// transformation from taking place, though currently the analysis cannot
// support moving any really useful instructions (only dead ones).
// 2. This pass transforms functions that are prevented from being tail
-// recursive by an associative expression to use an accumulator variable,
-// thus compiling the typical naive factorial or 'fib' implementation into
-// efficient code.
+// recursive by an associative and commutative expression to use an
+// accumulator variable, thus compiling the typical naive factorial or
+// 'fib' implementation into efficient code.
// 3. TRE is performed if the function returns void, if the return
// returns the result returned by the call, or if the function returns a
// run-time constant on all exits from the function. It is possible, though
// evaluated each time through the tail recursion. Safely keeping allocas
// in the entry block requires analysis to proves that the tail-called
// function does not read or write the stack object.
-// 2. Tail recursion is only performed if the call immediately preceeds the
+// 2. Tail recursion is only performed if the call immediately precedes the
// return instruction. It's possible that there could be a jump between
// the call and the return.
// 3. There can be intervening operations between the call and the return that
//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "tailcallelim"
#include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Constants.h"
-#include "llvm/DerivedTypes.h"
-#include "llvm/Function.h"
-#include "llvm/Instructions.h"
-#include "llvm/Pass.h"
-#include "llvm/Analysis/CaptureTracking.h"
-#include "llvm/Support/CFG.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/InlineCost.h"
+#include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/CallSite.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/ValueHandle.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
+#define DEBUG_TYPE "tailcallelim"
+
STATISTIC(NumEliminated, "Number of tail calls removed");
+STATISTIC(NumRetDuped, "Number of return duplicated");
STATISTIC(NumAccumAdded, "Number of accumulators introduced");
namespace {
struct TailCallElim : public FunctionPass {
+ const TargetTransformInfo *TTI;
+
static char ID; // Pass identification, replacement for typeid
- TailCallElim() : FunctionPass(&ID) {}
+ TailCallElim() : FunctionPass(ID) {
+ initializeTailCallElimPass(*PassRegistry::getPassRegistry());
+ }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override;
- virtual bool runOnFunction(Function &F);
+ bool runOnFunction(Function &F) override;
private:
+ bool runTRE(Function &F);
+ bool markTails(Function &F, bool &AllCallsAreTailCalls);
+
+ CallInst *FindTRECandidate(Instruction *I,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
+ BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool FoldReturnAndProcessPred(BasicBlock *BB,
+ ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail);
bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
bool &TailCallsAreMarkedTail,
- SmallVector<PHINode*, 8> &ArgumentPHIs,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
bool CannotTailCallElimCallsMarkedTail);
bool CanMoveAboveCall(Instruction *I, CallInst *CI);
Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
}
char TailCallElim::ID = 0;
-static RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination");
+INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim",
+ "Tail Call Elimination", false, false)
+INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
+INITIALIZE_PASS_END(TailCallElim, "tailcallelim",
+ "Tail Call Elimination", false, false)
// Public interface to the TailCallElimination pass
FunctionPass *llvm::createTailCallEliminationPass() {
return new TailCallElim();
}
-/// 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 |= PointerMayBeCaptured(AI, true);
-
- // 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;
+void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<TargetTransformInfo>();
+}
+
+/// \brief Scan the specified function for alloca instructions.
+/// If it contains any dynamic allocas, returns false.
+static bool CanTRE(Function &F) {
+ // Because of PR962, we don't TRE dynamic allocas.
+ for (auto &BB : F) {
+ for (auto &I : BB) {
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
+ if (!AI->isStaticAlloca())
+ return false;
+ }
}
- return RetVal;
+ }
+
+ return true;
}
bool TailCallElim::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
+ bool AllCallsAreTailCalls = false;
+ bool Modified = markTails(F, AllCallsAreTailCalls);
+ if (AllCallsAreTailCalls)
+ Modified |= runTRE(F);
+ return Modified;
+}
+
+namespace {
+struct AllocaDerivedValueTracker {
+ // Start at a root value and walk its use-def chain to mark calls that use the
+ // value or a derived value in AllocaUsers, and places where it may escape in
+ // EscapePoints.
+ void walk(Value *Root) {
+ SmallVector<Use *, 32> Worklist;
+ SmallPtrSet<Use *, 32> Visited;
+
+ auto AddUsesToWorklist = [&](Value *V) {
+ for (auto &U : V->uses()) {
+ if (!Visited.insert(&U))
+ continue;
+ Worklist.push_back(&U);
+ }
+ };
+
+ AddUsesToWorklist(Root);
+
+ while (!Worklist.empty()) {
+ Use *U = Worklist.pop_back_val();
+ Instruction *I = cast<Instruction>(U->getUser());
+
+ switch (I->getOpcode()) {
+ case Instruction::Call:
+ case Instruction::Invoke: {
+ CallSite CS(I);
+ bool IsNocapture = !CS.isCallee(U) &&
+ CS.doesNotCapture(CS.getArgumentNo(U));
+ callUsesLocalStack(CS, IsNocapture);
+ if (IsNocapture) {
+ // If the alloca-derived argument is passed in as nocapture, then it
+ // can't propagate to the call's return. That would be capturing.
+ continue;
+ }
+ break;
+ }
+ case Instruction::Load: {
+ // The result of a load is not alloca-derived (unless an alloca has
+ // otherwise escaped, but this is a local analysis).
+ continue;
+ }
+ case Instruction::Store: {
+ if (U->getOperandNo() == 0)
+ EscapePoints.insert(I);
+ continue; // Stores have no users to analyze.
+ }
+ case Instruction::BitCast:
+ case Instruction::GetElementPtr:
+ case Instruction::PHI:
+ case Instruction::Select:
+ case Instruction::AddrSpaceCast:
+ break;
+ default:
+ EscapePoints.insert(I);
+ break;
+ }
+
+ AddUsesToWorklist(I);
+ }
+ }
+
+ void callUsesLocalStack(CallSite CS, bool IsNocapture) {
+ // Add it to the list of alloca users. If it's already there, skip further
+ // processing.
+ if (!AllocaUsers.insert(CS.getInstruction()))
+ return;
+
+ // If it's nocapture then it can't capture the alloca.
+ if (IsNocapture)
+ return;
+
+ // If it can write to memory, it can leak the alloca value.
+ if (!CS.onlyReadsMemory())
+ EscapePoints.insert(CS.getInstruction());
+ }
+
+ SmallPtrSet<Instruction *, 32> AllocaUsers;
+ SmallPtrSet<Instruction *, 32> EscapePoints;
+};
+}
+
+bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
+ if (F.callsFunctionThatReturnsTwice())
+ return false;
+ AllCallsAreTailCalls = true;
+
+ // The local stack holds all alloca instructions and all byval arguments.
+ AllocaDerivedValueTracker Tracker;
+ for (Argument &Arg : F.args()) {
+ if (Arg.hasByValAttr())
+ Tracker.walk(&Arg);
+ }
+ for (auto &BB : F) {
+ for (auto &I : BB)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
+ Tracker.walk(AI);
+ }
+
+ bool Modified = false;
+
+ // Track whether a block is reachable after an alloca has escaped. Blocks that
+ // contain the escaping instruction will be marked as being visited without an
+ // escaped alloca, since that is how the block began.
+ enum VisitType {
+ UNVISITED,
+ UNESCAPED,
+ ESCAPED
+ };
+ DenseMap<BasicBlock *, VisitType> Visited;
+
+ // We propagate the fact that an alloca has escaped from block to successor.
+ // Visit the blocks that are propagating the escapedness first. To do this, we
+ // maintain two worklists.
+ SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
+
+ // We may enter a block and visit it thinking that no alloca has escaped yet,
+ // then see an escape point and go back around a loop edge and come back to
+ // the same block twice. Because of this, we defer setting tail on calls when
+ // we first encounter them in a block. Every entry in this list does not
+ // statically use an alloca via use-def chain analysis, but may find an alloca
+ // through other means if the block turns out to be reachable after an escape
+ // point.
+ SmallVector<CallInst *, 32> DeferredTails;
+
+ BasicBlock *BB = &F.getEntryBlock();
+ VisitType Escaped = UNESCAPED;
+ do {
+ for (auto &I : *BB) {
+ if (Tracker.EscapePoints.count(&I))
+ Escaped = ESCAPED;
+
+ CallInst *CI = dyn_cast<CallInst>(&I);
+ if (!CI || CI->isTailCall())
+ continue;
+
+ if (CI->doesNotAccessMemory()) {
+ // A call to a readnone function whose arguments are all things computed
+ // outside this function can be marked tail. Even if you stored the
+ // alloca address into a global, a readnone function can't load the
+ // global anyhow.
+ //
+ // Note that this runs whether we know an alloca has escaped or not. If
+ // it has, then we can't trust Tracker.AllocaUsers to be accurate.
+ bool SafeToTail = true;
+ for (auto &Arg : CI->arg_operands()) {
+ if (isa<Constant>(Arg.getUser()))
+ continue;
+ if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
+ if (!A->hasByValAttr())
+ continue;
+ SafeToTail = false;
+ break;
+ }
+ if (SafeToTail) {
+ emitOptimizationRemark(
+ F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
+ "marked this readnone call a tail call candidate");
+ CI->setTailCall();
+ Modified = true;
+ continue;
+ }
+ }
+
+ if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
+ DeferredTails.push_back(CI);
+ } else {
+ AllCallsAreTailCalls = false;
+ }
+ }
+
+ for (auto *SuccBB : successors(BB)) {
+ auto &State = Visited[SuccBB];
+ if (State < Escaped) {
+ State = Escaped;
+ if (State == ESCAPED)
+ WorklistEscaped.push_back(SuccBB);
+ else
+ WorklistUnescaped.push_back(SuccBB);
+ }
+ }
+
+ if (!WorklistEscaped.empty()) {
+ BB = WorklistEscaped.pop_back_val();
+ Escaped = ESCAPED;
+ } else {
+ BB = nullptr;
+ while (!WorklistUnescaped.empty()) {
+ auto *NextBB = WorklistUnescaped.pop_back_val();
+ if (Visited[NextBB] == UNESCAPED) {
+ BB = NextBB;
+ Escaped = UNESCAPED;
+ break;
+ }
+ }
+ }
+ } while (BB);
+
+ for (CallInst *CI : DeferredTails) {
+ if (Visited[CI->getParent()] != ESCAPED) {
+ // If the escape point was part way through the block, calls after the
+ // escape point wouldn't have been put into DeferredTails.
+ emitOptimizationRemark(F.getContext(), "tailcallelim", F,
+ CI->getDebugLoc(),
+ "marked this call a tail call candidate");
+ CI->setTailCall();
+ Modified = true;
+ } else {
+ AllCallsAreTailCalls = false;
+ }
+ }
+
+ return Modified;
+}
+
+bool TailCallElim::runTRE(Function &F) {
// If this function is a varargs function, we won't be able to PHI the args
// right, so don't even try to convert it...
if (F.getFunctionType()->isVarArg()) return false;
- BasicBlock *OldEntry = 0;
+ TTI = &getAnalysis<TargetTransformInfo>();
+ BasicBlock *OldEntry = nullptr;
bool TailCallsAreMarkedTail = false;
SmallVector<PHINode*, 8> ArgumentPHIs;
bool MadeChange = false;
- bool FunctionContainsEscapingAllocas = false;
-
- // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
+ // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
// marked with the 'tail' attribute, because doing so would cause the stack
- // size to increase (real TCE would deallocate variable sized allocas, TCE
+ // size to increase (real TRE would deallocate variable sized allocas, TRE
// doesn't).
- bool CannotTCETailMarkedCall = false;
+ bool CanTRETailMarkedCall = CanTRE(F);
- // Loop over the function, looking for any returning blocks, and keeping track
- // of whether this function has any non-trivially used allocas.
+ // Change any tail recursive calls to loops.
+ //
+ // FIXME: The code generator produces really bad code when an 'escaping
+ // alloca' is changed from being a static alloca to being a dynamic alloca.
+ // Until this is resolved, disable this transformation if that would ever
+ // happen. This bug is PR962.
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
- if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
- break;
-
- FunctionContainsEscapingAllocas |=
- CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
+ if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
+ bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs, !CanTRETailMarkedCall);
+ if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
+ Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
+ TailCallsAreMarkedTail, ArgumentPHIs,
+ !CanTRETailMarkedCall);
+ MadeChange |= Change;
+ }
}
-
- /// 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()))
- MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
- ArgumentPHIs,CannotTCETailMarkedCall);
// If we eliminated any tail recursions, it's possible that we inserted some
// silly PHI nodes which just merge an initial value (the incoming operand)
// with themselves. Check to see if we did and clean up our mess if so. This
// occurs when a function passes an argument straight through to its tail
// call.
- if (!ArgumentPHIs.empty()) {
- for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
- PHINode *PN = ArgumentPHIs[i];
-
- // If the PHI Node is a dynamic constant, replace it with the value it is.
- if (Value *PNV = PN->hasConstantValue()) {
- PN->replaceAllUsesWith(PNV);
- PN->eraseFromParent();
- }
+ for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
+ PHINode *PN = ArgumentPHIs[i];
+
+ // If the PHI Node is a dynamic constant, replace it with the value it is.
+ if (Value *PNV = SimplifyInstruction(PN)) {
+ PN->replaceAllUsesWith(PNV);
+ PN->eraseFromParent();
}
}
- // Finally, if this function contains no non-escaping allocas, mark all calls
- // in the function as eligible for tail calls (there is no stack memory for
- // them to access).
- if (!FunctionContainsEscapingAllocas)
- for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
- for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
- if (CallInst *CI = dyn_cast<CallInst>(I)) {
- CI->setTailCall();
- MadeChange = true;
- }
-
return MadeChange;
}
// 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()) {
// FIXME: Writes to memory only matter if they may alias the pointer
// being loaded from.
if (CI->mayWriteToMemory() ||
- !isSafeToLoadUnconditionally(L->getPointerOperand(), L))
+ !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
+ L->getAlignment()))
return false;
}
}
// We currently handle static constants and arguments that are not modified as
// part of the recursion.
//
-static bool isDynamicConstant(Value *V, CallInst *CI) {
+static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
if (isa<Constant>(V)) return true; // Static constants are always dyn consts
// Check to see if this is an immutable argument, if so, the value
// If we are passing this argument into call as the corresponding
// argument operand, then the argument is dynamically constant.
// Otherwise, we cannot transform this function safely.
- if (CI->getOperand(ArgNo+1) == Arg)
+ if (CI->getArgOperand(ArgNo) == Arg)
return true;
}
+
+ // Switch cases are always constant integers. If the value is being switched
+ // on and the return is only reachable from one of its cases, it's
+ // effectively constant.
+ if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
+ if (SwitchInst *SI = dyn_cast<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
-// return instruction and tail call consistently returns the same
-// runtime-constant value at all exit points. If so, return the returned value.
+// tail call consistently returns the same runtime-constant value at all exit
+// points except for IgnoreRI. If so, return the returned value.
//
-static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
- Function *F = TheRI->getParent()->getParent();
- Value *ReturnedValue = 0;
-
- for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
- if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
- if (RI != TheRI) {
- Value *RetOp = RI->getOperand(0);
-
- // We can only perform this transformation if the value returned is
- // evaluatable at the start of the initial invocation of the function,
- // instead of at the end of the evaluation.
- //
- if (!isDynamicConstant(RetOp, CI))
- return 0;
-
- if (ReturnedValue && RetOp != ReturnedValue)
- return 0; // Cannot transform if differing values are returned.
- ReturnedValue = RetOp;
- }
+static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
+ Function *F = CI->getParent()->getParent();
+ Value *ReturnedValue = nullptr;
+
+ for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
+ ReturnInst *RI = dyn_cast<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;
}
///
Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
CallInst *CI) {
- if (!I->isAssociative()) return 0;
+ if (!I->isAssociative() || !I->isCommutative()) return nullptr;
assert(I->getNumOperands() == 2 &&
- "Associative operations should have 2 args!");
+ "Associative/commutative operations should have 2 args!");
- // Exactly one operand should be the result of the call instruction...
+ // Exactly one operand should be the result of the call instruction.
if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
(I->getOperand(0) != CI && I->getOperand(1) != CI))
- return 0;
+ return nullptr;
// The only user of this instruction we allow is a single return instruction.
- if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
- return 0;
+ 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->use_back()), CI);
+ return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
}
-bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
- bool &TailCallsAreMarkedTail,
- SmallVector<PHINode*, 8> &ArgumentPHIs,
- bool CannotTailCallElimCallsMarkedTail) {
- BasicBlock *BB = Ret->getParent();
+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() == Ret) // Make sure there is something before the ret...
- return false;
-
- // If the return is in the entry block, then making this transformation would
- // turn infinite recursion into an infinite loop. This transformation is ok
- // in theory, but breaks some code like:
- // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
- // disable this xform in this case, because the code generator will lower the
- // call to fabs into inline code.
- if (BB == &F->getEntryBlock())
- return false;
+ if (&BB->front() == TI) // Make sure there is something before the terminator.
+ return nullptr;
// Scan backwards from the return, checking to see if there is a tail call in
// this block. If so, set CI to it.
- CallInst *CI;
- BasicBlock::iterator BBI = Ret;
- while (1) {
+ CallInst *CI = nullptr;
+ BasicBlock::iterator BBI = TI;
+ while (true) {
CI = dyn_cast<CallInst>(BBI);
if (CI && CI->getCalledFunction() == F)
break;
if (BBI == BB->begin())
- return false; // Didn't find a potential tail call.
+ return nullptr; // Didn't find a potential tail call.
--BBI;
}
// If this call is marked as a tail call, and if there are dynamic allocas in
// the function, we cannot perform this optimization.
if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
- return false;
+ return nullptr;
- // If we are introducing accumulator recursion to eliminate associative
- // operations after the call instruction, this variable contains the initial
- // value for the accumulator. If this value is set, we actually perform
- // accumulator recursion elimination instead of simple tail recursion
- // elimination.
- Value *AccumulatorRecursionEliminationInitVal = 0;
- Instruction *AccumulatorRecursionInstr = 0;
+ // As a special case, detect code like this:
+ // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
+ // and disable this xform in this case, because the code generator will
+ // lower the call to fabs into inline code.
+ if (BB == &F->getEntryBlock() &&
+ FirstNonDbg(BB->front()) == CI &&
+ FirstNonDbg(std::next(BB->begin())) == TI &&
+ CI->getCalledFunction() &&
+ !TTI->isLoweredToCall(CI->getCalledFunction())) {
+ // A single-block function with just a call and a return. Check that
+ // the arguments match.
+ CallSite::arg_iterator I = CallSite(CI).arg_begin(),
+ E = CallSite(CI).arg_end();
+ Function::arg_iterator FI = F->arg_begin(),
+ FE = F->arg_end();
+ for (; I != E && FI != FE; ++I, ++FI)
+ if (*I != &*FI) break;
+ if (I == E && FI == FE)
+ return nullptr;
+ }
+
+ return CI;
+}
+
+bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
+ BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ // If we are introducing accumulator recursion to eliminate operations after
+ // the call instruction that are both associative and commutative, the initial
+ // value for the accumulator is placed in this variable. If this value is set
+ // then we actually perform accumulator recursion elimination instead of
+ // simple tail recursion elimination. If the operation is an LLVM instruction
+ // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
+ // we are handling the case when the return instruction returns a constant C
+ // which is different to the constant returned by other return instructions
+ // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
+ // special case of accumulator recursion, the operation being "return C".
+ Value *AccumulatorRecursionEliminationInitVal = nullptr;
+ Instruction *AccumulatorRecursionInstr = nullptr;
// Ok, we found a potential tail call. We can currently only transform the
// tail call if all of the instructions between the call and the return are
// movable to above the call itself, leaving the call next to the return.
// Check that this is the case now.
- for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
- if (!CanMoveAboveCall(BBI, CI)) {
- // If we can't move the instruction above the call, it might be because it
- // is an associative operation that could be tranformed using accumulator
- // recursion elimination. Check to see if this is the case, and if so,
- // remember the initial accumulator value for later.
- if ((AccumulatorRecursionEliminationInitVal =
- CanTransformAccumulatorRecursion(BBI, CI))) {
- // Yes, this is accumulator recursion. Remember which instruction
- // accumulates.
- AccumulatorRecursionInstr = BBI;
- } else {
- return false; // Otherwise, we cannot eliminate the tail recursion!
- }
+ BasicBlock::iterator BBI = CI;
+ for (++BBI; &*BBI != Ret; ++BBI) {
+ if (CanMoveAboveCall(BBI, CI)) continue;
+
+ // If we can't move the instruction above the call, it might be because it
+ // is an associative and commutative operation that could be transformed
+ // using accumulator recursion elimination. Check to see if this is the
+ // case, and if so, remember the initial accumulator value for later.
+ if ((AccumulatorRecursionEliminationInitVal =
+ CanTransformAccumulatorRecursion(BBI, CI))) {
+ // Yes, this is accumulator recursion. Remember which instruction
+ // accumulates.
+ AccumulatorRecursionInstr = BBI;
+ } else {
+ return false; // Otherwise, we cannot eliminate the tail recursion!
}
+ }
// We can only transform call/return pairs that either ignore the return value
// of the call and return void, ignore the value of the call and return a
// accumulator recursion variable eliminated.
if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
!isa<UndefValue>(Ret->getReturnValue()) &&
- AccumulatorRecursionEliminationInitVal == 0 &&
- !getCommonReturnValue(Ret, CI))
- return false;
+ AccumulatorRecursionEliminationInitVal == nullptr &&
+ !getCommonReturnValue(nullptr, CI)) {
+ // One case remains that we are able to handle: the current return
+ // instruction returns a constant, and all other return instructions
+ // return a different constant.
+ if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
+ return false; // Current return instruction does not return a constant.
+ // Check that all other return instructions return a common constant. If
+ // so, record it in AccumulatorRecursionEliminationInitVal.
+ AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
+ if (!AccumulatorRecursionEliminationInitVal)
+ return false;
+ }
+
+ BasicBlock *BB = Ret->getParent();
+ Function *F = BB->getParent();
+
+ emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(),
+ "transforming tail recursion to loop");
// OK! We can transform this tail call. If this is the first one found,
// create the new entry block, allowing us to branch back to the old entry.
- if (OldEntry == 0) {
+ if (!OldEntry) {
OldEntry = &F->getEntryBlock();
BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
NewEntry->takeName(OldEntry);
Instruction *InsertPos = OldEntry->begin();
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
I != E; ++I) {
- PHINode *PN = PHINode::Create(I->getType(),
+ PHINode *PN = PHINode::Create(I->getType(), 2,
I->getName() + ".tr", InsertPos);
I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
PN->addIncoming(I, NewEntry);
// Ok, now that we know we have a pseudo-entry block WITH all of the
// required PHI nodes, add entries into the PHI node for the actual
// parameters passed into the tail-recursive call.
- for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
- ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
+ for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
+ ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
// If we are introducing an accumulator variable to eliminate the recursion,
// do so now. Note that we _know_ that no subsequent tail recursion
if (AccumulatorRecursionEliminationInitVal) {
Instruction *AccRecInstr = AccumulatorRecursionInstr;
// Start by inserting a new PHI node for the accumulator.
- PHINode *AccPN = PHINode::Create(AccRecInstr->getType(), "accumulator.tr",
- OldEntry->begin());
+ pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
+ PHINode *AccPN =
+ PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
+ std::distance(PB, PE) + 1,
+ "accumulator.tr", OldEntry->begin());
// Loop over all of the predecessors of the tail recursion block. For the
// real entry into the function we seed the PHI with the initial value,
// other tail recursions eliminated) the accumulator is not modified.
// Because we haven't added the branch in the current block to OldEntry yet,
// it will not show up as a predecessor.
- for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
- PI != PE; ++PI) {
- if (*PI == &F->getEntryBlock())
- AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
+ for (pred_iterator PI = PB; PI != PE; ++PI) {
+ BasicBlock *P = *PI;
+ if (P == &F->getEntryBlock())
+ AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
else
- AccPN->addIncoming(AccPN, *PI);
+ AccPN->addIncoming(AccPN, P);
}
- // Add an incoming argument for the current block, which is computed by our
- // associative accumulator instruction.
- AccPN->addIncoming(AccRecInstr, BB);
-
- // Next, rewrite the accumulator recursion instruction so that it does not
- // use the result of the call anymore, instead, use the PHI node we just
- // inserted.
- AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
+ if (AccRecInstr) {
+ // Add an incoming argument for the current block, which is computed by
+ // our associative and commutative accumulator instruction.
+ AccPN->addIncoming(AccRecInstr, BB);
+
+ // Next, rewrite the accumulator recursion instruction so that it does not
+ // use the result of the call anymore, instead, use the PHI node we just
+ // inserted.
+ AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
+ } else {
+ // Add an incoming argument for the current block, which is just the
+ // constant returned by the current return instruction.
+ AccPN->addIncoming(Ret->getReturnValue(), BB);
+ }
// Finally, rewrite any return instructions in the program to return the PHI
// node instead of the "initval" that they do currently. This loop will
// Now that all of the PHI nodes are in place, remove the call and
// ret instructions, replacing them with an unconditional branch.
- BranchInst::Create(OldEntry, Ret);
+ BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
+ NewBI->setDebugLoc(CI->getDebugLoc());
+
BB->getInstList().erase(Ret); // Remove return.
BB->getInstList().erase(CI); // Remove call.
++NumEliminated;
return true;
}
+
+bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB,
+ ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ bool Change = false;
+
+ // If the return block contains nothing but the return and PHI's,
+ // there might be an opportunity to duplicate the return in its
+ // predecessors and perform TRC there. Look for predecessors that end
+ // in unconditional branch and recursive call(s).
+ SmallVector<BranchInst*, 8> UncondBranchPreds;
+ for (BasicBlock *Pred : predecessors(BB)) {
+ TerminatorInst *PTI = Pred->getTerminator();
+ if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
+ if (BI->isUnconditional())
+ UncondBranchPreds.push_back(BI);
+ }
+
+ while (!UncondBranchPreds.empty()) {
+ BranchInst *BI = UncondBranchPreds.pop_back_val();
+ BasicBlock *Pred = BI->getParent();
+ if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
+ DEBUG(dbgs() << "FOLDING: " << *BB
+ << "INTO UNCOND BRANCH PRED: " << *Pred);
+ EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred),
+ OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+ ++NumRetDuped;
+ Change = true;
+ }
+ }
+
+ return Change;
+}
+
+bool
+TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
+ if (!CI)
+ return false;
+
+ return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+}
projects / oota-llvm.git / blobdiff