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//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains routines that help determine which pointers are captured.
// A pointer value is captured if the function makes a copy of any part of the
// pointer that outlives the call. Not being captured means, more or less, that
// the pointer is only dereferenced and not stored in a global. Returning part
// of the pointer as the function return value may or may not count as capturing
// the pointer, depending on the context.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Instructions.h"
#include "llvm/Value.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/CallSite.h"
using namespace llvm;
/// PointerMayBeCaptured - Return true if this pointer value may be captured
/// by the enclosing function (which is required to exist). This routine can
/// be expensive, so consider caching the results. The boolean ReturnCaptures
/// specifies whether returning the value (or part of it) from the function
/// counts as capturing it or not.
bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
SmallVector<Use*, 16> Worklist;
SmallSet<Use*, 16> Visited;
for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
UI != UE; ++UI) {
Use *U = &UI.getUse();
Visited.insert(U);
Worklist.push_back(U);
}
while (!Worklist.empty()) {
Use *U = Worklist.pop_back_val();
Instruction *I = cast<Instruction>(U->getUser());
V = U->get();
switch (I->getOpcode()) {
case Instruction::Call:
case Instruction::Invoke: {
CallSite CS = CallSite::get(I);
// Not captured if the callee is readonly, doesn't return a copy through
// its return value and doesn't unwind (a readonly function can leak bits
// by throwing an exception or not depending on the input value).
if (CS.onlyReadsMemory() && CS.doesNotThrow() &&
I->getType() == Type::getVoidTy(V->getContext()))
break;
// Not captured if only passed via 'nocapture' arguments. Note that
// calling a function pointer does not in itself cause the pointer to
// be captured. This is a subtle point considering that (for example)
// the callee might return its own address. It is analogous to saying
// 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).
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.
break;
}
case Instruction::Free:
// Freeing a pointer does not cause it to be captured.
break;
case Instruction::Load:
// Loading from a pointer does not cause it to be captured.
break;
case Instruction::Ret:
if (ReturnCaptures)
return true;
break;
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.
return true;
}
}
// All uses examined - not captured.
return false;
}
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