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Teach TailRecursionElimination to consider 'nocapture' when deciding whether

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calls can be marked tail.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166405 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2012-10-21 23:51:22 +00:00
parent 3d39fb8a3f
commit 241d1398e0
2 changed files with 69 additions and 56 deletions
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119
lib/Transforms/Scalar/TailRecursionElimination.cpp
View File

@ -69,6 +69,7 @@
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/ValueHandle.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;
@ -117,34 +118,45 @@ 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;
/// 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());
}
/// 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);
struct AllocaCaptureTracker : public CaptureTracker {
AllocaCaptureTracker() : Captured(false) {}
// 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;
}
void tooManyUses() { Captured = true; }
bool shouldExplore(Use *U) {
Value *V = U->get();
if (isa<CallInst>(V) || isa<InvokeInst>(V))
UsesAlloca.push_back(V);
return true;
}
bool captured(Use *U) {
if (isa<ReturnInst>(U->getUser()))
return false;
Captured = true;
return true;
}
SmallVector<WeakVH, 64> UsesAlloca;
bool Captured;
};
bool TailCallElim::runOnFunction(Function &F) {
// If this function is a varargs function, we won't be able to PHI the args
@ -157,38 +169,34 @@ bool TailCallElim::runOnFunction(Function &F) {
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 = true;
// 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);
// 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(I, &ACT);
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.
// Second pass, change any tail recursive calls to loops.
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs,CannotTCETailMarkedCall);
ArgumentPHIs, !CanTRETailMarkedCall);
if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
TailCallsAreMarkedTail, ArgumentPHIs,
CannotTCETailMarkedCall);
!CanTRETailMarkedCall);
MadeChange |= Change;
}
}
@ -210,21 +218,24 @@ bool TailCallElim::runOnFunction(Function &F) {
}
}
// Finally, if this function contains no non-escaping allocas, or calls
// setjmp, mark all calls in the function as eligible for tail calls
//(there is no stack memory for them to access).
// Finally, if this function contains no non-escaping allocas and doesn't
// call setjmp, mark all calls in the function as eligible for tail calls
// (there is no stack memory for them to access).
std::sort(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end());
if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice())
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (CallInst *CI = dyn_cast<CallInst>(I)) {
CI->setTailCall();
MadeChange = true;
}
if (CallInst *CI = dyn_cast<CallInst>(I))
if (!std::binary_search(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end(),
CI)) {
CI->setTailCall();
MadeChange = true;
}
return MadeChange;
}
/// CanMoveAboveCall - Return true if it is safe to move the specified
/// instruction from after the call to before the call, assuming that all
/// instructions between the call and this instruction are movable.

6
test/Transforms/TailCallElim/nocapture.ll
View File

@ -1,7 +1,7 @@
; RUN: opt %s -tailcallelim -S | FileCheck %s
; XFAIL: *
declare void @use(i8* nocapture, i8* nocapture)
declare void @boring()
define i8* @foo(i8* nocapture %A, i1 %cond) {
; CHECK: tailrecurse:
@ -19,7 +19,9 @@ cond_true:
cond_false:
; CHECK: cond_false
call void @use(i8* %A, i8* %B)
; CHECK: tail call void @use(i8* %A.tr, i8* %B)
; CHECK: call void @use(i8* %A.tr, i8* %B)
call void @boring()
; CHECK: tail call void @boring()
ret i8* null
; CHECK: ret i8* null
}