X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FTailRecursionElimination.cpp;h=b7580255150c48fae28ff2dbb02a62bb6ffafcca;hb=ec51f453381c4d03d67911730bfadd6eedf8d5e8;hp=ad92a5961432e5f63888255f429258ef02f6fbec;hpb=eb3d76da81e2148ed7c577594c873ba147f4f435;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/TailRecursionElimination.cpp b/lib/Transforms/Scalar/TailRecursionElimination.cpp index ad92a596143..b7580255150 100644 --- a/lib/Transforms/Scalar/TailRecursionElimination.cpp +++ b/lib/Transforms/Scalar/TailRecursionElimination.cpp @@ -50,32 +50,35 @@ // //===----------------------------------------------------------------------===// -#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/Analysis/TargetTransformInfo.h" +#include "llvm/IR/CFG.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/IR/Module.h" #include "llvm/IR/ValueHandle.h" #include "llvm/Pass.h" -#include "llvm/Support/CFG.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Transforms/Utils/BasicBlockUtils.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"); @@ -89,11 +92,14 @@ namespace { initializeTailCallElimPass(*PassRegistry::getPassRegistry()); } - virtual void getAnalysisUsage(AnalysisUsage &AU) const; + void getAnalysisUsage(AnalysisUsage &AU) const override; - virtual bool runOnFunction(Function &F); + 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, @@ -131,55 +137,253 @@ void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const { AU.addRequired(); } -/// 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(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(&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(Use *U) override { - Value *V = U->getUser(); - if (isa(V) || isa(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 Worklist; + SmallPtrSet 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(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(Use *U) override { - if (isa(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 UsesAlloca; + SmallPtrSet AllocaUsers; + SmallPtrSet 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(&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 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 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 DeferredTails; + + BasicBlock *BB = &F.getEntryBlock(); + VisitType Escaped = UNESCAPED; + do { + for (auto &I : *BB) { + if (Tracker.EscapePoints.count(&I)) + Escaped = ESCAPED; + + CallInst *CI = dyn_cast(&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(Arg.getUser())) + continue; + if (Argument *A = dyn_cast(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(); - BasicBlock *OldEntry = 0; + BasicBlock *OldEntry = nullptr; bool TailCallsAreMarkedTail = false; SmallVector ArgumentPHIs; bool MadeChange = false; @@ -188,39 +392,23 @@ bool TailCallElim::runOnFunction(Function &F) { // 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(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(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(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; } } @@ -229,34 +417,13 @@ bool TailCallElim::runOnFunction(Function &F) { // 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(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(); } } @@ -343,11 +510,11 @@ static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) { // 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(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, @@ -355,10 +522,10 @@ static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { // 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; @@ -370,23 +537,23 @@ static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { /// 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(I->use_back())) - return 0; + if (!I->hasOneUse() || !isa(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(I->use_back()), CI); + return getCommonReturnValue(cast(I->user_back()), CI); } static Instruction *FirstNonDbg(BasicBlock::iterator I) { @@ -402,11 +569,11 @@ TailCallElim::FindTRECandidate(Instruction *TI, 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(BBI); @@ -414,14 +581,14 @@ TailCallElim::FindTRECandidate(Instruction *TI, 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 @@ -441,7 +608,7 @@ TailCallElim::FindTRECandidate(Instruction *TI, for (; I != E && FI != FE; ++I, ++FI) if (*I != &*FI) break; if (I == E && FI == FE) - return 0; + return nullptr; } return CI; @@ -462,8 +629,8 @@ bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, // 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 @@ -493,8 +660,8 @@ bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, // accumulator recursion variable eliminated. if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI && !isa(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. @@ -510,9 +677,12 @@ bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, 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);