//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // 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.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/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 TailCallElim : public FunctionPass { const TargetTransformInfo *TTI; static char ID; // Pass identification, replacement for typeid TailCallElim() : FunctionPass(ID) { initializeTailCallElimPass(*PassRegistry::getPassRegistry()); } void getAnalysisUsage(AnalysisUsage &AU) const override; 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<PHINode *> &ArgumentPHIs, bool CannotTailCallElimCallsMarkedTail); bool FoldReturnAndProcessPred(BasicBlock *BB, ReturnInst *Ret, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs, bool CannotTailCallElimCallsMarkedTail); bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs, bool CannotTailCallElimCallsMarkedTail); bool CanMoveAboveCall(Instruction *I, CallInst *CI); Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); }; } char TailCallElim::ID = 0; 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<TargetTransformInfo>(); } /// \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; } 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<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); } } 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<Instruction *, 32> AllocaUsers; SmallPtrSet<Instruction *, 32> EscapePoints; }; } 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 = nullptr; bool TailCallsAreMarkedTail = false; SmallVector<PHINode*, 8> ArgumentPHIs; bool MadeChange = false; // 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 TRE would deallocate variable sized allocas, TRE // doesn't). bool CanTRETailMarkedCall = CanTRE(F); // Change any tail recursive calls to loops. // // FIXME: The code generator produces really bad code when an 'escaping // alloca' is changed from being a static alloca to being a dynamic alloca. // Until this is resolved, disable this transformation if that would ever // happen. This bug is PR962. for (Function::iterator 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; } } // 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 (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(); } } return MadeChange; } /// CanMoveAboveCall - 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. /// bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { // 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. // FIXME: Writes to memory only matter if they may alias the pointer // being loaded from. if (CI->mayWriteToMemory() || !isSafeToLoadUnconditionally(L->getPointerOperand(), L, L->getAlignment())) 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. for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) if (I->getOperand(i) == CI) return false; return true; } // isDynamicConstant - Return true if the specified value is the same when the // return would exit as it was when the initial iteration of the recursive // function was executed. // // We currently handle static constants and arguments that are not modified as // part of the recursion. // static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) { if (isa<Constant>(V)) return true; // Static constants are always dyn consts // Check to see if this is an immutable argument, if so, the value // will be available to initialize the accumulator. if (Argument *Arg = dyn_cast<Argument>(V)) { // Figure out which argument number this is... unsigned ArgNo = 0; Function *F = CI->getParent()->getParent(); for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) ++ArgNo; // If we are passing this argument into call as the corresponding // argument operand, then the argument is dynamically constant. // Otherwise, we cannot transform this function safely. if (CI->getArgOperand(ArgNo) == Arg) return true; } // Switch cases are always constant integers. If the value is being switched // on and the return is only reachable from one of its cases, it's // effectively constant. if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor()) if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator())) if (SI->getCondition() == V) return SI->getDefaultDest() != RI->getParent(); // Not a constant or immutable argument, we can't safely transform. return false; } // getCommonReturnValue - Check to see if the function containing the specified // tail call consistently returns the same runtime-constant value at all exit // points except for IgnoreRI. If so, return the returned value. // static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { Function *F = CI->getParent()->getParent(); Value *ReturnedValue = nullptr; for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()); if (RI == nullptr || RI == IgnoreRI) continue; // We can only perform this transformation if the value returned is // evaluatable at the start of the initial invocation of the function, // instead of at the end of the evaluation. // Value *RetOp = RI->getOperand(0); if (!isDynamicConstant(RetOp, CI, RI)) return nullptr; if (ReturnedValue && RetOp != ReturnedValue) return nullptr; // Cannot transform if differing values are returned. ReturnedValue = RetOp; } return ReturnedValue; } /// CanTransformAccumulatorRecursion - If the specified instruction can be /// transformed using accumulator recursion elimination, return the constant /// which is the start of the accumulator value. Otherwise return null. /// Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI) { if (!I->isAssociative() || !I->isCommutative()) return nullptr; assert(I->getNumOperands() == 2 && "Associative/commutative operations should have 2 args!"); // Exactly one operand should be the result of the call instruction. if ((I->getOperand(0) == CI && I->getOperand(1) == CI) || (I->getOperand(0) != CI && I->getOperand(1) != CI)) return nullptr; // The only user of this instruction we allow is a single return instruction. if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back())) return nullptr; // Ok, now we have to check all of the other return instructions in this // function. If they return non-constants or differing values, then we cannot // transform the function safely. return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI); } static Instruction *FirstNonDbg(BasicBlock::iterator I) { while (isa<DbgInfoIntrinsic>(I)) ++I; return &*I; } CallInst* TailCallElim::FindTRECandidate(Instruction *TI, bool CannotTailCallElimCallsMarkedTail) { BasicBlock *BB = TI->getParent(); Function *F = BB->getParent(); if (&BB->front() == TI) // Make sure there is something before the terminator. return nullptr; // Scan backwards from the return, checking to see if there is a tail call in // this block. If so, set CI to it. CallInst *CI = nullptr; BasicBlock::iterator BBI = TI; while (true) { CI = dyn_cast<CallInst>(BBI); if (CI && CI->getCalledFunction() == F) break; if (BBI == BB->begin()) 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 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()) == 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<PHINode *> &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. 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 // constant, return the value returned by the tail call, or that are being // accumulator recursion variable eliminated. if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI && !isa<UndefValue>(Ret->getReturnValue()) && AccumulatorRecursionEliminationInitVal == nullptr && !getCommonReturnValue(nullptr, CI)) { // One case remains that we are able to handle: the current return // instruction returns a constant, and all other return instructions // return a different constant. if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret)) return false; // Current return instruction does not return a constant. // Check that all other return instructions return a common constant. If // so, record it in AccumulatorRecursionEliminationInitVal. AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI); if (!AccumulatorRecursionEliminationInitVal) return false; } BasicBlock *BB = Ret->getParent(); Function *F = BB->getParent(); 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) { OldEntry = &F->getEntryBlock(); BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry); NewEntry->takeName(OldEntry); OldEntry->setName("tailrecurse"); BranchInst::Create(OldEntry, NewEntry); // If this tail call is marked 'tail' and if there are any allocas in the // entry block, move them up to the new entry block. TailCallsAreMarkedTail = CI->isTailCall(); if (TailCallsAreMarkedTail) // Move all fixed sized allocas from OldEntry to NewEntry. for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), NEBI = NewEntry->begin(); OEBI != E; ) if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) if (isa<ConstantInt>(AI->getArraySize())) AI->moveBefore(NEBI); // Now that we have created a new block, which jumps to the entry // block, insert a PHI node for each argument of the function. // For now, we initialize each PHI to only have the real arguments // which are passed in. Instruction *InsertPos = OldEntry->begin(); 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); ArgumentPHIs.push_back(PN); } } // If this function has self recursive calls in the tail position where some // are marked tail and some are not, only transform one flavor or another. We // have to choose whether we move allocas in the entry block to the new entry // block or not, so we can't make a good choice for both. NOTE: We could do // slightly better here in the case that the function has no entry block // allocas. if (TailCallsAreMarkedTail && !CI->isTailCall()) return false; // Ok, now that we know we have a pseudo-entry block WITH all of the // required PHI nodes, add entries into the PHI node for the actual // parameters passed into the tail-recursive call. for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i) ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB); // If we are introducing an accumulator variable to eliminate the recursion, // do so now. Note that we _know_ that no subsequent tail recursion // eliminations will happen on this function because of the way the // accumulator recursion predicate is set up. // if (AccumulatorRecursionEliminationInitVal) { Instruction *AccRecInstr = AccumulatorRecursionInstr; // Start by inserting a new PHI node for the accumulator. pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry); PHINode *AccPN = PHINode::Create(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, // computed earlier. For any other existing branches to this block (due to // other tail recursions eliminated) the accumulator is not modified. // Because we haven't added the branch in the current block to OldEntry yet, // it will not show up as a predecessor. for (pred_iterator PI = PB; PI != PE; ++PI) { BasicBlock *P = *PI; if (P == &F->getEntryBlock()) AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P); else AccPN->addIncoming(AccPN, P); } if (AccRecInstr) { // Add an incoming argument for the current block, which is computed by // our associative and commutative accumulator instruction. AccPN->addIncoming(AccRecInstr, BB); // Next, rewrite the accumulator recursion instruction so that it does not // use the result of the call anymore, instead, use the PHI node we just // inserted. AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); } else { // Add an incoming argument for the current block, which is just the // constant returned by the current return instruction. AccPN->addIncoming(Ret->getReturnValue(), BB); } // Finally, rewrite any return instructions in the program to return the PHI // node instead of the "initval" that they do currently. This loop will // actually rewrite the return value we are destroying, but that's ok. for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) RI->setOperand(0, AccPN); ++NumAccumAdded; } // Now that all of the PHI nodes are in place, remove the call and // ret instructions, replacing them with an unconditional branch. BranchInst *NewBI = BranchInst::Create(OldEntry, Ret); 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<PHINode *> &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<BranchInst*, 8> UncondBranchPreds; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *Pred = *PI; TerminatorInst *PTI = Pred->getTerminator(); if (BranchInst *BI = dyn_cast<BranchInst>(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<PHINode *> &ArgumentPHIs, bool CannotTailCallElimCallsMarkedTail) { CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail); if (!CI) return false; return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, ArgumentPHIs, CannotTailCallElimCallsMarkedTail); }