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//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
// 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 transforms calls of the current function (self recursion) followed
// by a return instruction with a branch to the entry of the function, creating
// a loop. This pass also implements the following extensions to the basic
// algorithm:
// 1. Trivial instructions between the call and return do not prevent the
// transformation from taking place, though currently the analysis cannot
// support moving any really useful instructions (only dead ones).
// 2. This pass transforms functions that are prevented from being tail
// recursive by an associative and commutative expression to use an
// accumulator variable, thus compiling the typical naive factorial or
// 'fib' implementation into efficient code.
// 3. TRE is performed if the function returns void, if the return
// returns the result returned by the call, or if the function returns a
// run-time constant on all exits from the function. It is possible, though
// unlikely, that the return returns something else (like constant 0), and
// can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
// the function return the exact same value.
// 4. If it can prove that callees do not access their caller stack frame,
// they are marked as eligible for tail call elimination (by the code
// generator).
// There are several improvements that could be made:
// 1. If the function has any alloca instructions, these instructions will be
// moved out of the entry block of the function, causing them to be
// evaluated each time through the tail recursion. Safely keeping allocas
// in the entry block requires analysis to proves that the tail-called
// function does not read or write the stack object.
// 2. Tail recursion is only performed if the call immediately precedes the
// return instruction. It's possible that there could be a jump between
// the call and the return.
// 3. There can be intervening operations between the call and the return that
// prevent the TRE from occurring. For example, there could be GEP's and
// stores to memory that will not be read or written by the call. This
// requires some substantial analysis (such as with DSA) to prove safe to
// move ahead of the call, but doing so could allow many more TREs to be
// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
// 4. The algorithm we use to detect if callees access their caller stack
// frames is very primitive.
#include "llvm/Transforms/Scalar/TailRecursionElimination.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
#define DEBUG_TYPE "tailcallelim"
STATISTIC(NumEliminated, "Number of tail calls removed");
STATISTIC(NumRetDuped, "Number of return duplicated");
STATISTIC(NumAccumAdded, "Number of accumulators introduced");
/// Scan the specified function for alloca instructions.
/// If it contains any dynamic allocas, returns false.
static bool canTRE(Function &F) {
// Because of PR962, we don't TRE dynamic allocas.
return llvm::all_of(instructions(F), [](Instruction &I) {
auto *AI = dyn_cast<AllocaInst>(&I);
return !AI || AI->isStaticAlloca();
namespace {
struct AllocaDerivedValueTracker {
// Start at a root value and walk its use-def chain to mark calls that use the
// value or a derived value in AllocaUsers, and places where it may escape in
// EscapePoints.
void walk(Value *Root) {
SmallVector<Use *, 32> Worklist;
SmallPtrSet<Use *, 32> Visited;
auto AddUsesToWorklist = [&](Value *V) {
for (auto &U : V->uses()) {
if (!Visited.insert(&U).second)
while (!Worklist.empty()) {
Use *U = Worklist.pop_back_val();
Instruction *I = cast<Instruction>(U->getUser());
switch (I->getOpcode()) {
case Instruction::Call:
case Instruction::Invoke: {
CallSite CS(I);
// If the alloca-derived argument is passed byval it is not an escape
// point, or a use of an alloca. Calling with byval copies the contents
// of the alloca into argument registers or stack slots, which exist
// beyond the lifetime of the current frame.
if (CS.isArgOperand(U) && CS.isByValArgument(CS.getArgumentNo(U)))
bool IsNocapture =
CS.isDataOperand(U) && CS.doesNotCapture(CS.getDataOperandNo(U));
callUsesLocalStack(CS, IsNocapture);
if (IsNocapture) {
// If the alloca-derived argument is passed in as nocapture, then it
// can't propagate to the call's return. That would be capturing.
case Instruction::Load: {
// The result of a load is not alloca-derived (unless an alloca has
// otherwise escaped, but this is a local analysis).
case Instruction::Store: {
if (U->getOperandNo() == 0)
continue; // Stores have no users to analyze.
case Instruction::BitCast:
case Instruction::GetElementPtr:
case Instruction::PHI:
case Instruction::Select:
case Instruction::AddrSpaceCast:
void callUsesLocalStack(CallSite CS, bool IsNocapture) {
// Add it to the list of alloca users.
// If it's nocapture then it can't capture this alloca.
if (IsNocapture)
// If it can write to memory, it can leak the alloca value.
if (!CS.onlyReadsMemory())
SmallPtrSet<Instruction *, 32> AllocaUsers;
SmallPtrSet<Instruction *, 32> EscapePoints;
static bool markTails(Function &F, bool &AllCallsAreTailCalls,
OptimizationRemarkEmitter *ORE) {
if (F.callsFunctionThatReturnsTwice())
return false;
AllCallsAreTailCalls = true;
// The local stack holds all alloca instructions and all byval arguments.
AllocaDerivedValueTracker Tracker;
for (Argument &Arg : F.args()) {
if (Arg.hasByValAttr())
for (auto &BB : F) {
for (auto &I : BB)
if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
bool Modified = false;
// Track whether a block is reachable after an alloca has escaped. Blocks that
// contain the escaping instruction will be marked as being visited without an
// escaped alloca, since that is how the block began.
enum VisitType {
DenseMap<BasicBlock *, VisitType> Visited;
// We propagate the fact that an alloca has escaped from block to successor.
// Visit the blocks that are propagating the escapedness first. To do this, we
// maintain two worklists.
SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
// We may enter a block and visit it thinking that no alloca has escaped yet,
// then see an escape point and go back around a loop edge and come back to
// the same block twice. Because of this, we defer setting tail on calls when
// we first encounter them in a block. Every entry in this list does not
// statically use an alloca via use-def chain analysis, but may find an alloca
// through other means if the block turns out to be reachable after an escape
// point.
SmallVector<CallInst *, 32> DeferredTails;
BasicBlock *BB = &F.getEntryBlock();
VisitType Escaped = UNESCAPED;
do {
for (auto &I : *BB) {
if (Tracker.EscapePoints.count(&I))
Escaped = ESCAPED;
CallInst *CI = dyn_cast<CallInst>(&I);
if (!CI || CI->isTailCall() || isa<DbgInfoIntrinsic>(&I))
bool IsNoTail = CI->isNoTailCall() || CI->hasOperandBundles();
if (!IsNoTail && CI->doesNotAccessMemory()) {
// A call to a readnone function whose arguments are all things computed
// outside this function can be marked tail. Even if you stored the
// alloca address into a global, a readnone function can't load the
// global anyhow.
// Note that this runs whether we know an alloca has escaped or not. If
// it has, then we can't trust Tracker.AllocaUsers to be accurate.
bool SafeToTail = true;
for (auto &Arg : CI->arg_operands()) {
if (isa<Constant>(Arg.getUser()))
if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
if (!A->hasByValAttr())
SafeToTail = false;
if (SafeToTail) {
using namespace ore;
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "tailcall-readnone", CI)
<< "marked as tail call candidate (readnone)";
Modified = true;
if (!IsNoTail && Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
} else {
AllCallsAreTailCalls = false;
for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
auto &State = Visited[SuccBB];
if (State < Escaped) {
State = Escaped;
if (State == ESCAPED)
if (!WorklistEscaped.empty()) {
BB = WorklistEscaped.pop_back_val();
Escaped = ESCAPED;
} else {
BB = nullptr;
while (!WorklistUnescaped.empty()) {
auto *NextBB = WorklistUnescaped.pop_back_val();
if (Visited[NextBB] == UNESCAPED) {
BB = NextBB;
Escaped = UNESCAPED;
} while (BB);
for (CallInst *CI : DeferredTails) {
if (Visited[CI->getParent()] != ESCAPED) {
// If the escape point was part way through the block, calls after the
// escape point wouldn't have been put into DeferredTails.
LLVM_DEBUG(dbgs() << "Marked as tail call candidate: " << *CI << "\n");
Modified = true;
} else {
AllCallsAreTailCalls = false;
return Modified;
/// Return true if it is safe to move the specified
/// instruction from after the call to before the call, assuming that all
/// instructions between the call and this instruction are movable.
static bool canMoveAboveCall(Instruction *I, CallInst *CI, AliasAnalysis *AA) {
// FIXME: We can move load/store/call/free instructions above the call if the
// call does not mod/ref the memory location being processed.
if (I->mayHaveSideEffects()) // This also handles volatile loads.
return false;
if (LoadInst *L = dyn_cast<LoadInst>(I)) {
// Loads may always be moved above calls without side effects.
if (CI->mayHaveSideEffects()) {
// Non-volatile loads may be moved above a call with side effects if it
// does not write to memory and the load provably won't trap.
// Writes to memory only matter if they may alias the pointer
// being loaded from.
const DataLayout &DL = L->getModule()->getDataLayout();
if (isModSet(AA->getModRefInfo(CI, MemoryLocation::get(L))) ||
!isSafeToLoadUnconditionally(L->getPointerOperand(), L->getType(),
MaybeAlign(L->getAlignment()), DL, L))
return false;
// Otherwise, if this is a side-effect free instruction, check to make sure
// that it does not use the return value of the call. If it doesn't use the
// return value of the call, it must only use things that are defined before
// the call, or movable instructions between the call and the instruction
// itself.
return !is_contained(I->operands(), CI);
/// Return true if the specified value is the same when the return would exit
/// as it was when the initial iteration of the recursive function was executed.
/// We currently handle static constants and arguments that are not modified as
/// part of the recursion.
static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
if (isa<Constant>(V)) return true; // Static constants are always dyn consts
// Check to see if this is an immutable argument, if so, the value
// will be available to initialize the accumulator.
if (Argument *Arg = dyn_cast<Argument>(V)) {
// Figure out which argument number this is...
unsigned ArgNo = 0;
Function *F = CI->getParent()->getParent();
for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
// If we are passing this argument into call as the corresponding
// argument operand, then the argument is dynamically constant.
// Otherwise, we cannot transform this function safely.
if (CI->getArgOperand(ArgNo) == Arg)
return true;
// Switch cases are always constant integers. If the value is being switched
// on and the return is only reachable from one of its cases, it's
// effectively constant.
if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
if (SI->getCondition() == V)
return SI->getDefaultDest() != RI->getParent();
// Not a constant or immutable argument, we can't safely transform.
return false;
/// Check to see if the function containing the specified tail call consistently
/// returns the same runtime-constant value at all exit points except for
/// IgnoreRI. If so, return the returned value.
static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
Function *F = CI->getParent()->getParent();
Value *ReturnedValue = nullptr;
for (BasicBlock &BBI : *F) {
ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator());
if (RI == nullptr || RI == IgnoreRI) continue;
// We can only perform this transformation if the value returned is
// evaluatable at the start of the initial invocation of the function,
// instead of at the end of the evaluation.
Value *RetOp = RI->getOperand(0);
if (!isDynamicConstant(RetOp, CI, RI))
return nullptr;
if (ReturnedValue && RetOp != ReturnedValue)
return nullptr; // Cannot transform if differing values are returned.
ReturnedValue = RetOp;
return ReturnedValue;
/// If the specified instruction can be transformed using accumulator recursion
/// elimination, return the constant which is the start of the accumulator
/// value. Otherwise return null.
static Value *canTransformAccumulatorRecursion(Instruction *I, CallInst *CI) {
if (!I->isAssociative() || !I->isCommutative()) return nullptr;
assert(I->getNumOperands() == 2 &&
"Associative/commutative operations should have 2 args!");
// Exactly one operand should be the result of the call instruction.
if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
(I->getOperand(0) != CI && I->getOperand(1) != CI))
return nullptr;
// The only user of this instruction we allow is a single return instruction.
if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
return nullptr;
// Ok, now we have to check all of the other return instructions in this
// function. If they return non-constants or differing values, then we cannot
// transform the function safely.
return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
static Instruction *firstNonDbg(BasicBlock::iterator I) {
while (isa<DbgInfoIntrinsic>(I))
return &*I;
static CallInst *findTRECandidate(Instruction *TI,
bool CannotTailCallElimCallsMarkedTail,
const TargetTransformInfo *TTI) {
BasicBlock *BB = TI->getParent();
Function *F = BB->getParent();
if (&BB->front() == TI) // Make sure there is something before the terminator.
return nullptr;
// Scan backwards from the return, checking to see if there is a tail call in
// this block. If so, set CI to it.
CallInst *CI = nullptr;
BasicBlock::iterator BBI(TI);
while (true) {
CI = dyn_cast<CallInst>(BBI);
if (CI && CI->getCalledFunction() == F)
if (BBI == BB->begin())
return nullptr; // Didn't find a potential tail call.
// If this call is marked as a tail call, and if there are dynamic allocas in
// the function, we cannot perform this optimization.
if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
return nullptr;
// As a special case, detect code like this:
// double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
// and disable this xform in this case, because the code generator will
// lower the call to fabs into inline code.
if (BB == &F->getEntryBlock() &&
firstNonDbg(BB->front().getIterator()) == CI &&
firstNonDbg(std::next(BB->begin())) == TI && CI->getCalledFunction() &&
!TTI->isLoweredToCall(CI->getCalledFunction())) {
// A single-block function with just a call and a return. Check that
// the arguments match.
CallSite::arg_iterator I = CallSite(CI).arg_begin(),
E = CallSite(CI).arg_end();
Function::arg_iterator FI = F->arg_begin(),
FE = F->arg_end();
for (; I != E && FI != FE; ++I, ++FI)
if (*I != &*FI) break;
if (I == E && FI == FE)
return nullptr;
return CI;
static bool eliminateRecursiveTailCall(
CallInst *CI, ReturnInst *Ret, BasicBlock *&OldEntry,
bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs,
AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
// If we are introducing accumulator recursion to eliminate operations after
// the call instruction that are both associative and commutative, the initial
// value for the accumulator is placed in this variable. If this value is set
// then we actually perform accumulator recursion elimination instead of
// simple tail recursion elimination. If the operation is an LLVM instruction
// (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
// we are handling the case when the return instruction returns a constant C
// which is different to the constant returned by other return instructions
// (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
// special case of accumulator recursion, the operation being "return C".
Value *AccumulatorRecursionEliminationInitVal = nullptr;
Instruction *AccumulatorRecursionInstr = nullptr;
// Ok, we found a potential tail call. We can currently only transform the
// tail call if all of the instructions between the call and the return are
// movable to above the call itself, leaving the call next to the return.
// Check that this is the case now.
BasicBlock::iterator BBI(CI);
for (++BBI; &*BBI != Ret; ++BBI) {
if (canMoveAboveCall(&*BBI, CI, AA))
// If we can't move the instruction above the call, it might be because it
// is an associative and commutative operation that could be transformed
// using accumulator recursion elimination. Check to see if this is the
// case, and if so, remember the initial accumulator value for later.
if ((AccumulatorRecursionEliminationInitVal =
canTransformAccumulatorRecursion(&*BBI, CI))) {
// Yes, this is accumulator recursion. Remember which instruction
// accumulates.
AccumulatorRecursionInstr = &*BBI;
} else {
return false; // Otherwise, we cannot eliminate the tail recursion!
// We can only transform call/return pairs that either ignore the return value
// of the call and return void, ignore the value of the call and return a
// constant, return the value returned by the tail call, or that are being
// accumulator recursion variable eliminated.
if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
!isa<UndefValue>(Ret->getReturnValue()) &&
AccumulatorRecursionEliminationInitVal == nullptr &&
!getCommonReturnValue(nullptr, CI)) {
// One case remains that we are able to handle: the current return
// instruction returns a constant, and all other return instructions
// return a different constant.
if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
return false; // Current return instruction does not return a constant.
// Check that all other return instructions return a common constant. If
// so, record it in AccumulatorRecursionEliminationInitVal.
AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
if (!AccumulatorRecursionEliminationInitVal)
return false;
BasicBlock *BB = Ret->getParent();
Function *F = BB->getParent();
using namespace ore;
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "tailcall-recursion", CI)
<< "transforming tail recursion into loop";
// OK! We can transform this tail call. If this is the first one found,
// create the new entry block, allowing us to branch back to the old entry.
if (!OldEntry) {
OldEntry = &F->getEntryBlock();
BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
BranchInst *BI = BranchInst::Create(OldEntry, NewEntry);
// If this tail call is marked 'tail' and if there are any allocas in the
// entry block, move them up to the new entry block.
TailCallsAreMarkedTail = CI->isTailCall();
if (TailCallsAreMarkedTail)
// Move all fixed sized allocas from OldEntry to NewEntry.
for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
NEBI = NewEntry->begin(); OEBI != E; )
if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
if (isa<ConstantInt>(AI->getArraySize()))
// Now that we have created a new block, which jumps to the entry
// block, insert a PHI node for each argument of the function.
// For now, we initialize each PHI to only have the real arguments
// which are passed in.
Instruction *InsertPos = &OldEntry->front();
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
I != E; ++I) {
PHINode *PN = PHINode::Create(I->getType(), 2,
I->getName() + ".tr", InsertPos);
I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
PN->addIncoming(&*I, NewEntry);
// The entry block was changed from OldEntry to NewEntry.
// The forward DominatorTree needs to be recalculated when the EntryBB is
// changed. In this corner-case we recalculate the entire tree.
// If this function has self recursive calls in the tail position where some
// are marked tail and some are not, only transform one flavor or another. We
// have to choose whether we move allocas in the entry block to the new entry
// block or not, so we can't make a good choice for both. NOTE: We could do
// slightly better here in the case that the function has no entry block
// allocas.
if (TailCallsAreMarkedTail && !CI->isTailCall())
return false;
// Ok, now that we know we have a pseudo-entry block WITH all of the
// required PHI nodes, add entries into the PHI node for the actual
// parameters passed into the tail-recursive call.
for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
// If we are introducing an accumulator variable to eliminate the recursion,
// do so now. Note that we _know_ that no subsequent tail recursion
// eliminations will happen on this function because of the way the
// accumulator recursion predicate is set up.
if (AccumulatorRecursionEliminationInitVal) {
Instruction *AccRecInstr = AccumulatorRecursionInstr;
// Start by inserting a new PHI node for the accumulator.
pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
PHINode *AccPN = PHINode::Create(
std::distance(PB, PE) + 1, "", &OldEntry->front());
// Loop over all of the predecessors of the tail recursion block. For the
// real entry into the function we seed the PHI with the initial value,
// computed earlier. For any other existing branches to this block (due to
// other tail recursions eliminated) the accumulator is not modified.
// Because we haven't added the branch in the current block to OldEntry yet,
// it will not show up as a predecessor.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
if (P == &F->getEntryBlock())
AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
AccPN->addIncoming(AccPN, P);
if (AccRecInstr) {
// Add an incoming argument for the current block, which is computed by
// our associative and commutative accumulator instruction.
AccPN->addIncoming(AccRecInstr, BB);
// Next, rewrite the accumulator recursion instruction so that it does not
// use the result of the call anymore, instead, use the PHI node we just
// inserted.
AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
} else {
// Add an incoming argument for the current block, which is just the
// constant returned by the current return instruction.
AccPN->addIncoming(Ret->getReturnValue(), BB);
// Finally, rewrite any return instructions in the program to return the PHI
// node instead of the "initval" that they do currently. This loop will
// actually rewrite the return value we are destroying, but that's ok.
for (BasicBlock &BBI : *F)
if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator()))
RI->setOperand(0, AccPN);
// Now that all of the PHI nodes are in place, remove the call and
// ret instructions, replacing them with an unconditional branch.
BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
BB->getInstList().erase(Ret); // Remove return.
BB->getInstList().erase(CI); // Remove call.
DTU.applyUpdates({{DominatorTree::Insert, BB, OldEntry}});
return true;
static bool foldReturnAndProcessPred(
BasicBlock *BB, ReturnInst *Ret, BasicBlock *&OldEntry,
bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs,
bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI,
AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
bool Change = false;
// Make sure this block is a trivial return block.
assert(BB->getFirstNonPHIOrDbg() == Ret &&
"Trying to fold non-trivial return block");
// If the return block contains nothing but the return and PHI's,
// there might be an opportunity to duplicate the return in its
// predecessors and perform TRE there. Look for predecessors that end
// in unconditional branch and recursive call(s).
SmallVector<BranchInst*, 8> UncondBranchPreds;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *Pred = *PI;
Instruction *PTI = Pred->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
if (BI->isUnconditional())
while (!UncondBranchPreds.empty()) {
BranchInst *BI = UncondBranchPreds.pop_back_val();
BasicBlock *Pred = BI->getParent();
if (CallInst *CI = findTRECandidate(BI, CannotTailCallElimCallsMarkedTail, TTI)){
LLVM_DEBUG(dbgs() << "FOLDING: " << *BB
ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred, &DTU);
// Cleanup: if all predecessors of BB have been eliminated by
// FoldReturnIntoUncondBranch, delete it. It is important to empty it,
// because the ret instruction in there is still using a value which
// eliminateRecursiveTailCall will attempt to remove.
if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB))
eliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs, AA, ORE, DTU);
Change = true;
return Change;
static bool processReturningBlock(
ReturnInst *Ret, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail,
SmallVectorImpl<PHINode *> &ArgumentPHIs,
bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI,
AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
CallInst *CI = findTRECandidate(Ret, CannotTailCallElimCallsMarkedTail, TTI);
if (!CI)
return false;
return eliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs, AA, ORE, DTU);
static bool eliminateTailRecursion(Function &F, const TargetTransformInfo *TTI,
AliasAnalysis *AA,
OptimizationRemarkEmitter *ORE,
DomTreeUpdater &DTU) {
if (F.getFnAttribute("disable-tail-calls").getValueAsString() == "true")
return false;
bool MadeChange = false;
bool AllCallsAreTailCalls = false;
MadeChange |= markTails(F, AllCallsAreTailCalls, ORE);
if (!AllCallsAreTailCalls)
return MadeChange;
// If this function is a varargs function, we won't be able to PHI the args
// right, so don't even try to convert it...
if (F.getFunctionType()->isVarArg())
return false;
BasicBlock *OldEntry = nullptr;
bool TailCallsAreMarkedTail = false;
SmallVector<PHINode*, 8> ArgumentPHIs;
// If false, we cannot perform TRE on tail calls marked with the 'tail'
// attribute, because doing so would cause the stack size to increase (real
// TRE would deallocate variable sized allocas, TRE doesn't).
bool CanTRETailMarkedCall = canTRE(F);
// Change any tail recursive calls to loops.
// FIXME: The code generator produces really bad code when an 'escaping
// alloca' is changed from being a static alloca to being a dynamic alloca.
// Until this is resolved, disable this transformation if that would ever
// happen. This bug is PR962.
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; /*in loop*/) {
BasicBlock *BB = &*BBI++; // foldReturnAndProcessPred may delete BB.
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
bool Change = processReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs, !CanTRETailMarkedCall,
if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
Change = foldReturnAndProcessPred(
BB, Ret, OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
!CanTRETailMarkedCall, TTI, AA, ORE, DTU);
MadeChange |= Change;
// If we eliminated any tail recursions, it's possible that we inserted some
// silly PHI nodes which just merge an initial value (the incoming operand)
// with themselves. Check to see if we did and clean up our mess if so. This
// occurs when a function passes an argument straight through to its tail
// call.
for (PHINode *PN : ArgumentPHIs) {
// If the PHI Node is a dynamic constant, replace it with the value it is.
if (Value *PNV = SimplifyInstruction(PN, F.getParent()->getDataLayout())) {
return MadeChange;
namespace {
struct TailCallElim : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
TailCallElim() : FunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
// There is no noticable performance difference here between Lazy and Eager
// UpdateStrategy based on some test results. It is feasible to switch the
// UpdateStrategy to Lazy if we find it profitable later.
DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
return eliminateTailRecursion(
F, &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F),
&getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE(), DTU);
char TailCallElim::ID = 0;
INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", "Tail Call Elimination",
false, false)
INITIALIZE_PASS_END(TailCallElim, "tailcallelim", "Tail Call Elimination",
false, false)
// Public interface to the TailCallElimination pass
FunctionPass *llvm::createTailCallEliminationPass() {
return new TailCallElim();
PreservedAnalyses TailCallElimPass::run(Function &F,
FunctionAnalysisManager &AM) {
TargetTransformInfo &TTI = AM.getResult<TargetIRAnalysis>(F);
AliasAnalysis &AA = AM.getResult<AAManager>(F);
auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
auto *PDT = AM.getCachedResult<PostDominatorTreeAnalysis>(F);
// There is no noticable performance difference here between Lazy and Eager
// UpdateStrategy based on some test results. It is feasible to switch the
// UpdateStrategy to Lazy if we find it profitable later.
DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
bool Changed = eliminateTailRecursion(F, &TTI, &AA, &ORE, DTU);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;