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//===-- GlobalStatus.cpp - Compute status info for globals -----------------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/GlobalStatus.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/AtomicOrdering.h"
#include "llvm/Support/Casting.h"
#include <algorithm>
#include <cassert>
using namespace llvm;
/// Return the stronger of the two ordering. If the two orderings are acquire
/// and release, then return AcquireRelease.
///
static AtomicOrdering strongerOrdering(AtomicOrdering X, AtomicOrdering Y) {
if ((X == AtomicOrdering::Acquire && Y == AtomicOrdering::Release) ||
(Y == AtomicOrdering::Acquire && X == AtomicOrdering::Release))
return AtomicOrdering::AcquireRelease;
return (AtomicOrdering)std::max((unsigned)X, (unsigned)Y);
}
/// It is safe to destroy a constant iff it is only used by constants itself.
/// Note that while constants cannot be cyclic, they can be tree-like, so we
/// should keep a visited set to avoid exponential runtime.
bool llvm::isSafeToDestroyConstant(const Constant *C) {
SmallVector<const Constant *, 8> Worklist;
SmallPtrSet<const Constant *, 8> Visited;
Worklist.push_back(C);
while (!Worklist.empty()) {
const Constant *C = Worklist.pop_back_val();
if (!Visited.insert(C).second)
continue;
if (isa<GlobalValue>(C) || isa<ConstantData>(C))
return false;
for (const User *U : C->users()) {
if (const Constant *CU = dyn_cast<Constant>(U))
Worklist.push_back(CU);
else
return false;
}
}
return true;
}
static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
SmallPtrSetImpl<const Value *> &VisitedUsers) {
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
if (GV->isExternallyInitialized())
GS.StoredType = GlobalStatus::StoredOnce;
for (const Use &U : V->uses()) {
const User *UR = U.getUser();
if (const Constant *C = dyn_cast<Constant>(UR)) {
const ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
if (CE && isa<PointerType>(CE->getType())) {
// Recursively analyze pointer-typed constant expressions.
// FIXME: Do we need to add constexpr selects to VisitedUsers?
if (analyzeGlobalAux(CE, GS, VisitedUsers))
return true;
} else {
// Ignore dead constant users.
if (!isSafeToDestroyConstant(C))
return true;
}
} else if (const Instruction *I = dyn_cast<Instruction>(UR)) {
if (!GS.HasMultipleAccessingFunctions) {
const Function *F = I->getParent()->getParent();
if (!GS.AccessingFunction)
GS.AccessingFunction = F;
else if (GS.AccessingFunction != F)
GS.HasMultipleAccessingFunctions = true;
}
if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
GS.IsLoaded = true;
// Don't hack on volatile loads.
if (LI->isVolatile())
return true;
GS.Ordering = strongerOrdering(GS.Ordering, LI->getOrdering());
} else if (const StoreInst *SI = dyn_cast<StoreInst>(I)) {
// Don't allow a store OF the address, only stores TO the address.
if (SI->getOperand(0) == V)
return true;
// Don't hack on volatile stores.
if (SI->isVolatile())
return true;
++GS.NumStores;
GS.Ordering = strongerOrdering(GS.Ordering, SI->getOrdering());
// If this is a direct store to the global (i.e., the global is a scalar
// value, not an aggregate), keep more specific information about
// stores.
if (GS.StoredType != GlobalStatus::Stored) {
const Value *Ptr = SI->getPointerOperand()->stripPointerCasts();
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
Value *StoredVal = SI->getOperand(0);
if (Constant *C = dyn_cast<Constant>(StoredVal)) {
if (C->isThreadDependent()) {
// The stored value changes between threads; don't track it.
return true;
}
}
if (GV->hasInitializer() && StoredVal == GV->getInitializer()) {
if (GS.StoredType < GlobalStatus::InitializerStored)
GS.StoredType = GlobalStatus::InitializerStored;
} else if (isa<LoadInst>(StoredVal) &&
cast<LoadInst>(StoredVal)->getOperand(0) == GV) {
if (GS.StoredType < GlobalStatus::InitializerStored)
GS.StoredType = GlobalStatus::InitializerStored;
} else if (GS.StoredType < GlobalStatus::StoredOnce) {
GS.StoredType = GlobalStatus::StoredOnce;
GS.StoredOnceStore = SI;
} else if (GS.StoredType == GlobalStatus::StoredOnce &&
GS.getStoredOnceValue() == StoredVal) {
// noop.
} else {
GS.StoredType = GlobalStatus::Stored;
}
} else {
GS.StoredType = GlobalStatus::Stored;
}
}
} else if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
isa<AddrSpaceCastInst>(I)) {
// Skip over bitcasts and GEPs; we don't care about the type or offset
// of the pointer.
if (analyzeGlobalAux(I, GS, VisitedUsers))
return true;
} else if (isa<SelectInst>(I) || isa<PHINode>(I)) {
// Look through selects and PHIs to find if the pointer is
// conditionally accessed. Make sure we only visit an instruction
// once; otherwise, we can get infinite recursion or exponential
// compile time.
if (VisitedUsers.insert(I).second)
if (analyzeGlobalAux(I, GS, VisitedUsers))
return true;
} else if (isa<CmpInst>(I)) {
GS.IsCompared = true;
} else if (const MemTransferInst *MTI = dyn_cast<MemTransferInst>(I)) {
if (MTI->isVolatile())
return true;
if (MTI->getArgOperand(0) == V)
GS.StoredType = GlobalStatus::Stored;
if (MTI->getArgOperand(1) == V)
GS.IsLoaded = true;
} else if (const MemSetInst *MSI = dyn_cast<MemSetInst>(I)) {
assert(MSI->getArgOperand(0) == V && "Memset only takes one pointer!");
if (MSI->isVolatile())
return true;
GS.StoredType = GlobalStatus::Stored;
} else if (const auto *CB = dyn_cast<CallBase>(I)) {
if (!CB->isCallee(&U))
return true;
GS.IsLoaded = true;
} else {
return true; // Any other non-load instruction might take address!
}
} else {
// Otherwise must be some other user.
return true;
}
}
return false;
}
GlobalStatus::GlobalStatus() = default;
bool GlobalStatus::analyzeGlobal(const Value *V, GlobalStatus &GS) {
SmallPtrSet<const Value *, 16> VisitedUsers;
return analyzeGlobalAux(V, GS, VisitedUsers);
}
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