// 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/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
-#include "llvm/Support/CallSite.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/ADT/Statistic.h"
+#include "llvm/Support/CallSite.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ValueHandle.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
STATISTIC(NumEliminated, "Number of tail calls removed");
+STATISTIC(NumRetDuped, "Number of return duplicated");
STATISTIC(NumAccumAdded, "Number of accumulators introduced");
namespace {
struct TailCallElim : public FunctionPass {
+ const TargetTransformInfo *TTI;
+
static char ID; // Pass identification, replacement for typeid
TailCallElim() : FunctionPass(ID) {
initializeTailCallElimPass(*PassRegistry::getPassRegistry());
}
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+
virtual bool runOnFunction(Function &F);
private:
+ CallInst *FindTRECandidate(Instruction *I,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
+ BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<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;
-INITIALIZE_PASS(TailCallElim, "tailcallelim",
- "Tail Call Elimination", false, false)
+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();
}
-/// 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;
+void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<TargetTransformInfo>();
}
-/// 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;
+/// CanTRE - Scan the specified basic block for alloca instructions.
+/// If it contains any that are variable-sized or not in the entry block,
+/// returns false.
+static bool CanTRE(AllocaInst *AI) {
+ // Because of PR962, we don't TRE allocas outside the entry block.
+
+ // If this alloca is in the body of the function, or if it is a variable
+ // sized allocation, we cannot tail call eliminate calls marked 'tail'
+ // with this mechanism.
+ BasicBlock *BB = AI->getParent();
+ return BB == &BB->getParent()->getEntryBlock() &&
+ isa<ConstantInt>(AI->getArraySize());
}
+namespace {
+struct AllocaCaptureTracker : public CaptureTracker {
+ AllocaCaptureTracker() : Captured(false) {}
+
+ void tooManyUses() LLVM_OVERRIDE { Captured = true; }
+
+ bool shouldExplore(Use *U) LLVM_OVERRIDE {
+ Value *V = U->getUser();
+ if (isa<CallInst>(V) || isa<InvokeInst>(V))
+ UsesAlloca.insert(V);
+ return true;
+ }
+
+ bool captured(Use *U) LLVM_OVERRIDE {
+ if (isa<ReturnInst>(U->getUser()))
+ return false;
+ Captured = true;
+ return true;
+ }
+
+ bool Captured;
+ SmallPtrSet<const Value *, 16> UsesAlloca;
+};
+} // end anonymous namespace
+
bool TailCallElim::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
// If this function is a varargs function, we won't be able to PHI the args
// right, so don't even try to convert it...
if (F.getFunctionType()->isVarArg()) return false;
+ TTI = &getAnalysis<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
+ // 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;
-
- // 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);
+ bool CanTRETailMarkedCall = true;
+
+ // Find calls that can be marked tail.
+ AllocaCaptureTracker ACT;
+ for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
+ CanTRETailMarkedCall &= CanTRE(AI);
+ PointerMayBeCaptured(AI, &ACT);
+ // If any allocas are captured, exit.
+ if (ACT.Captured)
+ return false;
+ }
+ }
}
-
- /// 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);
+ // Second pass, change any tail recursive calls to loops.
+ //
+ // FIXME: The code generator produces really bad code when an 'escaping
+ // alloca' is changed from being a static alloca to being a dynamic alloca.
+ // Until this is resolved, disable this transformation if that would ever
+ // happen. This bug is PR962.
+ if (ACT.UsesAlloca.empty()) {
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
+ if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
+ bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs, !CanTRETailMarkedCall);
+ if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
+ Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
+ TailCallsAreMarkedTail, ArgumentPHIs,
+ !CanTRETailMarkedCall);
+ MadeChange |= Change;
+ }
+ }
+ }
// If we eliminated any tail recursions, it's possible that we inserted some
// silly PHI nodes which just merge an initial value (the incoming operand)
}
}
- // 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)
+ // At this point, we know that the function does not have any captured
+ // allocas. If additionally the function does not call setjmp, mark all calls
+ // in the function that do not access stack memory with the tail keyword. This
+ // implies ensuring that there does not exist any path from a call that takes
+ // in an alloca but does not capture it and the call which we wish to mark
+ // with "tail".
+ if (!F.callsFunctionThatReturnsTwice()) {
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
if (CallInst *CI = dyn_cast<CallInst>(I)) {
- CI->setTailCall();
- MadeChange = true;
+ if (!ACT.UsesAlloca.count(CI)) {
+ 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()) {
return getCommonReturnValue(cast<ReturnInst>(I->use_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 (&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;
- BasicBlock::iterator BBI = Ret;
- while (1) {
+ 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 false; // Didn't find a potential tail call.
+ 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 false;
+ 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() &&
- &BB->front() == CI && &*++BB->begin() == Ret &&
- callIsSmall(F)) {
+ 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(),
for (; I != E && FI != FE; ++I, ++FI)
if (*I != &*FI) break;
if (I == E && FI == FE)
- return false;
+ return 0;
}
+ 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
// 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) {
+ 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 tranformed
+ // 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 =
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) {
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);
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
// 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) {
+ for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
if (P == &F->getEntryBlock())
AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
// 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 (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
+ TerminatorInst *PTI = Pred->getTerminator();
+ if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
+ if (BI->isUnconditional())
+ UncondBranchPreds.push_back(BI);
+ }
+
+ while (!UncondBranchPreds.empty()) {
+ BranchInst *BI = UncondBranchPreds.pop_back_val();
+ BasicBlock *Pred = BI->getParent();
+ if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
+ DEBUG(dbgs() << "FOLDING: " << *BB
+ << "INTO UNCOND BRANCH PRED: " << *Pred);
+ EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred),
+ OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+ ++NumRetDuped;
+ Change = true;
+ }
+ }
+
+ return Change;
+}
+
+bool
+TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVectorImpl<PHINode *> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
+ if (!CI)
+ return false;
+
+ return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+}
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