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As it turns out, the capture tracker named CaptureBefore used by AA, and now available via the PointerMayBeCapturedBefore function, would have been more-aptly named CapturedBeforeOrAt, because it considers captures at the instruction provided. This is not always what one wants, and it is difficult to get the strictly-before behavior given only the current interface. This adds an additional parameter which controls whether or not you want to include captures at the provided instruction. The default is not to include the instruction provided, so that 'Before' matches its name. No functionality change intended. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213582 91177308-0d34-0410-b5e6-96231b3b80d8
271 lines
10 KiB
C++
271 lines
10 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/ADT/SmallSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/CaptureTracking.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Instructions.h"
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using namespace llvm;
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CaptureTracker::~CaptureTracker() {}
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bool CaptureTracker::shouldExplore(const Use *U) { return true; }
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namespace {
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struct SimpleCaptureTracker : public CaptureTracker {
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explicit SimpleCaptureTracker(bool ReturnCaptures)
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: ReturnCaptures(ReturnCaptures), Captured(false) {}
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void tooManyUses() override { Captured = true; }
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bool captured(const Use *U) override {
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if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
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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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bool ReturnCaptures;
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bool Captured;
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};
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/// Only find pointer captures which happen before the given instruction. Uses
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/// the dominator tree to determine whether one instruction is before another.
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/// Only support the case where the Value is defined in the same basic block
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/// as the given instruction and the use.
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struct CapturesBefore : public CaptureTracker {
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CapturesBefore(bool ReturnCaptures, const Instruction *I, DominatorTree *DT,
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bool IncludeI)
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: BeforeHere(I), DT(DT), ReturnCaptures(ReturnCaptures),
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IncludeI(IncludeI), Captured(false) {}
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void tooManyUses() override { Captured = true; }
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bool shouldExplore(const Use *U) override {
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Instruction *I = cast<Instruction>(U->getUser());
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if (BeforeHere == I && !IncludeI)
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return false;
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BasicBlock *BB = I->getParent();
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// We explore this usage only if the usage can reach "BeforeHere".
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// If use is not reachable from entry, there is no need to explore.
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if (BeforeHere != I && !DT->isReachableFromEntry(BB))
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return false;
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// If the value is defined in the same basic block as use and BeforeHere,
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// there is no need to explore the use if BeforeHere dominates use.
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// Check whether there is a path from I to BeforeHere.
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if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
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!isPotentiallyReachable(I, BeforeHere, DT))
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return false;
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return true;
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}
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bool captured(const Use *U) override {
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if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
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return false;
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Instruction *I = cast<Instruction>(U->getUser());
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if (BeforeHere == I && !IncludeI)
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return false;
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BasicBlock *BB = I->getParent();
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// Same logic as in shouldExplore.
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if (BeforeHere != I && !DT->isReachableFromEntry(BB))
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return false;
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if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
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!isPotentiallyReachable(I, BeforeHere, DT))
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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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const Instruction *BeforeHere;
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DominatorTree *DT;
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bool ReturnCaptures;
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bool IncludeI;
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bool Captured;
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};
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}
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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. The boolean StoreCaptures specified whether
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/// storing the value (or part of it) into memory anywhere automatically
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/// counts as capturing it or not.
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bool llvm::PointerMayBeCaptured(const Value *V,
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bool ReturnCaptures, bool StoreCaptures) {
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assert(!isa<GlobalValue>(V) &&
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"It doesn't make sense to ask whether a global is captured.");
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// TODO: If StoreCaptures is not true, we could do Fancy analysis
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// to determine whether this store is not actually an escape point.
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// In that case, BasicAliasAnalysis should be updated as well to
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// take advantage of this.
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(void)StoreCaptures;
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SimpleCaptureTracker SCT(ReturnCaptures);
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PointerMayBeCaptured(V, &SCT);
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return SCT.Captured;
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}
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/// PointerMayBeCapturedBefore - Return true if this pointer value may be
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/// captured by the enclosing function (which is required to exist). If a
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/// DominatorTree is provided, only captures which happen before the given
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/// instruction are considered. This routine can be expensive, so consider
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/// caching the results. The boolean ReturnCaptures specifies whether
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/// returning the value (or part of it) from the function counts as capturing
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/// it or not. The boolean StoreCaptures specified whether storing the value
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/// (or part of it) into memory anywhere automatically counts as capturing it
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/// or not.
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bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
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bool StoreCaptures, const Instruction *I,
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DominatorTree *DT, bool IncludeI) {
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assert(!isa<GlobalValue>(V) &&
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"It doesn't make sense to ask whether a global is captured.");
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if (!DT)
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return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures);
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// TODO: See comment in PointerMayBeCaptured regarding what could be done
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// with StoreCaptures.
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CapturesBefore CB(ReturnCaptures, I, DT, IncludeI);
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PointerMayBeCaptured(V, &CB);
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return CB.Captured;
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}
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/// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
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/// a cache. Then we can move the code from BasicAliasAnalysis into
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/// that path, and remove this threshold.
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static int const Threshold = 20;
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void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
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assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
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SmallVector<const Use *, Threshold> Worklist;
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SmallSet<const Use *, Threshold> Visited;
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int Count = 0;
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for (const Use &U : V->uses()) {
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// If there are lots of uses, conservatively say that the value
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// is captured to avoid taking too much compile time.
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if (Count++ >= Threshold)
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return Tracker->tooManyUses();
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if (!Tracker->shouldExplore(&U)) continue;
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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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const 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 the callee is readonly, doesn't return a copy through
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// its return value and doesn't unwind (a readonly function can leak bits
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// by throwing an exception or not depending on the input value).
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if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
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break;
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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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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.doesNotCapture(A - B))
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// The parameter is not marked 'nocapture' - captured.
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if (Tracker->captured(U))
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return;
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break;
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}
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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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break;
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case Instruction::VAArg:
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// "va-arg" from a pointer does not cause it to be captured.
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break;
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case Instruction::Store:
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if (V == I->getOperand(0))
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// Stored the pointer - conservatively assume it may be captured.
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if (Tracker->captured(U))
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return;
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// Storing to the pointee does not cause the pointer to be captured.
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break;
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case Instruction::BitCast:
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case Instruction::GetElementPtr:
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case Instruction::PHI:
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case Instruction::Select:
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case Instruction::AddrSpaceCast:
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// The original value is not captured via this if the new value isn't.
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Count = 0;
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for (Use &UU : I->uses()) {
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// If there are lots of uses, conservatively say that the value
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// is captured to avoid taking too much compile time.
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if (Count++ >= Threshold)
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return Tracker->tooManyUses();
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if (Visited.insert(&UU))
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if (Tracker->shouldExplore(&UU))
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Worklist.push_back(&UU);
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}
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break;
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case Instruction::ICmp:
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// Don't count comparisons of a no-alias return value against null as
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// captures. This allows us to ignore comparisons of malloc results
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// with null, for example.
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if (ConstantPointerNull *CPN =
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dyn_cast<ConstantPointerNull>(I->getOperand(1)))
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if (CPN->getType()->getAddressSpace() == 0)
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if (isNoAliasCall(V->stripPointerCasts()))
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break;
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// Otherwise, be conservative. There are crazy ways to capture pointers
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// using comparisons.
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if (Tracker->captured(U))
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return;
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break;
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default:
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// Something else - be conservative and say it is captured.
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if (Tracker->captured(U))
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return;
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break;
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}
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}
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// All uses examined.
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}
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