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Teach capture tracking that readonly functions can

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only capture their arguments by returning them or
throwing an exception or not based on the argument
value.  Patch essentially by Frits van Bommel.

llvm-svn: 70876
This commit is contained in:
Duncan Sands 2009-05-04 16:50:29 +00:00
parent b77e5b9e2e
commit 4c7021febf
2 changed files with 67 additions and 32 deletions
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75
lib/Analysis/CaptureTracking.cpp
View File

@ -49,11 +49,7 @@ bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
switch (I->getOpcode()) {
case Instruction::Call:
case Instruction::Invoke: {
CallSite CS = CallSite::get(I);
// Not captured if the callee is readonly and doesn't return a copy
// through its return value.
if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
break;
CallSite CS(I);
// Not captured if only passed via 'nocapture' arguments. Note that
// calling a function pointer does not in itself cause the pointer to
@ -62,46 +58,69 @@ bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
// that loading a value from a pointer does not cause the pointer to be
// captured, even though the loaded value might be the pointer itself
// (think of self-referential objects).
bool MayBeCaptured = false;
CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
for (CallSite::arg_iterator A = B; A != E; ++A)
if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
// The parameter is not marked 'nocapture' - captured.
return true;
// Only passed via 'nocapture' arguments, or is the called function - not
// captured.
if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture)) {
// The parameter is not marked 'nocapture' - handled by generic code
// below.
MayBeCaptured = true;
break;
}
if (!MayBeCaptured)
// Only passed via 'nocapture' arguments, or is the called function -
// not captured.
continue;
if (!CS.doesNotThrow())
// Even a readonly function can leak bits by throwing an exception or
// not depending on the input value.
return true;
// Fall through to the generic code.
break;
}
case Instruction::Free:
// Freeing a pointer does not cause it to be captured.
break;
continue;
case Instruction::Load:
// Loading from a pointer does not cause it to be captured.
break;
continue;
case Instruction::Ret:
if (ReturnCaptures)
return true;
break;
continue;
case Instruction::Store:
if (V == I->getOperand(0))
// Stored the pointer - it may be captured.
return true;
// Storing to the pointee does not cause the pointer to be captured.
break;
case Instruction::BitCast:
case Instruction::GetElementPtr:
case Instruction::PHI:
case Instruction::Select:
// The original value is not captured via this if the new value isn't.
for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
Use *U = &UI.getUse();
if (Visited.insert(U))
Worklist.push_back(U);
}
break;
default:
// Something else - be conservative and say it is captured.
continue;
}
// If it may write to memory and isn't one of the special cases above,
// be conservative and assume the pointer is captured.
if (I->mayWriteToMemory())
return true;
// If the instruction doesn't write memory, it can only capture by
// having its own value depend on the input value.
const Type* Ty = I->getType();
if (Ty == Type::VoidTy)
// The value of an instruction can't be a copy if it can't contain any
// information.
continue;
if (!isa<PointerType>(Ty))
// At the moment, we don't track non-pointer values, so be conservative
// and assume the pointer is captured.
// FIXME: Track these too. This would need to be done very carefully as
// it is easy to leak bits via control flow if integer values are allowed.
return true;
// The original value is not captured via this if the new value isn't.
for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
Use *U = &UI.getUse();
if (Visited.insert(U))
Worklist.push_back(U);
}
}

24
test/Transforms/FunctionAttrs/2008-12-31-NoCapture.ll
View File

@ -39,6 +39,16 @@ define i1 @c5(i32* %q, i32 %bitno) {
ret i1 %val
}
declare void @throw_if_bit_set(i8*, i8) readonly
define i1 @c6(i8* %q, i8 %bit) {
invoke void @throw_if_bit_set(i8* %q, i8 %bit)
to label %ret0 unwind label %ret1
ret0:
ret i1 0
ret1:
ret i1 1
}
define i32 @nc1(i32* %q, i32* %p, i1 %b) {
e:
br label %l
@ -63,14 +73,20 @@ define void @nc3(void ()* %p) {
ret void
}
declare void @external(i8*) readonly
declare void @external(i8*) readonly nounwind
define void @nc4(i8* %p) {
call void @external(i8* %p)
ret void
}
define void @nc5(void (i8*)* %f, i8* %p) {
call void %f(i8* %p) readonly
call void %f(i8* nocapture %p)
define void @nc5(void (i8*)* %p, i8* %r) {
call void %p(i8* %r)
call void %p(i8* nocapture %r)
ret void
}
declare i8* @external_identity(i8*) readonly nounwind
define void @nc6(i8* %p) {
call i8* @external_identity(i8* %p)
ret void
}