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/CFG.h"
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#include "llvm/Analysis/CaptureTracking.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).second)
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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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