//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "tailcallelim"
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Transforms/Utils/Local.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");
bool runOnFunction(Function &F) override;
private:
+ bool runTRE(Function &F);
+ bool markTails(Function &F, bool &AllCallsAreTailCalls);
+
CallInst *FindTRECandidate(Instruction *I,
bool CannotTailCallElimCallsMarkedTail);
bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
AU.addRequired<TargetTransformInfo>();
}
-/// CanTRE - Scan the specified basic block for alloca instructions.
-/// If it contains any that are variable-sized or not in the entry block,
-/// returns false.
-static bool CanTRE(AllocaInst *AI) {
- // Because of PR962, we don't TRE allocas outside the entry block.
-
- // If this alloca is in the body of the function, or if it is a variable
- // sized allocation, we cannot tail call eliminate calls marked 'tail'
- // with this mechanism.
- BasicBlock *BB = AI->getParent();
- return BB == &BB->getParent()->getEntryBlock() &&
- isa<ConstantInt>(AI->getArraySize());
+/// \brief 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.
+ for (auto &BB : F) {
+ for (auto &I : BB) {
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
+ if (!AI->isStaticAlloca())
+ return false;
+ }
+ }
+ }
+
+ return true;
}
-namespace {
-struct AllocaCaptureTracker : public CaptureTracker {
- AllocaCaptureTracker() : Captured(false) {}
+bool TailCallElim::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
- void tooManyUses() override { Captured = true; }
+ bool AllCallsAreTailCalls = false;
+ bool Modified = markTails(F, AllCallsAreTailCalls);
+ if (AllCallsAreTailCalls)
+ Modified |= runTRE(F);
+ return Modified;
+}
- bool shouldExplore(const Use *U) override {
- Value *V = U->getUser();
- if (isa<CallInst>(V) || isa<InvokeInst>(V))
- UsesAlloca.insert(V);
- return true;
+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))
+ continue;
+ Worklist.push_back(&U);
+ }
+ };
+
+ AddUsesToWorklist(Root);
+
+ 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);
+ bool IsNocapture = !CS.isCallee(U) &&
+ CS.doesNotCapture(CS.getArgumentNo(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.
+ continue;
+ }
+ break;
+ }
+ case Instruction::Load: {
+ // The result of a load is not alloca-derived (unless an alloca has
+ // otherwise escaped, but this is a local analysis).
+ continue;
+ }
+ case Instruction::Store: {
+ if (U->getOperandNo() == 0)
+ EscapePoints.insert(I);
+ continue; // Stores have no users to analyze.
+ }
+ case Instruction::BitCast:
+ case Instruction::GetElementPtr:
+ case Instruction::PHI:
+ case Instruction::Select:
+ case Instruction::AddrSpaceCast:
+ break;
+ default:
+ EscapePoints.insert(I);
+ break;
+ }
+
+ AddUsesToWorklist(I);
+ }
}
- bool captured(const Use *U) override {
- if (isa<ReturnInst>(U->getUser()))
- return false;
- Captured = true;
- return true;
+ void callUsesLocalStack(CallSite CS, bool IsNocapture) {
+ // Add it to the list of alloca users.
+ AllocaUsers.insert(CS.getInstruction());
+
+ // If it's nocapture then it can't capture this alloca.
+ if (IsNocapture)
+ return;
+
+ // If it can write to memory, it can leak the alloca value.
+ if (!CS.onlyReadsMemory())
+ EscapePoints.insert(CS.getInstruction());
}
- bool Captured;
- SmallPtrSet<const Value *, 16> UsesAlloca;
+ SmallPtrSet<Instruction *, 32> AllocaUsers;
+ SmallPtrSet<Instruction *, 32> EscapePoints;
};
-} // end anonymous namespace
+}
-bool TailCallElim::runOnFunction(Function &F) {
- if (skipOptnoneFunction(F))
+bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
+ 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())
+ Tracker.walk(&Arg);
+ }
+ for (auto &BB : F) {
+ for (auto &I : BB)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
+ Tracker.walk(AI);
+ }
+ 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 {
+ UNVISITED,
+ UNESCAPED,
+ ESCAPED
+ };
+ 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())
+ continue;
+
+ if (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()))
+ continue;
+ if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
+ if (!A->hasByValAttr())
+ continue;
+ SafeToTail = false;
+ break;
+ }
+ if (SafeToTail) {
+ emitOptimizationRemark(
+ F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
+ "marked this readnone call a tail call candidate");
+ CI->setTailCall();
+ Modified = true;
+ continue;
+ }
+ }
+
+ if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
+ DeferredTails.push_back(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)
+ WorklistEscaped.push_back(SuccBB);
+ else
+ WorklistUnescaped.push_back(SuccBB);
+ }
+ }
+
+ 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;
+ break;
+ }
+ }
+ }
+ } 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.
+ emitOptimizationRemark(F.getContext(), "tailcallelim", F,
+ CI->getDebugLoc(),
+ "marked this call a tail call candidate");
+ CI->setTailCall();
+ Modified = true;
+ } else {
+ AllCallsAreTailCalls = false;
+ }
+ }
+
+ return Modified;
+}
+
+bool TailCallElim::runTRE(Function &F) {
// 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;
TTI = &getAnalysis<TargetTransformInfo>();
- BasicBlock *OldEntry = 0;
+ BasicBlock *OldEntry = nullptr;
bool TailCallsAreMarkedTail = false;
SmallVector<PHINode*, 8> ArgumentPHIs;
bool MadeChange = false;
// 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 = true;
-
- // Find calls that can be marked tail.
- AllocaCaptureTracker ACT;
- for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
- for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
- if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
- CanTRETailMarkedCall &= CanTRE(AI);
- PointerMayBeCaptured(AI, &ACT);
- // If any allocas are captured, exit.
- if (ACT.Captured)
- return false;
- }
- }
- }
+ bool CanTRETailMarkedCall = CanTRE(F);
- // Second pass, change any tail recursive calls to loops.
+ // 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.
- if (ACT.UsesAlloca.empty()) {
- for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++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);
- MadeChange |= Change;
- }
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++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);
+ MadeChange |= Change;
}
}
// 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.
- if (!ArgumentPHIs.empty()) {
- for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
- PHINode *PN = ArgumentPHIs[i];
-
- // If the PHI Node is a dynamic constant, replace it with the value it is.
- if (Value *PNV = SimplifyInstruction(PN)) {
- PN->replaceAllUsesWith(PNV);
- PN->eraseFromParent();
- }
- }
- }
+ for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
+ PHINode *PN = ArgumentPHIs[i];
- // At this point, we know that the function does not have any captured
- // allocas. If additionally the function does not call setjmp, mark all calls
- // in the function that do not access stack memory with the tail keyword. This
- // implies ensuring that there does not exist any path from a call that takes
- // in an alloca but does not capture it and the call which we wish to mark
- // with "tail".
- if (!F.callsFunctionThatReturnsTwice()) {
- for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
- for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
- if (CallInst *CI = dyn_cast<CallInst>(I)) {
- if (!ACT.UsesAlloca.count(CI)) {
- CI->setTailCall();
- MadeChange = true;
- }
- }
- }
+ // If the PHI Node is a dynamic constant, replace it with the value it is.
+ if (Value *PNV = SimplifyInstruction(PN)) {
+ PN->replaceAllUsesWith(PNV);
+ PN->eraseFromParent();
}
}
//
static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
Function *F = CI->getParent()->getParent();
- Value *ReturnedValue = 0;
+ Value *ReturnedValue = nullptr;
for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
- if (RI == 0 || RI == IgnoreRI) continue;
+ 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,
//
Value *RetOp = RI->getOperand(0);
if (!isDynamicConstant(RetOp, CI, RI))
- return 0;
+ return nullptr;
if (ReturnedValue && RetOp != ReturnedValue)
- return 0; // Cannot transform if differing values are returned.
+ return nullptr; // Cannot transform if differing values are returned.
ReturnedValue = RetOp;
}
return ReturnedValue;
///
Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
CallInst *CI) {
- if (!I->isAssociative() || !I->isCommutative()) return 0;
+ 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 0;
+ return nullptr;
// The only user of this instruction we allow is a single return instruction.
if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
- return 0;
+ 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
Function *F = BB->getParent();
if (&BB->front() == TI) // Make sure there is something before the terminator.
- return 0;
+ 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 = 0;
+ CallInst *CI = nullptr;
BasicBlock::iterator BBI = TI;
while (true) {
CI = dyn_cast<CallInst>(BBI);
break;
if (BBI == BB->begin())
- return 0; // Didn't find a potential tail call.
+ return nullptr; // Didn't find a potential tail call.
--BBI;
}
// 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 0;
+ return nullptr;
// As a special case, detect code like this:
// double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
for (; I != E && FI != FE; ++I, ++FI)
if (*I != &*FI) break;
if (I == E && FI == FE)
- return 0;
+ return nullptr;
}
return CI;
// 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 = 0;
- Instruction *AccumulatorRecursionInstr = 0;
+ 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
// accumulator recursion variable eliminated.
if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
!isa<UndefValue>(Ret->getReturnValue()) &&
- AccumulatorRecursionEliminationInitVal == 0 &&
- !getCommonReturnValue(0, CI)) {
+ 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.
BasicBlock *BB = Ret->getParent();
Function *F = BB->getParent();
+ emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(),
+ "transforming tail recursion to 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 == 0) {
+ if (!OldEntry) {
OldEntry = &F->getEntryBlock();
BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
NewEntry->takeName(OldEntry);
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