| //===- 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. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "tailcallelim" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/ADT/STLExtras.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 "llvm/Support/CFG.h" |
| #include "llvm/Support/CallSite.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; |
| |
| 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, |
| SmallVector<PHINode*, 8> &ArgumentPHIs, |
| bool CannotTailCallElimCallsMarkedTail); |
| bool FoldReturnAndProcessPred(BasicBlock *BB, |
| ReturnInst *Ret, BasicBlock *&OldEntry, |
| bool &TailCallsAreMarkedTail, |
| SmallVector<PHINode*, 8> &ArgumentPHIs, |
| bool CannotTailCallElimCallsMarkedTail); |
| bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, |
| bool &TailCallsAreMarkedTail, |
| SmallVector<PHINode*, 8> &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>(); |
| } |
| |
| /// 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; |
| } |
| |
| /// CheckForEscapingAllocas - 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<AllocaInst>(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<ConstantInt>(AI->getArraySize())) |
| CannotTCETailMarkedCall = true; |
| } |
| return RetVal; |
| } |
| |
| bool TailCallElim::runOnFunction(Function &F) { |
| // If this function is a varargs function, we won't be able to PHI the args |
| // right, so don't even try to convert it... |
| if (F.getFunctionType()->isVarArg()) return false; |
| |
| TTI = &getAnalysis<TargetTransformInfo>(); |
| BasicBlock *OldEntry = 0; |
| bool TailCallsAreMarkedTail = false; |
| SmallVector<PHINode*, 8> ArgumentPHIs; |
| bool MadeChange = false; |
| bool FunctionContainsEscapingAllocas = false; |
| |
| // CannotTCETailMarkedCall - If true, we cannot perform TCE 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 |
| // doesn't). |
| bool CannotTCETailMarkedCall = false; |
| |
| // Loop over the function, looking for any returning blocks, and keeping track |
| // of whether this function has any non-trivially used allocas. |
| for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { |
| if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) |
| break; |
| |
| FunctionContainsEscapingAllocas |= |
| CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); |
| } |
| |
| /// 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<ReturnInst>(BB->getTerminator())) { |
| bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, |
| ArgumentPHIs,CannotTCETailMarkedCall); |
| if (!Change && BB->getFirstNonPHIOrDbg() == Ret) |
| Change = FoldReturnAndProcessPred(BB, Ret, OldEntry, |
| TailCallsAreMarkedTail, ArgumentPHIs, |
| CannotTCETailMarkedCall); |
| 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(); |
| } |
| } |
| } |
| |
| // Finally, if this function contains no non-escaping allocas, or calls |
| // setjmp, mark all calls in the function as eligible for tail calls |
| //(there is no stack memory for them to access). |
| if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice()) |
| for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) |
| for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) |
| if (CallInst *CI = dyn_cast<CallInst>(I)) { |
| CI->setTailCall(); |
| MadeChange = true; |
| } |
| |
| 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 = 0; |
| |
| for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { |
| ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()); |
| if (RI == 0 || RI == IgnoreRI) continue; |
| |
| // 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<ReturnInst>(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<ReturnInst>(I->use_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 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<CallInst>(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(llvm::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, |
| SmallVector<PHINode*, 8> &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<UndefValue>(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<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, |
| SmallVector<PHINode*, 8> &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, |
| SmallVector<PHINode*, 8> &ArgumentPHIs, |
| bool CannotTailCallElimCallsMarkedTail) { |
| CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail); |
| if (!CI) |
| return false; |
| |
| return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, |
| ArgumentPHIs, |
| CannotTailCallElimCallsMarkedTail); |
| } |
