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455 lines
17 KiB
455 lines
17 KiB
//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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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/ADT/SmallSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.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/ValueTracking.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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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/Support/CommandLine.h"
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using namespace llvm;
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#define DEBUG_TYPE "capture-tracking"
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STATISTIC(NumCaptured, "Number of pointers maybe captured");
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STATISTIC(NumNotCaptured, "Number of pointers not captured");
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STATISTIC(NumCapturedBefore, "Number of pointers maybe captured before");
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STATISTIC(NumNotCapturedBefore, "Number of pointers not captured before");
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/// The default value for MaxUsesToExplore argument. It's relatively small to
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/// keep the cost of analysis reasonable for clients like BasicAliasAnalysis,
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/// where the results can't be cached.
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/// TODO: we should probably introduce a caching CaptureTracking analysis and
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/// use it where possible. The caching version can use much higher limit or
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/// don't have this cap at all.
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static cl::opt<unsigned>
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DefaultMaxUsesToExplore("capture-tracking-max-uses-to-explore", cl::Hidden,
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cl::desc("Maximal number of uses to explore."),
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cl::init(20));
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unsigned llvm::getDefaultMaxUsesToExploreForCaptureTracking() {
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return DefaultMaxUsesToExplore;
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}
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CaptureTracker::~CaptureTracker() {}
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bool CaptureTracker::shouldExplore(const Use *U) { return true; }
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bool CaptureTracker::isDereferenceableOrNull(Value *O, const DataLayout &DL) {
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// An inbounds GEP can either be a valid pointer (pointing into
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// or to the end of an allocation), or be null in the default
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// address space. So for an inbounds GEP there is no way to let
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// the pointer escape using clever GEP hacking because doing so
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// would make the pointer point outside of the allocated object
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// and thus make the GEP result a poison value. Similarly, other
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// dereferenceable pointers cannot be manipulated without producing
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// poison.
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if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
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if (GEP->isInBounds())
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return true;
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bool CanBeNull;
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return O->getPointerDereferenceableBytes(DL, CanBeNull);
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}
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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, const DominatorTree *DT,
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bool IncludeI)
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: BeforeHere(I), DT(DT),
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ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
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void tooManyUses() override { Captured = true; }
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bool isSafeToPrune(Instruction *I) {
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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 true;
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// Compute the case where both instructions are inside the same basic
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// block.
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if (BB == BeforeHere->getParent()) {
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// 'I' dominates 'BeforeHere' => not safe to prune.
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//
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// The value defined by an invoke dominates an instruction only
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// if it dominates every instruction in UseBB. A PHI is dominated only
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// if the instruction dominates every possible use in the UseBB. Since
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// UseBB == BB, avoid pruning.
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if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
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return false;
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if (!BeforeHere->comesBefore(I))
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return false;
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// 'BeforeHere' comes before 'I', it's safe to prune if we also
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// guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
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// by its successors, i.e, prune if:
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//
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// (1) BB is an entry block or have no successors.
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// (2) There's no path coming back through BB successors.
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if (BB == &BB->getParent()->getEntryBlock() ||
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!BB->getTerminator()->getNumSuccessors())
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return true;
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SmallVector<BasicBlock*, 32> Worklist;
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Worklist.append(succ_begin(BB), succ_end(BB));
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return !isPotentiallyReachableFromMany(Worklist, BB, nullptr, DT);
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}
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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, nullptr, DT))
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return true;
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return false;
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}
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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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if (isSafeToPrune(I))
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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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Captured = true;
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return true;
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}
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const Instruction *BeforeHere;
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const 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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unsigned MaxUsesToExplore) {
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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, MaxUsesToExplore);
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if (SCT.Captured)
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++NumCaptured;
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else
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++NumNotCaptured;
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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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const DominatorTree *DT, bool IncludeI,
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unsigned MaxUsesToExplore) {
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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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MaxUsesToExplore);
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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, MaxUsesToExplore);
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if (CB.Captured)
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++NumCapturedBefore;
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else
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++NumNotCapturedBefore;
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return CB.Captured;
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}
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void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker,
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unsigned MaxUsesToExplore) {
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assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
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if (MaxUsesToExplore == 0)
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MaxUsesToExplore = DefaultMaxUsesToExplore;
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SmallVector<const Use *, 20> Worklist;
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Worklist.reserve(getDefaultMaxUsesToExploreForCaptureTracking());
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SmallSet<const Use *, 20> Visited;
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auto AddUses = [&](const Value *V) {
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unsigned 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++ >= MaxUsesToExplore) {
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Tracker->tooManyUses();
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return false;
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}
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if (!Visited.insert(&U).second)
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continue;
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if (!Tracker->shouldExplore(&U))
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continue;
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Worklist.push_back(&U);
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}
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return true;
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};
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if (!AddUses(V))
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return;
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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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switch (I->getOpcode()) {
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case Instruction::Call:
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case Instruction::Invoke: {
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auto *Call = cast<CallBase>(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 (Call->onlyReadsMemory() && Call->doesNotThrow() &&
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Call->getType()->isVoidTy())
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break;
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// The pointer is not captured if returned pointer is not captured.
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// NOTE: CaptureTracking users should not assume that only functions
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// marked with nocapture do not capture. This means that places like
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// getUnderlyingObject in ValueTracking or DecomposeGEPExpression
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// in BasicAA also need to know about this property.
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if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call,
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true)) {
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if (!AddUses(Call))
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return;
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break;
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}
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// Volatile operations effectively capture the memory location that they
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// load and store to.
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if (auto *MI = dyn_cast<MemIntrinsic>(Call))
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if (MI->isVolatile())
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if (Tracker->captured(U))
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return;
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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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if (Call->isDataOperand(U) &&
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!Call->doesNotCapture(Call->getDataOperandNo(U))) {
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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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}
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break;
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}
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case Instruction::Load:
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// Volatile loads make the address observable.
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if (cast<LoadInst>(I)->isVolatile())
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if (Tracker->captured(U))
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return;
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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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// Stored the pointer - conservatively assume it may be captured.
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// Volatile stores make the address observable.
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if (U->getOperandNo() == 0 || cast<StoreInst>(I)->isVolatile())
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if (Tracker->captured(U))
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return;
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break;
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case Instruction::AtomicRMW: {
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// atomicrmw conceptually includes both a load and store from
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// the same location.
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// As with a store, the location being accessed is not captured,
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// but the value being stored is.
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// Volatile stores make the address observable.
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auto *ARMWI = cast<AtomicRMWInst>(I);
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if (U->getOperandNo() == 1 || ARMWI->isVolatile())
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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::AtomicCmpXchg: {
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// cmpxchg conceptually includes both a load and store from
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// the same location.
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// As with a store, the location being accessed is not captured,
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// but the value being stored is.
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// Volatile stores make the address observable.
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auto *ACXI = cast<AtomicCmpXchgInst>(I);
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if (U->getOperandNo() == 1 || U->getOperandNo() == 2 ||
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ACXI->isVolatile())
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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::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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if (!AddUses(I))
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return;
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break;
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case Instruction::ICmp: {
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unsigned Idx = U->getOperandNo();
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unsigned OtherIdx = 1 - Idx;
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if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) {
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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 (CPN->getType()->getAddressSpace() == 0)
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if (isNoAliasCall(U->get()->stripPointerCasts()))
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break;
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if (!I->getFunction()->nullPointerIsDefined()) {
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auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation();
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// Comparing a dereferenceable_or_null pointer against null cannot
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// lead to pointer escapes, because if it is not null it must be a
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// valid (in-bounds) pointer.
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if (Tracker->isDereferenceableOrNull(O, I->getModule()->getDataLayout()))
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break;
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}
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}
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// Comparison against value stored in global variable. Given the pointer
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// does not escape, its value cannot be guessed and stored separately in a
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// global variable.
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auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx));
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if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
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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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}
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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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bool llvm::isNonEscapingLocalObject(
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const Value *V, SmallDenseMap<const Value *, bool, 8> *IsCapturedCache) {
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SmallDenseMap<const Value *, bool, 8>::iterator CacheIt;
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if (IsCapturedCache) {
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bool Inserted;
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std::tie(CacheIt, Inserted) = IsCapturedCache->insert({V, false});
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if (!Inserted)
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// Found cached result, return it!
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return CacheIt->second;
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}
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// If this is a local allocation, check to see if it escapes.
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if (isa<AllocaInst>(V) || isNoAliasCall(V)) {
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// Set StoreCaptures to True so that we can assume in our callers that the
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// pointer is not the result of a load instruction. Currently
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// PointerMayBeCaptured doesn't have any special analysis for the
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// StoreCaptures=false case; if it did, our callers could be refined to be
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// more precise.
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auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
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if (IsCapturedCache)
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CacheIt->second = Ret;
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return Ret;
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}
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// If this is an argument that corresponds to a byval or noalias argument,
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// then it has not escaped before entering the function. Check if it escapes
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// inside the function.
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if (const Argument *A = dyn_cast<Argument>(V))
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if (A->hasByValAttr() || A->hasNoAliasAttr()) {
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// Note even if the argument is marked nocapture, we still need to check
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// for copies made inside the function. The nocapture attribute only
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// specifies that there are no copies made that outlive the function.
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auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
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if (IsCapturedCache)
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CacheIt->second = Ret;
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return Ret;
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}
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return false;
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}
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