X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FTailRecursionElimination.cpp;h=d9280ac6c9c11084785a59316401606d6fa83a3c;hp=b84a1f04ca38704df204b205752332fbd25c8e29;hb=a4697dad1926a8c91c12cd6663f5ddc7c4cd16c7;hpb=1d0be15f89cb5056e20e2d24faa8d6afb1573bca diff --git a/lib/Transforms/Scalar/TailRecursionElimination.cpp b/lib/Transforms/Scalar/TailRecursionElimination.cpp index b84a1f04ca3..d9280ac6c9c 100644 --- a/lib/Transforms/Scalar/TailRecursionElimination.cpp +++ b/lib/Transforms/Scalar/TailRecursionElimination.cpp @@ -16,16 +16,16 @@ // 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 expression to use an accumulator variable, -// thus compiling the typical naive factorial or 'fib' implementation into -// efficient code. +// 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 theier caller stack frame, +// 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). // @@ -36,7 +36,7 @@ // 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 preceeds the +// 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 @@ -50,33 +50,71 @@ // //===----------------------------------------------------------------------===// -#define DEBUG_TYPE "tailcallelim" #include "llvm/Transforms/Scalar.h" -#include "llvm/Transforms/Utils/Local.h" -#include "llvm/Constants.h" -#include "llvm/DerivedTypes.h" -#include "llvm/Function.h" -#include "llvm/Instructions.h" -#include "llvm/Pass.h" -#include "llvm/Support/CFG.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/Statistic.h" -#include "llvm/Support/Compiler.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/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"); namespace { - struct VISIBILITY_HIDDEN TailCallElim : public FunctionPass { + struct TailCallElim : public FunctionPass { + const TargetTransformInfo *TTI; + static char ID; // Pass identification, replacement for typeid - TailCallElim() : FunctionPass(&ID) {} + TailCallElim() : FunctionPass(ID) { + initializeTailCallElimPass(*PassRegistry::getPassRegistry()); + } + + 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, + BasicBlock *&OldEntry, + bool &TailCallsAreMarkedTail, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail); + bool FoldReturnAndProcessPred(BasicBlock *BB, + ReturnInst *Ret, BasicBlock *&OldEntry, + bool &TailCallsAreMarkedTail, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail); bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail, - std::vector &ArgumentPHIs, + SmallVectorImpl &ArgumentPHIs, bool CannotTailCallElimCallsMarkedTail); bool CanMoveAboveCall(Instruction *I, CallInst *CI); Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); @@ -84,113 +122,313 @@ namespace { } char TailCallElim::ID = 0; -static RegisterPass X("tailcallelim", "Tail Call Elimination"); +INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", + "Tail Call Elimination", false, false) +INITIALIZE_AG_DEPENDENCY(TargetTransformInfo) +INITIALIZE_PASS_END(TailCallElim, "tailcallelim", + "Tail Call Elimination", false, false) // Public interface to the TailCallElimination pass FunctionPass *llvm::createTailCallEliminationPass() { return new TailCallElim(); } +void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const { + AU.addRequired(); +} + +/// \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; + } + } + } -/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by -/// callees of this function. We only do very simple analysis right now, this -/// could be expanded in the future to use mod/ref information for particular -/// call sites if desired. -static bool AllocaMightEscapeToCalls(AllocaInst *AI) { - // FIXME: do simple 'address taken' analysis. return true; } -/// FunctionContainsAllocas - Scan the specified basic block for alloca -/// instructions. If it contains any that might be accessed by calls, return -/// true. -static bool CheckForEscapingAllocas(BasicBlock *BB, - bool &CannotTCETailMarkedCall) { - bool RetVal = false; - for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) - if (AllocaInst *AI = dyn_cast(I)) { - RetVal |= AllocaMightEscapeToCalls(AI); - - // 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. - if (BB != &BB->getParent()->getEntryBlock() || - !isa(AI->getArraySize())) - CannotTCETailMarkedCall = true; +bool TailCallElim::runOnFunction(Function &F) { + if (skipOptnoneFunction(F)) + return false; + + bool AllCallsAreTailCalls = false; + bool Modified = markTails(F, AllCallsAreTailCalls); + if (AllCallsAreTailCalls) + Modified |= runTRE(F); + return Modified; +} + +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); } - return RetVal; + } + + void callUsesLocalStack(CallSite CS, bool IsNocapture) { + // Add it to the list of alloca users. If it's already there, skip further + // processing. + if (!AllocaUsers.insert(CS.getInstruction())) + return; + + // If it's nocapture then it can't capture the alloca. + if (IsNocapture) + return; + + // If it can write to memory, it can leak the alloca value. + if (!CS.onlyReadsMemory()) + EscapePoints.insert(CS.getInstruction()); + } + + SmallPtrSet AllocaUsers; + SmallPtrSet EscapePoints; +}; } -bool TailCallElim::runOnFunction(Function &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 : successors(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; - BasicBlock *OldEntry = 0; + TTI = &getAnalysis(); + BasicBlock *OldEntry = nullptr; bool TailCallsAreMarkedTail = false; - std::vector ArgumentPHIs; + SmallVector ArgumentPHIs; bool MadeChange = false; - bool FunctionContainsEscapingAllocas = false; - - // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls + // CanTRETailMarkedCall - 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 TCE would deallocate variable sized allocas, TCE + // size to increase (real TRE would deallocate variable sized allocas, TRE // doesn't). - bool CannotTCETailMarkedCall = false; + bool CanTRETailMarkedCall = CanTRE(F); - // Loop over the function, looking for any returning blocks, and keeping track - // of whether this function has any non-trivially used allocas. + // 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 BB = F.begin(), E = F.end(); BB != E; ++BB) { - if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) - break; - - FunctionContainsEscapingAllocas |= - CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); + 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; + } } - - /// 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 (FunctionContainsEscapingAllocas) - return false; - - - // Second pass, change any tail calls to loops. - for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) - if (ReturnInst *Ret = dyn_cast(BB->getTerminator())) - MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, - ArgumentPHIs,CannotTCETailMarkedCall); // 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. - 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 = PN->hasConstantValue()) { - PN->replaceAllUsesWith(PNV); - PN->eraseFromParent(); - } + 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(); } } - // Finally, if this function contains no non-escaping allocas, mark all calls - // in the function as eligible for tail calls (there is no stack memory for - // them to access). - if (!FunctionContainsEscapingAllocas) - 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)) { - CI->setTailCall(); - MadeChange = true; - } - return MadeChange; } @@ -204,8 +442,8 @@ bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { // 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(I)) { + + if (LoadInst *L = dyn_cast(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 @@ -213,7 +451,8 @@ bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { // FIXME: Writes to memory only matter if they may alias the pointer // being loaded from. if (CI->mayWriteToMemory() || - !isSafeToLoadUnconditionally(L->getPointerOperand(), L)) + !isSafeToLoadUnconditionally(L->getPointerOperand(), L, + L->getAlignment())) return false; } } @@ -236,7 +475,7 @@ bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { // We currently handle static constants and arguments that are not modified as // part of the recursion. // -static bool isDynamicConstant(Value *V, CallInst *CI) { +static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) { if (isa(V)) return true; // Static constants are always dyn consts // Check to see if this is an immutable argument, if so, the value @@ -251,41 +490,46 @@ static bool isDynamicConstant(Value *V, CallInst *CI) { // 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->getOperand(ArgNo+1) == Arg) + 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(UniquePred->getTerminator())) + if (SI->getCondition() == V) + return SI->getDefaultDest() != RI->getParent(); + // Not a constant or immutable argument, we can't safely transform. return false; } // getCommonReturnValue - Check to see if the function containing the specified -// return instruction and tail call consistently returns the same -// runtime-constant value at all exit points. If so, return the returned value. +// 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 *TheRI, CallInst *CI) { - Function *F = TheRI->getParent()->getParent(); - Value *ReturnedValue = 0; - - // TODO: Handle multiple value ret instructions; - if (isa(F->getReturnType())) - return 0; - - for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) - if (ReturnInst *RI = dyn_cast(BBI->getTerminator())) - if (RI != TheRI) { - Value *RetOp = RI->getOperand(0); - - // 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. - // - if (!isDynamicConstant(RetOp, CI)) - return 0; - - if (ReturnedValue && RetOp != ReturnedValue) - return 0; // Cannot transform if differing values are returned. - ReturnedValue = RetOp; - } +static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { + Function *F = CI->getParent()->getParent(); + Value *ReturnedValue = nullptr; + + for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { + ReturnInst *RI = dyn_cast(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; } @@ -295,90 +539,122 @@ static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) { /// Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI) { - if (!I->isAssociative()) return 0; + if (!I->isAssociative() || !I->isCommutative()) return nullptr; assert(I->getNumOperands() == 2 && - "Associative operations should have 2 args!"); + "Associative/commutative operations should have 2 args!"); - // Exactly one operand should be the result of the call instruction... + // 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); } -bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, - bool &TailCallsAreMarkedTail, - std::vector &ArgumentPHIs, - bool CannotTailCallElimCallsMarkedTail) { - BasicBlock *BB = Ret->getParent(); +static Instruction *FirstNonDbg(BasicBlock::iterator I) { + while (isa(I)) + ++I; + return &*I; +} + +CallInst* +TailCallElim::FindTRECandidate(Instruction *TI, + bool CannotTailCallElimCallsMarkedTail) { + BasicBlock *BB = TI->getParent(); Function *F = BB->getParent(); - if (&BB->front() == Ret) // Make sure there is something before the ret... - return false; - - // If the return is in the entry block, then making this transformation would - // turn infinite recursion into an infinite loop. This transformation is ok - // in theory, but breaks some code like: - // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call - // disable this xform in this case, because the code generator will lower the - // call to fabs into inline code. - if (BB == &F->getEntryBlock()) - return false; + 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; - BasicBlock::iterator BBI = Ret; - while (1) { + CallInst *CI = nullptr; + BasicBlock::iterator BBI = TI; + while (true) { CI = dyn_cast(BBI); if (CI && CI->getCalledFunction() == F) break; if (BBI == BB->begin()) - return false; // 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 false; + return nullptr; - // If we are introducing accumulator recursion to eliminate associative - // operations after the call instruction, this variable contains the initial - // value for the accumulator. If this value is set, we actually perform - // accumulator recursion elimination instead of simple tail recursion - // elimination. - Value *AccumulatorRecursionEliminationInitVal = 0; - Instruction *AccumulatorRecursionInstr = 0; + // 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()) == 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; +} + +bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, + BasicBlock *&OldEntry, + bool &TailCallsAreMarkedTail, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail) { + // 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. - for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) - if (!CanMoveAboveCall(BBI, CI)) { - // If we can't move the instruction above the call, it might be because it - // is an associative operation that could be tranformed 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! - } + BasicBlock::iterator BBI = CI; + for (++BBI; &*BBI != Ret; ++BBI) { + if (CanMoveAboveCall(BBI, CI)) continue; + + // 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 @@ -386,13 +662,29 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, // accumulator recursion variable eliminated. if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI && !isa(Ret->getReturnValue()) && - AccumulatorRecursionEliminationInitVal == 0 && - !getCommonReturnValue(Ret, CI)) - return false; + 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(); + + 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); @@ -417,7 +709,7 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, Instruction *InsertPos = OldEntry->begin(); for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) { - PHINode *PN = PHINode::Create(I->getType(), + PHINode *PN = PHINode::Create(I->getType(), 2, I->getName() + ".tr", InsertPos); I->replaceAllUsesWith(PN); // Everyone use the PHI node now! PN->addIncoming(I, NewEntry); @@ -437,8 +729,8 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, // 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->getNumOperands()-1; i != e; ++i) - ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB); + 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 @@ -448,8 +740,11 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, if (AccumulatorRecursionEliminationInitVal) { Instruction *AccRecInstr = AccumulatorRecursionInstr; // Start by inserting a new PHI node for the accumulator. - PHINode *AccPN = PHINode::Create(AccRecInstr->getType(), "accumulator.tr", - OldEntry->begin()); + pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry); + PHINode *AccPN = + PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(), + std::distance(PB, PE) + 1, + "accumulator.tr", OldEntry->begin()); // 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, @@ -457,22 +752,28 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, // 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 = pred_begin(OldEntry), PE = pred_end(OldEntry); - PI != PE; ++PI) { - if (*PI == &F->getEntryBlock()) - AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI); + for (pred_iterator PI = PB; PI != PE; ++PI) { + BasicBlock *P = *PI; + if (P == &F->getEntryBlock()) + AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P); else - AccPN->addIncoming(AccPN, *PI); + AccPN->addIncoming(AccPN, P); } - // Add an incoming argument for the current block, which is computed by our - // associative 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); + 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 @@ -485,9 +786,61 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, // Now that all of the PHI nodes are in place, remove the call and // ret instructions, replacing them with an unconditional branch. - BranchInst::Create(OldEntry, Ret); + BranchInst *NewBI = BranchInst::Create(OldEntry, Ret); + NewBI->setDebugLoc(CI->getDebugLoc()); + BB->getInstList().erase(Ret); // Remove return. BB->getInstList().erase(CI); // Remove call. ++NumEliminated; return true; } + +bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB, + ReturnInst *Ret, BasicBlock *&OldEntry, + bool &TailCallsAreMarkedTail, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail) { + bool Change = false; + + // 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 TRC there. Look for predecessors that end + // in unconditional branch and recursive call(s). + SmallVector UncondBranchPreds; + for (BasicBlock *Pred : predecessors(BB)) { + TerminatorInst *PTI = Pred->getTerminator(); + if (BranchInst *BI = dyn_cast(PTI)) + if (BI->isUnconditional()) + UncondBranchPreds.push_back(BI); + } + + while (!UncondBranchPreds.empty()) { + BranchInst *BI = UncondBranchPreds.pop_back_val(); + BasicBlock *Pred = BI->getParent(); + if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){ + DEBUG(dbgs() << "FOLDING: " << *BB + << "INTO UNCOND BRANCH PRED: " << *Pred); + EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred), + OldEntry, TailCallsAreMarkedTail, ArgumentPHIs, + CannotTailCallElimCallsMarkedTail); + ++NumRetDuped; + Change = true; + } + } + + return Change; +} + +bool +TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, + bool &TailCallsAreMarkedTail, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail) { + CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail); + if (!CI) + return false; + + return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, + ArgumentPHIs, + CannotTailCallElimCallsMarkedTail); +}