
dbgs() << " BB '" << BB->getName()
<< "': FOUND condition = " << *PredValue.first
<< " for pred '" << PredValue.second->getName() << "'.\n";
// Decide what we want to thread through. Convert our list of known values to
// a list of known destinations for each pred. This also discards duplicate
// predecessors and keeps track of the undefined inputs (which are represented
// as a null dest in the PredToDestList).
SmallPtrSet<BasicBlock*, 16> SeenPreds;
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
BasicBlock *OnlyDest = nullptr;
BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
Constant *OnlyVal = nullptr;
Constant *MultipleVal = (Constant *)(intptr_t)~0ULL;
for (const auto &PredValue : PredValues) {
BasicBlock *Pred = PredValue.second;
if (!SeenPreds.insert(Pred).second)
continue; // Duplicate predecessor entry.
Constant *Val = PredValue.first;
BasicBlock *DestBB;
if (isa<UndefValue>(Val))
DestBB = nullptr;
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
DestBB = SI->findCaseValue(cast<ConstantInt>(Val))->getCaseSuccessor();
} else {
&& "Unexpected terminator");
assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress");
DestBB = cast<BlockAddress>(Val)->getBasicBlock();
// If we have exactly one destination, remember it for efficiency below.
if (PredToDestList.empty()) {
OnlyDest = DestBB;
OnlyVal = Val;
} else {
if (OnlyDest != DestBB)
OnlyDest = MultipleDestSentinel;
// It possible we have same destination, but different value, e.g. default
// case in switchinst.
if (Val != OnlyVal)
OnlyVal = MultipleVal;
// If the predecessor ends with an indirect goto, we can't change its
// destination. Same for CallBr.
if (isa<IndirectBrInst>(Pred->getTerminator()) ||
PredToDestList.push_back(std::make_pair(Pred, DestBB));
// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
return false;
// If all the predecessors go to a single known successor, we want to fold,
// not thread. By doing so, we do not need to duplicate the current block and
// also miss potential opportunities in case we dont/cant duplicate.
if (OnlyDest && OnlyDest != MultipleDestSentinel) {
if (BB->hasNPredecessors(PredToDestList.size())) {
bool SeenFirstBranchToOnlyDest = false;
std::vector <DominatorTree::UpdateType> Updates;
Updates.reserve(BB->getTerminator()->getNumSuccessors() - 1);
for (BasicBlock *SuccBB : successors(BB)) {
if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest) {
SeenFirstBranchToOnlyDest = true; // Don't modify the first branch.
} else {
SuccBB->removePredecessor(BB, true); // This is unreachable successor.
Updates.push_back({DominatorTree::Delete, BB, SuccBB});
// Finally update the terminator.
Instruction *Term = BB->getTerminator();
BranchInst::Create(OnlyDest, Term);
// If the condition is now dead due to the removal of the old terminator,
// erase it.
if (auto *CondInst = dyn_cast<Instruction>(Cond)) {
if (CondInst->use_empty() && !CondInst->mayHaveSideEffects())
// We can safely replace *some* uses of the CondInst if it has
// exactly one value as returned by LVI. RAUW is incorrect in the
// presence of guards and assumes, that have the `Cond` as the use. This
// is because we use the guards/assume to reason about the `Cond` value
// at the end of block, but RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
else if (OnlyVal && OnlyVal != MultipleVal &&
CondInst->getParent() == BB)
ReplaceFoldableUses(CondInst, OnlyVal);
return true;
// Determine which is the most common successor. If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
// to the most popular destination first. If we only know about one
// threadable destination (the common case) we can avoid this.
BasicBlock *MostPopularDest = OnlyDest;
if (MostPopularDest == MultipleDestSentinel) {
// Remove any loop headers from the Dest list, ThreadEdge conservatively
// won't process them, but we might have other destination that are eligible
// and we still want to process.
[&](const std::pair<BasicBlock *, BasicBlock *> &PredToDest) {
return LoopHeaders.count(PredToDest.second) != 0;
if (PredToDestList.empty())
return false;
MostPopularDest = FindMostPopularDest(BB, PredToDestList);
// Now that we know what the most popular destination is, factor all
// predecessors that will jump to it into a single predecessor.
SmallVector<BasicBlock*, 16> PredsToFactor;
for (const auto &PredToDest : PredToDestList)
if (PredToDest.second == MostPopularDest) {
BasicBlock *Pred = PredToDest.first;
// This predecessor may be a switch or something else that has multiple
// edges to the block. Factor each of these edges by listing them
// according to # occurrences in PredsToFactor.
for (BasicBlock *Succ : successors(Pred))
if (Succ == BB)
// If the threadable edges are branching on an undefined value, we get to pick
// the destination that these predecessors should get to.
if (!MostPopularDest)
MostPopularDest = BB->getTerminator()->
// Ok, try to thread it!
return ThreadEdge(BB, PredsToFactor, MostPopularDest);
/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
/// a PHI node in the current block. See if there are any simplifications we
/// can do based on inputs to the phi node.
bool JumpThreadingPass::ProcessBranchOnPHI(PHINode *PN) {
BasicBlock *BB = PN->getParent();
// TODO: We could make use of this to do it once for blocks with common PHI
// values.
SmallVector<BasicBlock*, 1> PredBBs;
// If any of the predecessor blocks end in an unconditional branch, we can
// *duplicate* the conditional branch into that block in order to further
// encourage jump threading and to eliminate cases where we have branch on a
// phi of an icmp (branch on icmp is much better).
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
if (PredBr->isUnconditional()) {
PredBBs[0] = PredBB;
// Try to duplicate BB into PredBB.
if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
return true;
return false;
/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
/// a xor instruction in the current block. See if there are any
/// simplifications we can do based on inputs to the xor.
bool JumpThreadingPass::ProcessBranchOnXOR(BinaryOperator *BO) {
BasicBlock *BB = BO->getParent();
// If either the LHS or RHS of the xor is a constant, don't do this
// optimization.
if (isa<ConstantInt>(BO->getOperand(0)) ||
return false;
// If the first instruction in BB isn't a phi, we won't be able to infer
// anything special about any particular predecessor.
if (!isa<PHINode>(BB->front()))
return false;
// If this BB is a landing pad, we won't be able to split the edge into it.
if (BB->isEHPad())
return false;
// If we have a xor as the branch input to this block, and we know that the
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
// the condition into the predecessor and fix that value to true, saving some
// logical ops on that path and encouraging other paths to simplify.
// This copies something like this:
// BB:
// %X = phi i1 [1], [%X']
// %Y = icmp eq i32 %A, %B
// %Z = xor i1 %X, %Y
// br i1 %Z, ...
// Into:
// BB':
// %Y = icmp ne i32 %A, %B
// br i1 %Y, ...
PredValueInfoTy XorOpValues;
bool isLHS = true;
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
WantInteger, BO)) {
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
WantInteger, BO))
return false;
isLHS = false;
assert(!XorOpValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
// Scan the information to see which is most popular: true or false. The
// predecessors can be of the set true, false, or undef.
unsigned NumTrue = 0, NumFalse = 0;
for (const auto &XorOpValue : XorOpValues) {
if (isa<UndefValue>(XorOpValue.first))
// Ignore undefs for the count.
if (cast<ConstantInt>(XorOpValue.first)->isZero())
// Determine which value to split on, true, false, or undef if neither.
ConstantInt *SplitVal = nullptr;
if (NumTrue > NumFalse)
SplitVal = ConstantInt::getTrue(BB->getContext());
else if (NumTrue != 0 || NumFalse != 0)
SplitVal = ConstantInt::getFalse(BB->getContext());
// Collect all of the blocks that this can be folded into so that we can
// factor this once and clone it once.
SmallVector<BasicBlock*, 8> BlocksToFoldInto;
for (const auto &XorOpValue : XorOpValues) {
if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first))
// If we inferred a value for all of the predecessors, then duplication won't
// help us. However, we can just replace the LHS or RHS with the constant.
if (BlocksToFoldInto.size() ==
cast<PHINode>(BB->front()).getNumIncomingValues()) {
if (!SplitVal) {
// If all preds provide undef, just nuke the xor, because it is undef too.
} else if (SplitVal->isZero()) {
// If all preds provide 0, replace the xor with the other input.
} else {
// If all preds provide 1, set the computed value to 1.
BO->setOperand(!isLHS, SplitVal);
return true;
// Try to duplicate BB into PredBB.
return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
/// NewPred using the entries from OldPred (suitably mapped).
static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
BasicBlock *OldPred,
BasicBlock *NewPred,
DenseMap<Instruction*, Value*> &ValueMap) {
for (PHINode &PN : PHIBB->phis()) {
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN.getIncomingValueForBlock(OldPred);
// Remap the value if necessary.
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
if (I != ValueMap.end())
IV = I->second;
PN.addIncoming(IV, NewPred);
/// ThreadEdge - We have decided that it is safe and profitable to factor the
/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
/// across BB. Transform the IR to reflect this change.
bool JumpThreadingPass::ThreadEdge(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &PredBBs,
BasicBlock *SuccBB) {
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
<< "' - would thread to self!\n");
return false;
// If threading this would thread across a loop header, don't thread the edge.
// See the comments above FindLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
bool BBIsHeader = LoopHeaders.count(BB);
bool SuccIsHeader = LoopHeaders.count(SuccBB);
dbgs() << " Not threading across "
<< (BBIsHeader ? "loop header BB '" : "block BB '") << BB->getName()
<< "' to dest " << (SuccIsHeader ? "loop header BB '" : "block BB '")
<< SuccBB->getName() << "' - it might create an irreducible loop!\n";
return false;
unsigned JumpThreadCost =
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
if (JumpThreadCost > BBDupThreshold) {
LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
// And finally, do it! Start by factoring the predecessors if needed.
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
// And finally, do it!
LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB->getName()
<< "' to '" << SuccBB->getName()
<< "' with cost: " << JumpThreadCost
<< ", across block:\n " << *BB << "\n");
if (DTU->hasPendingDomTreeUpdates())
LVI->threadEdge(PredBB, BB, SuccBB);
// We are going to have to map operands from the original BB block to the new
// copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
// account for entry from PredBB.
DenseMap<Instruction*, Value*> ValueMapping;
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
BB->getParent(), BB);
// Set the block frequency of NewBB.
if (HasProfileData) {
auto NewBBFreq =
BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
// Clone the non-phi instructions of BB into NewBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; !BI->isTerminator(); ++BI) {
Instruction *New = BI->clone();
ValueMapping[&*BI] = New;
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
// We didn't copy the terminator from BB over to NewBB, because there is now
// an unconditional jump to SuccBB. Insert the unconditional jump.
BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
// PHI nodes for NewBB now.
AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
// Update the terminator of PredBB to jump to NewBB instead of BB. This
// eliminates predecessors from BB, which requires us to simplify any PHI
// nodes in BB.
Instruction *PredTerm = PredBB->getTerminator();
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
if (PredTerm->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB, true);
PredTerm->setSuccessor(i, NewBB);
// Enqueue required DT updates.
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, SuccBB},
{DominatorTree::Insert, PredBB, NewBB},
{DominatorTree::Delete, PredBB, BB}});
// If there were values defined in BB that are used outside the block, then we
// now have to update all uses of the value to use either the original value,
// the cloned value, or some PHI derived value. This can require arbitrary
// PHI insertion, of which we are prepared to do, clean these up now.
SSAUpdater SSAUpdate;
SmallVector<Use*, 16> UsesToRename;
for (Instruction &I : *BB) {
// Scan all uses of this instruction to see if their uses are no longer
// dominated by the previous def and if so, record them in UsesToRename.
// Also, skip phi operands from PredBB - we'll remove them anyway.
for (Use &U : I.uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(U) == BB)
} else if (User->getParent() == BB)
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
// We found a use of I outside of BB. Rename all uses of I that are outside
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
// with the two values we know.
SSAUpdate.Initialize(I.getType(), I.getName());
SSAUpdate.AddAvailableValue(BB, &I);
SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
while (!UsesToRename.empty())
LLVM_DEBUG(dbgs() << "\n");
// At this point, the IR is fully up to date and consistent. Do a quick scan
// over the new instructions and zap any that are constants or dead. This
// frequently happens because of phi translation.
SimplifyInstructionsInBlock(NewBB, TLI);
// Update the edge weight from BB to SuccBB, which should be less than before.
UpdateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
// Threaded an edge!
return true;
/// Create a new basic block that will be the predecessor of BB and successor of
/// all blocks in Preds. When profile data is available, update the frequency of
/// this new block.
BasicBlock *JumpThreadingPass::SplitBlockPreds(BasicBlock *BB,
ArrayRef<BasicBlock *> Preds,
const char *Suffix) {
SmallVector<BasicBlock *, 2> NewBBs;
// Collect the frequencies of all predecessors of BB, which will be used to
// update the edge weight of the result of splitting predecessors.
DenseMap<BasicBlock *, BlockFrequency> FreqMap;
if (HasProfileData)
for (auto Pred : Preds)
Pred, BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB)));
// In the case when BB is a LandingPad block we create 2 new predecessors
// instead of just one.
if (BB->isLandingPad()) {
std::string NewName = std::string(Suffix) + ".split-lp";
SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs);
} else {
NewBBs.push_back(SplitBlockPredecessors(BB, Preds, Suffix));
std::vector<DominatorTree::UpdateType> Updates;
Updates.reserve((2 * Preds.size()) + NewBBs.size());
for (auto NewBB : NewBBs) {
BlockFrequency NewBBFreq(0);
Updates.push_back({DominatorTree::Insert, NewBB, BB});
for (auto Pred : predecessors(NewBB)) {
Updates.push_back({DominatorTree::Delete, Pred, BB});
Updates.push_back({DominatorTree::Insert, Pred, NewBB});
if (HasProfileData) // Update frequencies between Pred -> NewBB.
NewBBFreq += FreqMap.lookup(Pred);
if (HasProfileData) // Apply the summed frequency to NewBB.
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
return NewBBs[0];
bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock *BB) {
const Instruction *TI = BB->getTerminator();
assert(TI->getNumSuccessors() > 1 && "not a split");
MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
if (!WeightsNode)
return false;
MDString *MDName = cast<MDString>(WeightsNode->getOperand(0));
if (MDName->getString() != "branch_weights")
return false;
// Ensure there are weights for all of the successors. Note that the first
// operand to the metadata node is a name, not a weight.
return WeightsNode->getNumOperands() == TI->getNumSuccessors() + 1;
/// Update the block frequency of BB and branch weight and the metadata on the
/// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
/// Freq(PredBB->BB) / Freq(BB->SuccBB).
void JumpThreadingPass::UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
BasicBlock *BB,
BasicBlock *NewBB,
BasicBlock *SuccBB) {
if (!HasProfileData)
assert(BFI && BPI && "BFI & BPI should have been created here");
// As the edge from PredBB to BB is deleted, we have to update the block
// frequency of BB.
auto BBOrigFreq = BFI->getBlockFreq(BB);
auto NewBBFreq = BFI->getBlockFreq(NewBB);
auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
auto BBNewFreq = BBOrigFreq - NewBBFreq;
BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
// Collect updated outgoing edges' frequencies from BB and use them to update
// edge probabilities.
SmallVector<uint64_t, 4> BBSuccFreq;
for (BasicBlock *Succ : successors(BB)) {
auto SuccFreq = (Succ == SuccBB)
? BB2SuccBBFreq - NewBBFreq
: BBOrigFreq * BPI->getEdgeProbability(BB, Succ);
uint64_t MaxBBSuccFreq =
*std::max_element(BBSuccFreq.begin(), BBSuccFreq.end());
SmallVector<BranchProbability, 4> BBSuccProbs;
if (MaxBBSuccFreq == 0)
{1, static_cast<unsigned>(BBSuccFreq.size())});
else {
for (uint64_t Freq : BBSuccFreq)
BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq));
// Normalize edge probabilities so that they sum up to one.
// Update edge probabilities in BPI.
for (int I = 0, E = BBSuccProbs.size(); I < E; I++)
BPI->setEdgeProbability(BB, I, BBSuccProbs[I]);
// Update the profile metadata as well.
// Don't do this if the profile of the transformed blocks was statically
// estimated. (This could occur despite the function having an entry
// frequency in completely cold parts of the CFG.)
// In this case we don't want to suggest to subsequent passes that the
// calculated weights are fully consistent. Consider this graph:
// check_1
// 50% / |
// eq_1 | 50%
// \ |
// check_2
// 50% / |
// eq_2 | 50%
// \ |
// check_3
// 50% / |
// eq_3 | 50%
// \ |
// Assuming the blocks check_* all compare the same value against 1, 2 and 3,
// the overall probabilities are inconsistent; the total probability that the
// value is either 1, 2 or 3 is 150%.
// As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3
// becomes 0%. This is even worse if the edge whose probability becomes 0% is
// the loop exit edge. Then based solely on static estimation we would assume
// the loop was extremely hot.
// FIXME this locally as well so that BPI and BFI are consistent as well. We
// shouldn't make edges extremely likely or unlikely based solely on static
// estimation.
if (BBSuccProbs.size() >= 2 && doesBlockHaveProfileData(BB)) {
SmallVector<uint32_t, 4> Weights;
for (auto Prob : BBSuccProbs)
auto TI = BB->getTerminator();
/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
/// If we can duplicate the contents of BB up into PredBB do so now, this
/// improves the odds that the branch will be on an analyzable instruction like
/// a compare.
bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred(
BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) {
assert(!PredBBs.empty() && "Can't handle an empty set");
// If BB is a loop header, then duplicating this block outside the loop would
// cause us to transform this into an irreducible loop, don't do this.
// See the comments above FindLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB)) {
LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
<< "' into predecessor block '" << PredBBs[0]->getName()
<< "' - it might create an irreducible loop!\n");
return false;
unsigned DuplicationCost =
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
if (DuplicationCost > BBDupThreshold) {
LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
<< "' - Cost is too high: " << DuplicationCost << "\n");
return false;
// And finally, do it! Start by factoring the predecessors if needed.
std::vector<DominatorTree::UpdateType> Updates;
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
Updates.push_back({DominatorTree::Delete, PredBB, BB});
// Okay, we decided to do this! Clone all the instructions in BB onto the end
// of PredBB.
LLVM_DEBUG(dbgs() << " Duplicating block '" << BB->getName()
<< "' into end of '" << PredBB->getName()
<< "' to eliminate branch on phi. Cost: "
<< DuplicationCost << " block is:" << *BB << "\n");
// Unless PredBB ends with an unconditional branch, split the edge so that we
// can just clone the bits from BB into the end of the new PredBB.
BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
BasicBlock *OldPredBB = PredBB;
PredBB = SplitEdge(OldPredBB, BB);
Updates.push_back({DominatorTree::Insert, OldPredBB, PredBB});
Updates.push_back({DominatorTree::Insert, PredBB, BB});
Updates.push_back({DominatorTree::Delete, OldPredBB, BB});
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
// We are going to have to map operands from the original BB block into the
// PredBB block. Evaluate PHI nodes in BB.
DenseMap<Instruction*, Value*> ValueMapping;
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
// Clone the non-phi instructions of BB into PredBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; BI != BB->end(); ++BI) {
Instruction *New = BI->clone();
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
// If this instruction can be simplified after the operands are updated,
// just use the simplified value instead. This frequently happens due to
// phi translation.
if (Value *IV = SimplifyInstruction(
{BB->getModule()->getDataLayout(), TLI, nullptr, nullptr, New})) {
ValueMapping[&*BI] = IV;
if (!New->mayHaveSideEffects()) {
New = nullptr;
} else {
ValueMapping[&*BI] = New;
if (New) {
// Otherwise, insert the new instruction into the block.
PredBB->getInstList().insert(OldPredBranch->getIterator(), New);
// Update Dominance from simplified New instruction operands.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (BasicBlock *SuccBB = dyn_cast<BasicBlock>(New->getOperand(i)))
Updates.push_back({DominatorTree::Insert, PredBB, SuccBB});
// Check to see if the targets of the branch had PHI nodes. If so, we need to
// add entries to the PHI nodes for branch from PredBB now.
BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
// If there were values defined in BB that are used outside the block, then we
// now have to update all uses of the value to use either the original value,
// the cloned value, or some PHI derived value. This can require arbitrary
// PHI insertion, of which we are prepared to do, clean these up now.
SSAUpdater SSAUpdate;
SmallVector<Use*, 16> UsesToRename;
for (Instruction &I : *BB) {
// Scan all uses of this instruction to see if it is used outside of its
// block, and if so, record them in UsesToRename.
for (Use &U : I.uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(U) == BB)
} else if (User->getParent() == BB)
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
// We found a use of I outside of BB. Rename all uses of I that are outside
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
// with the two values we know.
SSAUpdate.Initialize(I.getType(), I.getName());
SSAUpdate.AddAvailableValue(BB, &I);
SSAUpdate.AddAvailableValue(PredBB, ValueMapping[&I]);
while (!UsesToRename.empty())
LLVM_DEBUG(dbgs() << "\n");
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
// that we nuked.
BB->removePredecessor(PredBB, true);
// Remove the unconditional branch at the end of the PredBB block.
return true;
// Pred is a predecessor of BB with an unconditional branch to BB. SI is
// a Select instruction in Pred. BB has other predecessors and SI is used in
// a PHI node in BB. SI has no other use.
// A new basic block, NewBB, is created and SI is converted to compare and
// conditional branch. SI is erased from parent.
void JumpThreadingPass::UnfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
SelectInst *SI, PHINode *SIUse,
unsigned Idx) {
// Expand the select.
// Pred --
// | v
// | NewBB
// | |
// |-----
// v
// BB
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
BB->getParent(), BB);
// Move the unconditional branch to NewBB.
NewBB->getInstList().insert(NewBB->end(), PredTerm);
// Create a conditional branch and update PHI nodes.
BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
SIUse->setIncomingValue(Idx, SI->getFalseValue());
SIUse->addIncoming(SI->getTrueValue(), NewBB);
// The select is now dead.
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, BB},
{DominatorTree::Insert, Pred, NewBB}});
// Update any other PHI nodes in BB.
for (BasicBlock::iterator BI = BB->begin();
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
if (Phi != SIUse)
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
bool JumpThreadingPass::TryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) {
PHINode *CondPHI = dyn_cast<PHINode>(SI->getCondition());
if (!CondPHI || CondPHI->getParent() != BB)
return false;
for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondPHI->getIncomingBlock(I);
SelectInst *PredSI = dyn_cast<SelectInst>(CondPHI->getIncomingValue(I));
// The second and third condition can be potentially relaxed. Currently
// the conditions help to simplify the code and allow us to reuse existing
// code, developed for TryToUnfoldSelect(CmpInst *, BasicBlock *)
if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse())
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
UnfoldSelectInstr(Pred, BB, PredSI, CondPHI, I);
return true;
return false;
/// TryToUnfoldSelect - Look for blocks of the form
/// bb1:
/// %a = select
/// br bb2
/// bb2:
/// %p = phi [%a, %bb1] ...
/// %c = icmp %p
/// br i1 %c
/// And expand the select into a branch structure if one of its arms allows %c
/// to be folded. This later enables threading from bb1 over bb2.
bool JumpThreadingPass::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
if (!CondBr || !CondBr->isConditional() || !CondLHS ||