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Teach TailRecursionElimination to consider 'nocapture' when deciding whether
calls can be marked tail. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166405 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -69,6 +69,7 @@
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Support/ValueHandle.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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using namespace llvm;
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@ -117,34 +118,45 @@ FunctionPass *llvm::createTailCallEliminationPass() {
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return new TailCallElim();
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}
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/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
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/// callees of this function. We only do very simple analysis right now, this
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/// could be expanded in the future to use mod/ref information for particular
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/// call sites if desired.
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static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
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// FIXME: do simple 'address taken' analysis.
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return true;
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/// CanTRE - Scan the specified basic block for alloca instructions.
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/// If it contains any that are variable-sized or not in the entry block,
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/// returns false.
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static bool CanTRE(AllocaInst *AI) {
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// Because of PR962, we don't TRE allocas outside the entry block.
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// If this alloca is in the body of the function, or if it is a variable
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// sized allocation, we cannot tail call eliminate calls marked 'tail'
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// with this mechanism.
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BasicBlock *BB = AI->getParent();
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return BB == &BB->getParent()->getEntryBlock() &&
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isa<ConstantInt>(AI->getArraySize());
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}
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/// CheckForEscapingAllocas - Scan the specified basic block for alloca
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/// instructions. If it contains any that might be accessed by calls, return
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/// true.
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static bool CheckForEscapingAllocas(BasicBlock *BB,
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bool &CannotTCETailMarkedCall) {
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bool RetVal = false;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
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RetVal |= AllocaMightEscapeToCalls(AI);
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struct AllocaCaptureTracker : public CaptureTracker {
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AllocaCaptureTracker() : Captured(false) {}
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// If this alloca is in the body of the function, or if it is a variable
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// sized allocation, we cannot tail call eliminate calls marked 'tail'
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// with this mechanism.
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if (BB != &BB->getParent()->getEntryBlock() ||
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!isa<ConstantInt>(AI->getArraySize()))
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CannotTCETailMarkedCall = true;
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}
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return RetVal;
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}
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void tooManyUses() { Captured = true; }
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bool shouldExplore(Use *U) {
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Value *V = U->get();
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if (isa<CallInst>(V) || isa<InvokeInst>(V))
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UsesAlloca.push_back(V);
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return true;
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}
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bool captured(Use *U) {
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if (isa<ReturnInst>(U->getUser()))
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return false;
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Captured = true;
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return true;
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}
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SmallVector<WeakVH, 64> UsesAlloca;
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bool Captured;
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};
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bool TailCallElim::runOnFunction(Function &F) {
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// If this function is a varargs function, we won't be able to PHI the args
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@ -157,38 +169,34 @@ bool TailCallElim::runOnFunction(Function &F) {
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bool MadeChange = false;
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bool FunctionContainsEscapingAllocas = false;
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// CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
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// CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
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// marked with the 'tail' attribute, because doing so would cause the stack
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// size to increase (real TCE would deallocate variable sized allocas, TCE
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// size to increase (real TRE would deallocate variable sized allocas, TRE
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// doesn't).
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bool CannotTCETailMarkedCall = false;
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bool CanTRETailMarkedCall = true;
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// Loop over the function, looking for any returning blocks, and keeping track
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// of whether this function has any non-trivially used allocas.
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
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if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
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break;
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FunctionContainsEscapingAllocas |=
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CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
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// Find calls that can be marked tail.
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AllocaCaptureTracker ACT;
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for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
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if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
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CanTRETailMarkedCall &= CanTRE(AI);
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PointerMayBeCaptured(I, &ACT);
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if (ACT.Captured)
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return false;
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}
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}
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}
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/// FIXME: The code generator produces really bad code when an 'escaping
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/// alloca' is changed from being a static alloca to being a dynamic alloca.
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/// Until this is resolved, disable this transformation if that would ever
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/// happen. This bug is PR962.
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if (FunctionContainsEscapingAllocas)
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return false;
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// Second pass, change any tail calls to loops.
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// Second pass, change any tail recursive calls to loops.
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
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if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
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bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
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ArgumentPHIs,CannotTCETailMarkedCall);
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ArgumentPHIs, !CanTRETailMarkedCall);
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if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
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Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
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TailCallsAreMarkedTail, ArgumentPHIs,
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CannotTCETailMarkedCall);
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!CanTRETailMarkedCall);
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MadeChange |= Change;
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}
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}
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@ -210,21 +218,24 @@ bool TailCallElim::runOnFunction(Function &F) {
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}
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}
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// Finally, if this function contains no non-escaping allocas, or calls
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// setjmp, mark all calls in the function as eligible for tail calls
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//(there is no stack memory for them to access).
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// Finally, if this function contains no non-escaping allocas and doesn't
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// call setjmp, mark all calls in the function as eligible for tail calls
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// (there is no stack memory for them to access).
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std::sort(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end());
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if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice())
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (CallInst *CI = dyn_cast<CallInst>(I)) {
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CI->setTailCall();
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MadeChange = true;
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}
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if (CallInst *CI = dyn_cast<CallInst>(I))
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if (!std::binary_search(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end(),
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CI)) {
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CI->setTailCall();
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MadeChange = true;
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}
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return MadeChange;
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}
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/// CanMoveAboveCall - Return true if it is safe to move the specified
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/// instruction from after the call to before the call, assuming that all
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/// instructions between the call and this instruction are movable.
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@ -1,7 +1,7 @@
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; RUN: opt %s -tailcallelim -S | FileCheck %s
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; XFAIL: *
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declare void @use(i8* nocapture, i8* nocapture)
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declare void @boring()
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define i8* @foo(i8* nocapture %A, i1 %cond) {
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; CHECK: tailrecurse:
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@ -19,7 +19,9 @@ cond_true:
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cond_false:
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; CHECK: cond_false
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call void @use(i8* %A, i8* %B)
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; CHECK: tail call void @use(i8* %A.tr, i8* %B)
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; CHECK: call void @use(i8* %A.tr, i8* %B)
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call void @boring()
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; CHECK: tail call void @boring()
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ret i8* null
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; CHECK: ret i8* null
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}
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