blob: 4f1350e4ebb9c619c45f7b4fafa66e8fbb50eb27 [file] [log] [blame]
//===- 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/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.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/Analysis/ValueTracking.h"
#include "llvm/IR/CFG.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/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.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) {
// TODO: We don't do TRE if dynamic allocas are used.
// Dynamic allocas allocate stack space which should be
// deallocated before new iteration started. That is
// currently not implemented.
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: {
auto &CB = cast<CallBase>(*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 (CB.isArgOperand(U) && CB.isByValArgument(CB.getArgOperandNo(U)))
bool IsNocapture =
CB.isDataOperand(U) && CB.doesNotCapture(CB.getDataOperandNo(U));
callUsesLocalStack(CB, 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(CallBase &CB, 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 (!CB.onlyReadsMemory())
SmallPtrSet<Instruction *, 32> AllocaUsers;
SmallPtrSet<Instruction *, 32> EscapePoints;
static bool markTails(Function &F, OptimizationRemarkEmitter *ORE) {
if (F.callsFunctionThatReturnsTwice())
return false;
// 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);
// A PseudoProbeInst has the IntrInaccessibleMemOnly tag hence it is
// considered accessing memory and will be marked as a tail call if we
// don't bail out here.
if (!CI || CI->isTailCall() || isa<DbgInfoIntrinsic>(&I) ||
// Special-case operand bundles "clang.arc.attachedcall", "ptrauth", and
// "kcfi".
bool IsNoTail = CI->isNoTailCall() ||
LLVMContext::OB_ptrauth, LLVMContext::OB_kcfi});
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->args()) {
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))
for (auto *SuccBB : successors(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;
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) {
if (isa<DbgInfoIntrinsic>(I))
return true;
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
if (II->getIntrinsicID() == Intrinsic::lifetime_end &&
return true;
// 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(),
L->getAlign(), 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);
static bool canTransformAccumulatorRecursion(Instruction *I, CallInst *CI) {
if (!I->isAssociative() || !I->isCommutative())
return false;
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 false;
// The only user of this instruction we allow is a single return instruction.
if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
return false;
return true;
static Instruction *firstNonDbg(BasicBlock::iterator I) {
while (isa<DbgInfoIntrinsic>(I))
return &*I;
namespace {
class TailRecursionEliminator {
Function &F;
const TargetTransformInfo *TTI;
AliasAnalysis *AA;
OptimizationRemarkEmitter *ORE;
DomTreeUpdater &DTU;
// The below are shared state we want to have available when eliminating any
// calls in the function. There values should be populated by
// createTailRecurseLoopHeader the first time we find a call we can eliminate.
BasicBlock *HeaderBB = nullptr;
SmallVector<PHINode *, 8> ArgumentPHIs;
// PHI node to store our return value.
PHINode *RetPN = nullptr;
// i1 PHI node to track if we have a valid return value stored in RetPN.
PHINode *RetKnownPN = nullptr;
// Vector of select instructions we insereted. These selects use RetKnownPN
// to either propagate RetPN or select a new return value.
SmallVector<SelectInst *, 8> RetSelects;
// The below are shared state needed when performing accumulator recursion.
// There values should be populated by insertAccumulator the first time we
// find an elimination that requires an accumulator.
// PHI node to store our current accumulated value.
PHINode *AccPN = nullptr;
// The instruction doing the accumulating.
Instruction *AccumulatorRecursionInstr = nullptr;
TailRecursionEliminator(Function &F, const TargetTransformInfo *TTI,
AliasAnalysis *AA, OptimizationRemarkEmitter *ORE,
DomTreeUpdater &DTU)
CallInst *findTRECandidate(BasicBlock *BB);
void createTailRecurseLoopHeader(CallInst *CI);
void insertAccumulator(Instruction *AccRecInstr);
bool eliminateCall(CallInst *CI);
void cleanupAndFinalize();
bool processBlock(BasicBlock &BB);
void copyByValueOperandIntoLocalTemp(CallInst *CI, int OpndIdx);
void copyLocalTempOfByValueOperandIntoArguments(CallInst *CI, int OpndIdx);
static bool eliminate(Function &F, const TargetTransformInfo *TTI,
AliasAnalysis *AA, OptimizationRemarkEmitter *ORE,
DomTreeUpdater &DTU);
} // namespace
CallInst *TailRecursionEliminator::findTRECandidate(BasicBlock *BB) {
Instruction *TI = BB->getTerminator();
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.
assert((!CI->isTailCall() || !CI->isNoTailCall()) &&
"Incompatible call site attributes(Tail,NoTail)");
if (!CI->isTailCall())
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.
auto I = CI->arg_begin(), E = 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;
void TailRecursionEliminator::createTailRecurseLoopHeader(CallInst *CI) {
HeaderBB = &F.getEntryBlock();
BasicBlock *NewEntry = BasicBlock::Create(F.getContext(), "", &F, HeaderBB);
BranchInst *BI = BranchInst::Create(HeaderBB, NewEntry);
// Move all fixed sized allocas from HeaderBB to NewEntry.
for (BasicBlock::iterator OEBI = HeaderBB->begin(), E = HeaderBB->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 = &HeaderBB->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);
// If the function doen't return void, create the RetPN and RetKnownPN PHI
// nodes to track our return value. We initialize RetPN with poison and
// RetKnownPN with false since we can't know our return value at function
// entry.
Type *RetType = F.getReturnType();
if (!RetType->isVoidTy()) {
Type *BoolType = Type::getInt1Ty(F.getContext());
RetPN = PHINode::Create(RetType, 2, "", InsertPos);
RetKnownPN = PHINode::Create(BoolType, 2, "", InsertPos);
RetPN->addIncoming(PoisonValue::get(RetType), NewEntry);
RetKnownPN->addIncoming(ConstantInt::getFalse(BoolType), NewEntry);
// The entry block was changed from HeaderBB to NewEntry.
// The forward DominatorTree needs to be recalculated when the EntryBB is
// changed. In this corner-case we recalculate the entire tree.
void TailRecursionEliminator::insertAccumulator(Instruction *AccRecInstr) {
assert(!AccPN && "Trying to insert multiple accumulators");
AccumulatorRecursionInstr = AccRecInstr;
// Start by inserting a new PHI node for the accumulator.
pred_iterator PB = pred_begin(HeaderBB), PE = pred_end(HeaderBB);
AccPN = PHINode::Create(F.getReturnType(), std::distance(PB, PE) + 1,
"", &HeaderBB->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 identity constant for
// the accumulation operation. 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 HeaderBB yet,
// it will not show up as a predecessor.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
if (P == &F.getEntryBlock()) {
Constant *Identity = ConstantExpr::getBinOpIdentity(
AccRecInstr->getOpcode(), AccRecInstr->getType());
AccPN->addIncoming(Identity, P);
} else {
AccPN->addIncoming(AccPN, P);
// Creates a copy of contents of ByValue operand of the specified
// call instruction into the newly created temporarily variable.
void TailRecursionEliminator::copyByValueOperandIntoLocalTemp(CallInst *CI,
int OpndIdx) {
Type *AggTy = CI->getParamByValType(OpndIdx);
const DataLayout &DL = F.getParent()->getDataLayout();
// Get alignment of byVal operand.
Align Alignment(CI->getParamAlign(OpndIdx).valueOrOne());
// Create alloca for temporarily byval operands.
// Put alloca into the entry block.
Value *NewAlloca = new AllocaInst(
AggTy, DL.getAllocaAddrSpace(), nullptr, Alignment,
CI->getArgOperand(OpndIdx)->getName(), &*F.getEntryBlock().begin());
IRBuilder<> Builder(CI);
Value *Size = Builder.getInt64(DL.getTypeAllocSize(AggTy));
// Copy data from byvalue operand into the temporarily variable.
Builder.CreateMemCpy(NewAlloca, /*DstAlign*/ Alignment,
/*SrcAlign*/ Alignment, Size);
CI->setArgOperand(OpndIdx, NewAlloca);
// Creates a copy from temporarily variable(keeping value of ByVal argument)
// into the corresponding function argument location.
void TailRecursionEliminator::copyLocalTempOfByValueOperandIntoArguments(
CallInst *CI, int OpndIdx) {
Type *AggTy = CI->getParamByValType(OpndIdx);
const DataLayout &DL = F.getParent()->getDataLayout();
// Get alignment of byVal operand.
Align Alignment(CI->getParamAlign(OpndIdx).valueOrOne());
IRBuilder<> Builder(CI);
Value *Size = Builder.getInt64(DL.getTypeAllocSize(AggTy));
// Copy data from the temporarily variable into corresponding
// function argument location.
Builder.CreateMemCpy(F.getArg(OpndIdx), /*DstAlign*/ Alignment,
/*SrcAlign*/ Alignment, Size);
bool TailRecursionEliminator::eliminateCall(CallInst *CI) {
ReturnInst *Ret = cast<ReturnInst>(CI->getParent()->getTerminator());
// 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.
Instruction *AccRecInstr = nullptr;
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 which instruction accumulates for later.
if (AccPN || !canTransformAccumulatorRecursion(&*BBI, CI))
return false; // We cannot eliminate the tail recursion!
// Yes, this is accumulator recursion. Remember which instruction
// accumulates.
AccRecInstr = &*BBI;
BasicBlock *BB = Ret->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 (!HeaderBB)
// Copy values of ByVal operands into local temporarily variables.
for (unsigned I = 0, E = CI->arg_size(); I != E; ++I) {
if (CI->isByValArgument(I))
copyByValueOperandIntoLocalTemp(CI, I);
// 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->arg_size(); I != E; ++I) {
if (CI->isByValArgument(I)) {
copyLocalTempOfByValueOperandIntoArguments(CI, I);
ArgumentPHIs[I]->addIncoming(F.getArg(I), BB);
} else
ArgumentPHIs[I]->addIncoming(CI->getArgOperand(I), BB);
if (AccRecInstr) {
// 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);
// Update our return value tracking
if (RetPN) {
if (Ret->getReturnValue() == CI || AccRecInstr) {
// Defer selecting a return value
RetPN->addIncoming(RetPN, BB);
RetKnownPN->addIncoming(RetKnownPN, BB);
} else {
// We found a return value we want to use, insert a select instruction to
// select it if we don't already know what our return value will be and
// store the result in our return value PHI node.
SelectInst *SI = SelectInst::Create(
RetKnownPN, RetPN, Ret->getReturnValue(), "", Ret);
RetPN->addIncoming(SI, BB);
RetKnownPN->addIncoming(ConstantInt::getTrue(RetKnownPN->getType()), BB);
if (AccPN)
AccPN->addIncoming(AccRecInstr ? AccRecInstr : AccPN, BB);
// 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(HeaderBB, Ret);
Ret->eraseFromParent(); // Remove return.
CI->eraseFromParent(); // Remove call.
DTU.applyUpdates({{DominatorTree::Insert, BB, HeaderBB}});
return true;
void TailRecursionEliminator::cleanupAndFinalize() {
// 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())) {
if (RetPN) {
if (RetSelects.empty()) {
// If we didn't insert any select instructions, then we know we didn't
// store a return value and we can remove the PHI nodes we inserted.
if (AccPN) {
// We need to insert a copy of our accumulator instruction before any
// return in the function, and return its result instead.
Instruction *AccRecInstr = AccumulatorRecursionInstr;
for (BasicBlock &BB : F) {
ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator());
if (!RI)
Instruction *AccRecInstrNew = AccRecInstr->clone();
AccRecInstrNew->setOperand(AccRecInstr->getOperand(0) == AccPN,
RI->setOperand(0, AccRecInstrNew);
} else {
// We need to insert a select instruction before any return left in the
// function to select our stored return value if we have one.
for (BasicBlock &BB : F) {
ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator());
if (!RI)
SelectInst *SI = SelectInst::Create(
RetKnownPN, RetPN, RI->getOperand(0), "", RI);
RI->setOperand(0, SI);
if (AccPN) {
// We need to insert a copy of our accumulator instruction before any
// of the selects we inserted, and select its result instead.
Instruction *AccRecInstr = AccumulatorRecursionInstr;
for (SelectInst *SI : RetSelects) {
Instruction *AccRecInstrNew = AccRecInstr->clone();
AccRecInstrNew->setOperand(AccRecInstr->getOperand(0) == AccPN,
bool TailRecursionEliminator::processBlock(BasicBlock &BB) {
Instruction *TI = BB.getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isConditional())
return false;
BasicBlock *Succ = BI->getSuccessor(0);
ReturnInst *Ret = dyn_cast<ReturnInst>(Succ->getFirstNonPHIOrDbg(true));
if (!Ret)
return false;
CallInst *CI = findTRECandidate(&BB);
if (!CI)
return false;
LLVM_DEBUG(dbgs() << "FOLDING: " << *Succ
FoldReturnIntoUncondBranch(Ret, Succ, &BB, &DTU);
// If all predecessors of Succ 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
// eliminateCall will attempt to remove. This block can only contain
// instructions that can't have uses, therefore it is safe to remove.
if (pred_empty(Succ))
return true;
} else if (isa<ReturnInst>(TI)) {
CallInst *CI = findTRECandidate(&BB);
if (CI)
return eliminateCall(CI);
return false;
bool TailRecursionEliminator::eliminate(Function &F,
const TargetTransformInfo *TTI,
AliasAnalysis *AA,
OptimizationRemarkEmitter *ORE,
DomTreeUpdater &DTU) {
if (F.getFnAttribute("disable-tail-calls").getValueAsBool())
return false;
bool MadeChange = false;
MadeChange |= markTails(F, ORE);
// 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 MadeChange;
if (!canTRE(F))
return MadeChange;
// Change any tail recursive calls to loops.
TailRecursionEliminator TRE(F, TTI, AA, ORE, DTU);
for (BasicBlock &BB : F)
MadeChange |= TRE.processBlock(BB);
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 TailRecursionEliminator::eliminate(
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 = TailRecursionEliminator::eliminate(F, &TTI, &AA, &ORE, DTU);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;