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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70892 91177308-0d34-0410-b5e6-96231b3b80d8
130 lines
4.9 KiB
C++
130 lines
4.9 KiB
C++
//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains routines that help determine which pointers are captured.
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// A pointer value is captured if the function makes a copy of any part of the
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// pointer that outlives the call. Not being captured means, more or less, that
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// the pointer is only dereferenced and not stored in a global. Returning part
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// of the pointer as the function return value may or may not count as capturing
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// the pointer, depending on the context.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/CaptureTracking.h"
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#include "llvm/Instructions.h"
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#include "llvm/Value.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/Support/CallSite.h"
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using namespace llvm;
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/// PointerMayBeCaptured - Return true if this pointer value may be captured
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/// by the enclosing function (which is required to exist). This routine can
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/// be expensive, so consider caching the results. The boolean ReturnCaptures
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/// specifies whether returning the value (or part of it) from the function
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/// counts as capturing it or not.
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bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
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assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
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SmallVector<Use*, 16> Worklist;
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SmallSet<Use*, 16> Visited;
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for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
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UI != UE; ++UI) {
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Use *U = &UI.getUse();
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Visited.insert(U);
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Worklist.push_back(U);
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}
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while (!Worklist.empty()) {
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Use *U = Worklist.pop_back_val();
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Instruction *I = cast<Instruction>(U->getUser());
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V = U->get();
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switch (I->getOpcode()) {
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case Instruction::Call:
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case Instruction::Invoke: {
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CallSite CS(I);
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// Not captured if only passed via 'nocapture' arguments. Note that
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// calling a function pointer does not in itself cause the pointer to
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// be captured. This is a subtle point considering that (for example)
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// the callee might return its own address. It is analogous to saying
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// that loading a value from a pointer does not cause the pointer to be
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// captured, even though the loaded value might be the pointer itself
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// (think of self-referential objects).
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bool MayBeCaptured = false;
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CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
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for (CallSite::arg_iterator A = B; A != E; ++A)
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if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture)) {
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// The parameter is not marked 'nocapture' - handled by generic code
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// below.
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MayBeCaptured = true;
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break;
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}
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if (!MayBeCaptured)
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// Only passed via 'nocapture' arguments, or is the called function -
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// not captured.
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continue;
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if (!CS.doesNotThrow())
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// Even a readonly function can leak bits by throwing an exception or
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// not depending on the input value.
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return true;
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// Fall through to the generic code.
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break;
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}
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case Instruction::Free:
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// Freeing a pointer does not cause it to be captured.
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continue;
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case Instruction::Load:
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// Loading from a pointer does not cause it to be captured.
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continue;
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case Instruction::Ret:
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if (ReturnCaptures)
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return true;
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continue;
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case Instruction::Store:
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if (V == I->getOperand(0))
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// Stored the pointer - it may be captured.
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return true;
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// Storing to the pointee does not cause the pointer to be captured.
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continue;
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}
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// If it may write to memory and isn't one of the special cases above,
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// be conservative and assume the pointer is captured.
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if (I->mayWriteToMemory())
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return true;
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// If the instruction doesn't write memory, it can only capture by
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// having its own value depend on the input value.
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const Type* Ty = I->getType();
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if (Ty == Type::VoidTy)
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// The value of an instruction can't be a copy if it can't contain any
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// information.
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continue;
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if (!isa<PointerType>(Ty))
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// At the moment, we don't track non-pointer values, so be conservative
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// and assume the pointer is captured.
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// FIXME: Track these too. This would need to be done very carefully as
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// it is easy to leak bits via control flow if integer values are allowed.
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return true;
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// The original value is not captured via this if the new value isn't.
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for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI) {
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Use *U = &UI.getUse();
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if (Visited.insert(U))
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Worklist.push_back(U);
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}
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}
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// All uses examined - not captured.
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return false;
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}
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