X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FTailRecursionElimination.cpp;h=299b767eaa81a2b17ce778192a6f111612542580;hp=87cd27d22be8c5989150584e851acb8dff26843d;hb=d628f19f5df9e4033adce5af969049e90a90ae5d;hpb=f8485c643412dbff46fe87ea2867445169a5c28e diff --git a/lib/Transforms/Scalar/TailRecursionElimination.cpp b/lib/Transforms/Scalar/TailRecursionElimination.cpp index 87cd27d22be..299b767eaa8 100644 --- a/lib/Transforms/Scalar/TailRecursionElimination.cpp +++ b/lib/Transforms/Scalar/TailRecursionElimination.cpp @@ -1,111 +1,675 @@ //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// -// +// // The LLVM Compiler Infrastructure // -// This file was developed by the LLVM research group and is distributed under -// the University of Illinois Open Source License. See LICENSE.TXT for details. -// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// //===----------------------------------------------------------------------===// // -// This file implements tail recursion elimination. +// 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). // -// Caveats: The algorithm implemented is trivially simple. There are several -// improvements that could be made: +// There are several improvements that could be made: // -// 1. If the function has any alloca instructions, these instructions will not -// remain in the entry block of the function. Doing this requires analysis -// to prove that the alloca is not reachable by the recursively invoked -// function call. -// 2. Tail recursion is only performed if the call immediately preceeds the -// return instruction. Would it be useful to generalize this somehow? -// 3. TRE is only performed if the function returns void or if the return -// returns the result returned by the call. It is possible, but 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. +// 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. // //===----------------------------------------------------------------------===// +#define DEBUG_TYPE "tailcallelim" #include "llvm/Transforms/Scalar.h" -#include "llvm/DerivedTypes.h" -#include "llvm/Function.h" -#include "llvm/Instructions.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/CaptureTracking.h" +#include "llvm/Analysis/InlineCost.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/Loads.h" +#include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DerivedTypes.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Module.h" #include "llvm/Pass.h" -#include "Support/Statistic.h" - +#include "llvm/Support/CFG.h" +#include "llvm/Support/CallSite.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/ValueHandle.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Local.h" using namespace llvm; -namespace { - Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed"); +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()); + } + + virtual void getAnalysisUsage(AnalysisUsage &AU) const; + virtual bool runOnFunction(Function &F); + + private: + 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, + SmallVectorImpl &ArgumentPHIs, + bool CannotTailCallElimCallsMarkedTail); + bool CanMoveAboveCall(Instruction *I, CallInst *CI); + Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); }; - RegisterOpt X("tailcallelim", "Tail Call Elimination"); } +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(); +} + +/// 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()); +} + +namespace { +struct AllocaCaptureTracker : public CaptureTracker { + AllocaCaptureTracker() : Captured(false) {} + + void tooManyUses() override { Captured = true; } + + bool shouldExplore(Use *U) override { + Value *V = U->getUser(); + if (isa(V) || isa(V)) + UsesAlloca.insert(V); + return true; + } + + bool captured(Use *U) override { + if (isa(U->getUser())) + return false; + Captured = true; + return true; + } + + bool Captured; + SmallPtrSet UsesAlloca; +}; +} // end anonymous namespace bool TailCallElim::runOnFunction(Function &F) { + if (skipOptnoneFunction(F)) + return false; + // 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; - std::vector ArgumentPHIs; + bool TailCallsAreMarkedTail = false; + SmallVector ArgumentPHIs; bool MadeChange = false; - // Loop over the function, looking for any returning blocks... - for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) - if (ReturnInst *Ret = dyn_cast(BB->getTerminator())) - if (Ret != BB->begin()) // Make sure there is something before the ret... - if (CallInst *CI = dyn_cast(Ret->getPrev())) - // Make sure the tail call is to the current function, and that the - // return either returns void or returns the value computed by the - // call. - if (CI->getCalledFunction() == &F && - (Ret->getNumOperands() == 0 || Ret->getReturnValue() == CI)) { - // Ohh, it looks like we found a tail call, is this the first? - if (!OldEntry) { - // Ok, so this is the first tail call we have found in this - // function. Insert a new entry block into the function, allowing - // us to branch back to the old entry block. - OldEntry = &F.getEntryBlock(); - BasicBlock *NewEntry = new BasicBlock("tailrecurse", OldEntry); - new BranchInst(OldEntry, 0, 0, NewEntry); - - // 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::aiterator I = F.abegin(), E = F.aend(); I!=E; ++I){ - PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", - InsertPos); - I->replaceAllUsesWith(PN); // Everyone use the PHI node now! - PN->addIncoming(I, NewEntry); - ArgumentPHIs.push_back(PN); - } - } - - // 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); - - // Now that all of the PHI nodes are in place, remove the call and - // ret instructions, replacing them with an unconditional branch. - new BranchInst(OldEntry, CI); - BB->getInstList().pop_back(); // Remove return. - BB->getInstList().pop_back(); // Remove call. + // 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 = 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; + } + } + } + + // Second pass, 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; + } + } + } + + // 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 = SimplifyInstruction(PN)) { + PN->replaceAllUsesWith(PNV); + PN->eraseFromParent(); + } + } + } + + // 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; - NumEliminated++; } - + } + } + } + } + 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(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(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(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(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 = 0; + + for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { + ReturnInst *RI = dyn_cast(BBI->getTerminator()); + if (RI == 0 || 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 0; + + if (ReturnedValue && RetOp != ReturnedValue) + return 0; // 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 0; + 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; + + // The only user of this instruction we allow is a single return instruction. + if (!I->hasOneUse() || !isa(I->use_back())) + return 0; + + // 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); +} + +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() == TI) // Make sure there is something before the terminator. + return 0; + + // 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; + BasicBlock::iterator BBI = TI; + while (true) { + CI = dyn_cast(BBI); + if (CI && CI->getCalledFunction() == F) + break; + + if (BBI == BB->begin()) + return 0; // 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; + + // 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 0; + } + + 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 = 0; + Instruction *AccumulatorRecursionInstr = 0; + + // 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(Ret->getReturnValue()) && + AccumulatorRecursionEliminationInitVal == 0 && + !getCommonReturnValue(0, 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(); + + // 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) { + 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(OEBI++)) + if (isa(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(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 &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 (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(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); +}