| //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // 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/TailRecursionElimination.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/CFG.h" |
| #include "llvm/Analysis/CaptureTracking.h" |
| #include "llvm/Analysis/DomTreeUpdater.h" |
| #include "llvm/Analysis/GlobalsModRef.h" |
| #include "llvm/Analysis/InlineCost.h" |
| #include "llvm/Analysis/InstructionSimplify.h" |
| #include "llvm/Analysis/Loads.h" |
| #include "llvm/Analysis/OptimizationRemarkEmitter.h" |
| #include "llvm/Analysis/PostDominators.h" |
| #include "llvm/Analysis/TargetTransformInfo.h" |
| #include "llvm/IR/CFG.h" |
| #include "llvm/IR/CallSite.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/DerivedTypes.h" |
| #include "llvm/IR/DiagnosticInfo.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/InstIterator.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/Scalar.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.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"); |
| |
| /// 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. |
| return llvm::all_of(instructions(F), [](Instruction &I) { |
| auto *AI = dyn_cast<AllocaInst>(&I); |
| return !AI || AI->isStaticAlloca(); |
| }); |
| } |
| |
| 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).second) |
| 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); |
| // If the alloca-derived argument is passed byval it is not an escape |
| // point, or a use of an alloca. Calling with byval copies the contents |
| // of the alloca into argument registers or stack slots, which exist |
| // beyond the lifetime of the current frame. |
| if (CS.isArgOperand(U) && CS.isByValArgument(CS.getArgumentNo(U))) |
| continue; |
| bool IsNocapture = |
| CS.isDataOperand(U) && CS.doesNotCapture(CS.getDataOperandNo(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. |
| AllocaUsers.insert(CS.getInstruction()); |
| |
| // If it's nocapture then it can't capture this alloca. |
| if (IsNocapture) |
| return; |
| |
| // If it can write to memory, it can leak the alloca value. |
| if (!CS.onlyReadsMemory()) |
| EscapePoints.insert(CS.getInstruction()); |
| } |
| |
| SmallPtrSet<Instruction *, 32> AllocaUsers; |
| SmallPtrSet<Instruction *, 32> EscapePoints; |
| }; |
| } |
| |
| static bool markTails(Function &F, bool &AllCallsAreTailCalls, |
| OptimizationRemarkEmitter *ORE) { |
| 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() || isa<DbgInfoIntrinsic>(&I)) |
| continue; |
| |
| bool IsNoTail = CI->isNoTailCall() || CI->hasOperandBundles(); |
| |
| if (!IsNoTail && 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) { |
| using namespace ore; |
| ORE->emit([&]() { |
| return OptimizationRemark(DEBUG_TYPE, "tailcall-readnone", CI) |
| << "marked as tail call candidate (readnone)"; |
| }); |
| CI->setTailCall(); |
| Modified = true; |
| continue; |
| } |
| } |
| |
| if (!IsNoTail && 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. |
| LLVM_DEBUG(dbgs() << "Marked as tail call candidate: " << *CI << "\n"); |
| CI->setTailCall(); |
| Modified = true; |
| } else { |
| AllCallsAreTailCalls = false; |
| } |
| } |
| |
| return Modified; |
| } |
| |
| /// 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. |
| /// |
| static bool canMoveAboveCall(Instruction *I, CallInst *CI, AliasAnalysis *AA) { |
| // 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. |
| // Writes to memory only matter if they may alias the pointer |
| // being loaded from. |
| const DataLayout &DL = L->getModule()->getDataLayout(); |
| if (isModSet(AA->getModRefInfo(CI, MemoryLocation::get(L))) || |
| !isSafeToLoadUnconditionally(L->getPointerOperand(), |
| L->getAlignment(), DL, L)) |
| 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. |
| return !is_contained(I->operands(), CI); |
| } |
| |
| /// 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; |
| } |
| |
| /// 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 (BasicBlock &BBI : *F) { |
| 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; |
| } |
| |
| /// 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. |
| static Value *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; |
| } |
| |
| static CallInst *findTRECandidate(Instruction *TI, |
| bool CannotTailCallElimCallsMarkedTail, |
| const TargetTransformInfo *TTI) { |
| 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().getIterator()) == 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; |
| } |
| |
| static bool eliminateRecursiveTailCall( |
| CallInst *CI, ReturnInst *Ret, BasicBlock *&OldEntry, |
| bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs, |
| AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) { |
| // 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, AA)) |
| 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(); |
| |
| using namespace ore; |
| ORE->emit([&]() { |
| return OptimizationRemark(DEBUG_TYPE, "tailcall-recursion", CI) |
| << "transforming tail recursion into 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 *BI = BranchInst::Create(OldEntry, NewEntry); |
| BI->setDebugLoc(CI->getDebugLoc()); |
| |
| // 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->front(); |
| 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); |
| } |
| // The entry block was changed from OldEntry to NewEntry. |
| // The forward DominatorTree needs to be recalculated when the EntryBB is |
| // changed. In this corner-case we recalculate the entire tree. |
| DTU.recalculate(*NewEntry->getParent()); |
| } |
| |
| // 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->front()); |
| |
| // 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 (BasicBlock &BBI : *F) |
| 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. |
| DTU.insertEdge(BB, OldEntry); |
| ++NumEliminated; |
| return true; |
| } |
| |
| static bool foldReturnAndProcessPred( |
| BasicBlock *BB, ReturnInst *Ret, BasicBlock *&OldEntry, |
| bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs, |
| bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI, |
| AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) { |
| bool Change = false; |
| |
| // Make sure this block is a trivial return block. |
| assert(BB->getFirstNonPHIOrDbg() == Ret && |
| "Trying to fold non-trivial return block"); |
| |
| // 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 TRE 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; |
| Instruction *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, TTI)){ |
| LLVM_DEBUG(dbgs() << "FOLDING: " << *BB |
| << "INTO UNCOND BRANCH PRED: " << *Pred); |
| ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred, &DTU); |
| |
| // Cleanup: if all predecessors of BB have been eliminated by |
| // FoldReturnIntoUncondBranch, delete it. It is important to empty it, |
| // because the ret instruction in there is still using a value which |
| // eliminateRecursiveTailCall will attempt to remove. |
| if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB)) |
| DTU.deleteBB(BB); |
| |
| eliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail, |
| ArgumentPHIs, AA, ORE, DTU); |
| ++NumRetDuped; |
| Change = true; |
| } |
| } |
| |
| return Change; |
| } |
| |
| static bool processReturningBlock( |
| ReturnInst *Ret, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail, |
| SmallVectorImpl<PHINode *> &ArgumentPHIs, |
| bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI, |
| AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) { |
| CallInst *CI = findTRECandidate(Ret, CannotTailCallElimCallsMarkedTail, TTI); |
| if (!CI) |
| return false; |
| |
| return eliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, |
| ArgumentPHIs, AA, ORE, DTU); |
| } |
| |
| static bool eliminateTailRecursion(Function &F, const TargetTransformInfo *TTI, |
| AliasAnalysis *AA, |
| OptimizationRemarkEmitter *ORE, |
| DomTreeUpdater &DTU) { |
| if (F.getFnAttribute("disable-tail-calls").getValueAsString() == "true") |
| return false; |
| |
| bool MadeChange = false; |
| bool AllCallsAreTailCalls = false; |
| MadeChange |= markTails(F, AllCallsAreTailCalls, ORE); |
| if (!AllCallsAreTailCalls) |
| return MadeChange; |
| |
| // 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 = nullptr; |
| bool TailCallsAreMarkedTail = false; |
| SmallVector<PHINode*, 8> ArgumentPHIs; |
| |
| // 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 BBI = F.begin(), E = F.end(); BBI != E; /*in loop*/) { |
| BasicBlock *BB = &*BBI++; // foldReturnAndProcessPred may delete BB. |
| if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) { |
| bool Change = processReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, |
| ArgumentPHIs, !CanTRETailMarkedCall, |
| TTI, AA, ORE, DTU); |
| if (!Change && BB->getFirstNonPHIOrDbg() == Ret) |
| Change = foldReturnAndProcessPred( |
| BB, Ret, OldEntry, TailCallsAreMarkedTail, ArgumentPHIs, |
| !CanTRETailMarkedCall, TTI, AA, ORE, DTU); |
| 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 (PHINode *PN : ArgumentPHIs) { |
| // If the PHI Node is a dynamic constant, replace it with the value it is. |
| if (Value *PNV = SimplifyInstruction(PN, F.getParent()->getDataLayout())) { |
| PN->replaceAllUsesWith(PNV); |
| PN->eraseFromParent(); |
| } |
| } |
| |
| return MadeChange; |
| } |
| |
| namespace { |
| struct TailCallElim : public FunctionPass { |
| static char ID; // Pass identification, replacement for typeid |
| TailCallElim() : FunctionPass(ID) { |
| initializeTailCallElimPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.addRequired<TargetTransformInfoWrapperPass>(); |
| AU.addRequired<AAResultsWrapperPass>(); |
| AU.addRequired<OptimizationRemarkEmitterWrapperPass>(); |
| AU.addPreserved<GlobalsAAWrapperPass>(); |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| AU.addPreserved<PostDominatorTreeWrapperPass>(); |
| } |
| |
| bool runOnFunction(Function &F) override { |
| if (skipFunction(F)) |
| return false; |
| |
| auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); |
| auto *DT = DTWP ? &DTWP->getDomTree() : nullptr; |
| auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>(); |
| auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr; |
| // There is no noticable performance difference here between Lazy and Eager |
| // UpdateStrategy based on some test results. It is feasible to switch the |
| // UpdateStrategy to Lazy if we find it profitable later. |
| DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager); |
| |
| return eliminateTailRecursion( |
| F, &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F), |
| &getAnalysis<AAResultsWrapperPass>().getAAResults(), |
| &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE(), DTU); |
| } |
| }; |
| } |
| |
| char TailCallElim::ID = 0; |
| INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", "Tail Call Elimination", |
| false, false) |
| INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass) |
| INITIALIZE_PASS_END(TailCallElim, "tailcallelim", "Tail Call Elimination", |
| false, false) |
| |
| // Public interface to the TailCallElimination pass |
| FunctionPass *llvm::createTailCallEliminationPass() { |
| return new TailCallElim(); |
| } |
| |
| PreservedAnalyses TailCallElimPass::run(Function &F, |
| FunctionAnalysisManager &AM) { |
| |
| TargetTransformInfo &TTI = AM.getResult<TargetIRAnalysis>(F); |
| AliasAnalysis &AA = AM.getResult<AAManager>(F); |
| auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F); |
| auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F); |
| auto *PDT = AM.getCachedResult<PostDominatorTreeAnalysis>(F); |
| // There is no noticable performance difference here between Lazy and Eager |
| // UpdateStrategy based on some test results. It is feasible to switch the |
| // UpdateStrategy to Lazy if we find it profitable later. |
| DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager); |
| bool Changed = eliminateTailRecursion(F, &TTI, &AA, &ORE, DTU); |
| |
| if (!Changed) |
| return PreservedAnalyses::all(); |
| PreservedAnalyses PA; |
| PA.preserve<GlobalsAA>(); |
| PA.preserve<DominatorTreeAnalysis>(); |
| PA.preserve<PostDominatorTreeAnalysis>(); |
| return PA; |
| } |