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llvm/lib/Analysis/GlobalsModRef.cpp
James Molloy d56ad58fbc [GlobalsAA] An indirect global that is initialized is not fair game 
When checking if an indirect global (a global with pointer type) is only assigned by allocation functions, first check if the global is itself initialized. If it is, it's not only assigned by allocation functions.

This fixes PR25309. Thanks to David Majnemer for reducing the test case!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251508 91177308-0d34-0410-b5e6-96231b3b80d8
2015-10-28 10:41:29 +00:00

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//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This simple pass provides alias and mod/ref information for global values
// that do not have their address taken, and keeps track of whether functions
// read or write memory (are "pure"). For this simple (but very common) case,
// we can provide pretty accurate and useful information.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;
#define DEBUG_TYPE "globalsmodref-aa"
STATISTIC(NumNonAddrTakenGlobalVars,
"Number of global vars without address taken");
STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
// An option to enable unsafe alias results from the GlobalsModRef analysis.
// When enabled, GlobalsModRef will provide no-alias results which in extremely
// rare cases may not be conservatively correct. In particular, in the face of
// transforms which cause assymetry between how effective GetUnderlyingObject
// is for two pointers, it may produce incorrect results.
//
// These unsafe results have been returned by GMR for many years without
// causing significant issues in the wild and so we provide a mechanism to
// re-enable them for users of LLVM that have a particular performance
// sensitivity and no known issues. The option also makes it easy to evaluate
// the performance impact of these results.
static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
"enable-unsafe-globalsmodref-alias-results", cl::init(false), cl::Hidden);
/// The mod/ref information collected for a particular function.
///
/// We collect information about mod/ref behavior of a function here, both in
/// general and as pertains to specific globals. We only have this detailed
/// information when we know *something* useful about the behavior. If we
/// saturate to fully general mod/ref, we remove the info for the function.
class GlobalsAAResult::FunctionInfo {
typedef SmallDenseMap<const GlobalValue *, ModRefInfo, 16> GlobalInfoMapType;
/// Build a wrapper struct that has 8-byte alignment. All heap allocations
/// should provide this much alignment at least, but this makes it clear we
/// specifically rely on this amount of alignment.
struct LLVM_ALIGNAS(8) AlignedMap {
AlignedMap() {}
AlignedMap(const AlignedMap &Arg) : Map(Arg.Map) {}
GlobalInfoMapType Map;
};
/// Pointer traits for our aligned map.
struct AlignedMapPointerTraits {
static inline void *getAsVoidPointer(AlignedMap *P) { return P; }
static inline AlignedMap *getFromVoidPointer(void *P) {
return (AlignedMap *)P;
}
enum { NumLowBitsAvailable = 3 };
static_assert(AlignOf<AlignedMap>::Alignment >= (1 << NumLowBitsAvailable),
"AlignedMap insufficiently aligned to have enough low bits.");
};
/// The bit that flags that this function may read any global. This is
/// chosen to mix together with ModRefInfo bits.
enum { MayReadAnyGlobal = 4 };
/// Checks to document the invariants of the bit packing here.
static_assert((MayReadAnyGlobal & MRI_ModRef) == 0,
"ModRef and the MayReadAnyGlobal flag bits overlap.");
static_assert(((MayReadAnyGlobal | MRI_ModRef) >>
AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
"Insufficient low bits to store our flag and ModRef info.");
public:
FunctionInfo() : Info() {}
~FunctionInfo() {
delete Info.getPointer();
}
// Spell out the copy ond move constructors and assignment operators to get
// deep copy semantics and correct move semantics in the face of the
// pointer-int pair.
FunctionInfo(const FunctionInfo &Arg)
: Info(nullptr, Arg.Info.getInt()) {
if (const auto *ArgPtr = Arg.Info.getPointer())
Info.setPointer(new AlignedMap(*ArgPtr));
}
FunctionInfo(FunctionInfo &&Arg)
: Info(Arg.Info.getPointer(), Arg.Info.getInt()) {
Arg.Info.setPointerAndInt(nullptr, 0);
}
FunctionInfo &operator=(const FunctionInfo &RHS) {
delete Info.getPointer();
Info.setPointerAndInt(nullptr, RHS.Info.getInt());
if (const auto *RHSPtr = RHS.Info.getPointer())
Info.setPointer(new AlignedMap(*RHSPtr));
return *this;
}
FunctionInfo &operator=(FunctionInfo &&RHS) {
delete Info.getPointer();
Info.setPointerAndInt(RHS.Info.getPointer(), RHS.Info.getInt());
RHS.Info.setPointerAndInt(nullptr, 0);
return *this;
}
/// Returns the \c ModRefInfo info for this function.
ModRefInfo getModRefInfo() const {
return ModRefInfo(Info.getInt() & MRI_ModRef);
}
/// Adds new \c ModRefInfo for this function to its state.
void addModRefInfo(ModRefInfo NewMRI) {
Info.setInt(Info.getInt() | NewMRI);
}
/// Returns whether this function may read any global variable, and we don't
/// know which global.
bool mayReadAnyGlobal() const { return Info.getInt() & MayReadAnyGlobal; }
/// Sets this function as potentially reading from any global.
void setMayReadAnyGlobal() { Info.setInt(Info.getInt() | MayReadAnyGlobal); }
/// Returns the \c ModRefInfo info for this function w.r.t. a particular
/// global, which may be more precise than the general information above.
ModRefInfo getModRefInfoForGlobal(const GlobalValue &GV) const {
ModRefInfo GlobalMRI = mayReadAnyGlobal() ? MRI_Ref : MRI_NoModRef;
if (AlignedMap *P = Info.getPointer()) {
auto I = P->Map.find(&GV);
if (I != P->Map.end())
GlobalMRI = ModRefInfo(GlobalMRI | I->second);
}
return GlobalMRI;
}
/// Add mod/ref info from another function into ours, saturating towards
/// MRI_ModRef.
void addFunctionInfo(const FunctionInfo &FI) {
addModRefInfo(FI.getModRefInfo());
if (FI.mayReadAnyGlobal())
setMayReadAnyGlobal();
if (AlignedMap *P = FI.Info.getPointer())
for (const auto &G : P->Map)
addModRefInfoForGlobal(*G.first, G.second);
}
void addModRefInfoForGlobal(const GlobalValue &GV, ModRefInfo NewMRI) {
AlignedMap *P = Info.getPointer();
if (!P) {
P = new AlignedMap();
Info.setPointer(P);
}
auto &GlobalMRI = P->Map[&GV];
GlobalMRI = ModRefInfo(GlobalMRI | NewMRI);
}
/// Clear a global's ModRef info. Should be used when a global is being
/// deleted.
void eraseModRefInfoForGlobal(const GlobalValue &GV) {
if (AlignedMap *P = Info.getPointer())
P->Map.erase(&GV);
}
private:
/// All of the information is encoded into a single pointer, with a three bit
/// integer in the low three bits. The high bit provides a flag for when this
/// function may read any global. The low two bits are the ModRefInfo. And
/// the pointer, when non-null, points to a map from GlobalValue to
/// ModRefInfo specific to that GlobalValue.
PointerIntPair<AlignedMap *, 3, unsigned, AlignedMapPointerTraits> Info;
};
void GlobalsAAResult::DeletionCallbackHandle::deleted() {
Value *V = getValPtr();
if (auto *F = dyn_cast<Function>(V))
GAR->FunctionInfos.erase(F);
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
if (GAR->NonAddressTakenGlobals.erase(GV)) {
// This global might be an indirect global. If so, remove it and
// remove any AllocRelatedValues for it.
if (GAR->IndirectGlobals.erase(GV)) {
// Remove any entries in AllocsForIndirectGlobals for this global.
for (auto I = GAR->AllocsForIndirectGlobals.begin(),
E = GAR->AllocsForIndirectGlobals.end();
I != E; ++I)
if (I->second == GV)
GAR->AllocsForIndirectGlobals.erase(I);
}
// Scan the function info we have collected and remove this global
// from all of them.
for (auto &FIPair : GAR->FunctionInfos)
FIPair.second.eraseModRefInfoForGlobal(*GV);
}
}
// If this is an allocation related to an indirect global, remove it.
GAR->AllocsForIndirectGlobals.erase(V);
// And clear out the handle.
setValPtr(nullptr);
GAR->Handles.erase(I);
// This object is now destroyed!
}
FunctionModRefBehavior GlobalsAAResult::getModRefBehavior(const Function *F) {
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
if (FunctionInfo *FI = getFunctionInfo(F)) {
if (FI->getModRefInfo() == MRI_NoModRef)
Min = FMRB_DoesNotAccessMemory;
else if ((FI->getModRefInfo() & MRI_Mod) == 0)
Min = FMRB_OnlyReadsMemory;
}
return FunctionModRefBehavior(AAResultBase::getModRefBehavior(F) & Min);
}
FunctionModRefBehavior
GlobalsAAResult::getModRefBehavior(ImmutableCallSite CS) {
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
if (const Function *F = CS.getCalledFunction())
if (FunctionInfo *FI = getFunctionInfo(F)) {
if (FI->getModRefInfo() == MRI_NoModRef)
Min = FMRB_DoesNotAccessMemory;
else if ((FI->getModRefInfo() & MRI_Mod) == 0)
Min = FMRB_OnlyReadsMemory;
}
return FunctionModRefBehavior(AAResultBase::getModRefBehavior(CS) & Min);
}
/// Returns the function info for the function, or null if we don't have
/// anything useful to say about it.
GlobalsAAResult::FunctionInfo *
GlobalsAAResult::getFunctionInfo(const Function *F) {
auto I = FunctionInfos.find(F);
if (I != FunctionInfos.end())
return &I->second;
return nullptr;
}
/// AnalyzeGlobals - Scan through the users of all of the internal
/// GlobalValue's in the program. If none of them have their "address taken"
/// (really, their address passed to something nontrivial), record this fact,
/// and record the functions that they are used directly in.
void GlobalsAAResult::AnalyzeGlobals(Module &M) {
SmallPtrSet<Function *, 64> TrackedFunctions;
for (Function &F : M)
if (F.hasLocalLinkage())
if (!AnalyzeUsesOfPointer(&F)) {
// Remember that we are tracking this global.
NonAddressTakenGlobals.insert(&F);
TrackedFunctions.insert(&F);
Handles.emplace_front(*this, &F);
Handles.front().I = Handles.begin();
++NumNonAddrTakenFunctions;
}
SmallPtrSet<Function *, 64> Readers, Writers;
for (GlobalVariable &GV : M.globals())
if (GV.hasLocalLinkage()) {
if (!AnalyzeUsesOfPointer(&GV, &Readers,
GV.isConstant() ? nullptr : &Writers)) {
// Remember that we are tracking this global, and the mod/ref fns
NonAddressTakenGlobals.insert(&GV);
Handles.emplace_front(*this, &GV);
Handles.front().I = Handles.begin();
for (Function *Reader : Readers) {
if (TrackedFunctions.insert(Reader).second) {
Handles.emplace_front(*this, Reader);
Handles.front().I = Handles.begin();
}
FunctionInfos[Reader].addModRefInfoForGlobal(GV, MRI_Ref);
}
if (!GV.isConstant()) // No need to keep track of writers to constants
for (Function *Writer : Writers) {
if (TrackedFunctions.insert(Writer).second) {
Handles.emplace_front(*this, Writer);
Handles.front().I = Handles.begin();
}
FunctionInfos[Writer].addModRefInfoForGlobal(GV, MRI_Mod);
}
++NumNonAddrTakenGlobalVars;
// If this global holds a pointer type, see if it is an indirect global.
if (GV.getType()->getElementType()->isPointerTy() &&
AnalyzeIndirectGlobalMemory(&GV))
++NumIndirectGlobalVars;
}
Readers.clear();
Writers.clear();
}
}
/// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
/// If this is used by anything complex (i.e., the address escapes), return
/// true. Also, while we are at it, keep track of those functions that read and
/// write to the value.
///
/// If OkayStoreDest is non-null, stores into this global are allowed.
bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
SmallPtrSetImpl<Function *> *Readers,
SmallPtrSetImpl<Function *> *Writers,
GlobalValue *OkayStoreDest) {
if (!V->getType()->isPointerTy())
return true;
for (Use &U : V->uses()) {
User *I = U.getUser();
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
if (Readers)
Readers->insert(LI->getParent()->getParent());
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (V == SI->getOperand(1)) {
if (Writers)
Writers->insert(SI->getParent()->getParent());
} else if (SI->getOperand(1) != OkayStoreDest) {
return true; // Storing the pointer
}
} else if (Operator::getOpcode(I) == Instruction::GetElementPtr) {
if (AnalyzeUsesOfPointer(I, Readers, Writers))
return true;
} else if (Operator::getOpcode(I) == Instruction::BitCast) {
if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
return true;
} else if (auto CS = CallSite(I)) {
// Make sure that this is just the function being called, not that it is
// passing into the function.
if (!CS.isCallee(&U)) {
// Detect calls to free.
if (isFreeCall(I, &TLI)) {
if (Writers)
Writers->insert(CS->getParent()->getParent());
} else if (CS.doesNotCapture(CS.getArgumentNo(&U))) {
Function *ParentF = CS->getParent()->getParent();
// A nocapture argument may be read from or written to, but does not
// escape unless the call can somehow recurse.
//
// nocapture "indicates that the callee does not make any copies of
// the pointer that outlive itself". Therefore if we directly or
// indirectly recurse, we must treat the pointer as escaping.
if (FunctionToSCCMap[ParentF] ==
FunctionToSCCMap[CS.getCalledFunction()])
return true;
if (Readers)
Readers->insert(ParentF);
if (Writers)
Writers->insert(ParentF);
} else {
return true; // Argument of an unknown call.
}
}
} else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
return true; // Allow comparison against null.
} else {
return true;
}
}
return false;
}
/// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
/// which holds a pointer type. See if the global always points to non-aliased
/// heap memory: that is, all initializers of the globals are allocations, and
/// those allocations have no use other than initialization of the global.
/// Further, all loads out of GV must directly use the memory, not store the
/// pointer somewhere. If this is true, we consider the memory pointed to by
/// GV to be owned by GV and can disambiguate other pointers from it.
bool GlobalsAAResult::AnalyzeIndirectGlobalMemory(GlobalVariable *GV) {
// Keep track of values related to the allocation of the memory, f.e. the
// value produced by the malloc call and any casts.
std::vector<Value *> AllocRelatedValues;
// If the initializer is a valid pointer, bail.
if (Constant *C = GV->getInitializer())
if (!C->isNullValue())
return false;
// Walk the user list of the global. If we find anything other than a direct
// load or store, bail out.
for (User *U : GV->users()) {
if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
// The pointer loaded from the global can only be used in simple ways:
// we allow addressing of it and loading storing to it. We do *not* allow
// storing the loaded pointer somewhere else or passing to a function.
if (AnalyzeUsesOfPointer(LI))
return false; // Loaded pointer escapes.
// TODO: Could try some IP mod/ref of the loaded pointer.
} else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
// Storing the global itself.
if (SI->getOperand(0) == GV)
return false;
// If storing the null pointer, ignore it.
if (isa<ConstantPointerNull>(SI->getOperand(0)))
continue;
// Check the value being stored.
Value *Ptr = GetUnderlyingObject(SI->getOperand(0),
GV->getParent()->getDataLayout());
if (!isAllocLikeFn(Ptr, &TLI))
return false; // Too hard to analyze.
// Analyze all uses of the allocation. If any of them are used in a
// non-simple way (e.g. stored to another global) bail out.
if (AnalyzeUsesOfPointer(Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
GV))
return false; // Loaded pointer escapes.
// Remember that this allocation is related to the indirect global.
AllocRelatedValues.push_back(Ptr);
} else {
// Something complex, bail out.
return false;
}
}
// Okay, this is an indirect global. Remember all of the allocations for
// this global in AllocsForIndirectGlobals.
while (!AllocRelatedValues.empty()) {
AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
Handles.emplace_front(*this, AllocRelatedValues.back());
Handles.front().I = Handles.begin();
AllocRelatedValues.pop_back();
}
IndirectGlobals.insert(GV);
Handles.emplace_front(*this, GV);
Handles.front().I = Handles.begin();
return true;
}
void GlobalsAAResult::CollectSCCMembership(CallGraph &CG) {
// We do a bottom-up SCC traversal of the call graph. In other words, we
// visit all callees before callers (leaf-first).
unsigned SCCID = 0;
for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
const std::vector<CallGraphNode *> &SCC = *I;
assert(!SCC.empty() && "SCC with no functions?");
for (auto *CGN : SCC)
if (Function *F = CGN->getFunction())
FunctionToSCCMap[F] = SCCID;
++SCCID;
}
}
/// AnalyzeCallGraph - At this point, we know the functions where globals are
/// immediately stored to and read from. Propagate this information up the call
/// graph to all callers and compute the mod/ref info for all memory for each
/// function.
void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
// We do a bottom-up SCC traversal of the call graph. In other words, we
// visit all callees before callers (leaf-first).
for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
const std::vector<CallGraphNode *> &SCC = *I;
assert(!SCC.empty() && "SCC with no functions?");
if (!SCC[0]->getFunction() || SCC[0]->getFunction()->mayBeOverridden()) {
// Calls externally or is weak - can't say anything useful. Remove any existing
// function records (may have been created when scanning globals).
for (auto *Node : SCC)
FunctionInfos.erase(Node->getFunction());
continue;
}
FunctionInfo &FI = FunctionInfos[SCC[0]->getFunction()];
bool KnowNothing = false;
// Collect the mod/ref properties due to called functions. We only compute
// one mod-ref set.
for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
Function *F = SCC[i]->getFunction();
if (!F) {
KnowNothing = true;
break;
}
if (F->isDeclaration()) {
// Try to get mod/ref behaviour from function attributes.
if (F->doesNotAccessMemory()) {
// Can't do better than that!
} else if (F->onlyReadsMemory()) {
FI.addModRefInfo(MRI_Ref);
if (!F->isIntrinsic())
// This function might call back into the module and read a global -
// consider every global as possibly being read by this function.
FI.setMayReadAnyGlobal();
} else {
FI.addModRefInfo(MRI_ModRef);
// Can't say anything useful unless it's an intrinsic - they don't
// read or write global variables of the kind considered here.
KnowNothing = !F->isIntrinsic();
}
continue;
}
for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
CI != E && !KnowNothing; ++CI)
if (Function *Callee = CI->second->getFunction()) {
if (FunctionInfo *CalleeFI = getFunctionInfo(Callee)) {
// Propagate function effect up.
FI.addFunctionInfo(*CalleeFI);
} else {
// Can't say anything about it. However, if it is inside our SCC,
// then nothing needs to be done.
CallGraphNode *CalleeNode = CG[Callee];
if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end())
KnowNothing = true;
}
} else {
KnowNothing = true;
}
}
// If we can't say anything useful about this SCC, remove all SCC functions
// from the FunctionInfos map.
if (KnowNothing) {
for (auto *Node : SCC)
FunctionInfos.erase(Node->getFunction());
continue;
}
// Scan the function bodies for explicit loads or stores.
for (auto *Node : SCC) {
if (FI.getModRefInfo() == MRI_ModRef)
break; // The mod/ref lattice saturates here.
for (Instruction &I : instructions(Node->getFunction())) {
if (FI.getModRefInfo() == MRI_ModRef)
break; // The mod/ref lattice saturates here.
// We handle calls specially because the graph-relevant aspects are
// handled above.
if (auto CS = CallSite(&I)) {
if (isAllocationFn(&I, &TLI) || isFreeCall(&I, &TLI)) {
// FIXME: It is completely unclear why this is necessary and not
// handled by the above graph code.
FI.addModRefInfo(MRI_ModRef);
} else if (Function *Callee = CS.getCalledFunction()) {
// The callgraph doesn't include intrinsic calls.
if (Callee->isIntrinsic()) {
FunctionModRefBehavior Behaviour =
AAResultBase::getModRefBehavior(Callee);
FI.addModRefInfo(ModRefInfo(Behaviour & MRI_ModRef));
}
}
continue;
}
// All non-call instructions we use the primary predicates for whether
// thay read or write memory.
if (I.mayReadFromMemory())
FI.addModRefInfo(MRI_Ref);
if (I.mayWriteToMemory())
FI.addModRefInfo(MRI_Mod);
}
}
if ((FI.getModRefInfo() & MRI_Mod) == 0)
++NumReadMemFunctions;
if (FI.getModRefInfo() == MRI_NoModRef)
++NumNoMemFunctions;
// Finally, now that we know the full effect on this SCC, clone the
// information to each function in the SCC.
// FI is a reference into FunctionInfos, so copy it now so that it doesn't
// get invalidated if DenseMap decides to re-hash.
FunctionInfo CachedFI = FI;
for (unsigned i = 1, e = SCC.size(); i != e; ++i)
FunctionInfos[SCC[i]->getFunction()] = CachedFI;
}
}
// GV is a non-escaping global. V is a pointer address that has been loaded from.
// If we can prove that V must escape, we can conclude that a load from V cannot
// alias GV.
static bool isNonEscapingGlobalNoAliasWithLoad(const GlobalValue *GV,
const Value *V,
int &Depth,
const DataLayout &DL) {
SmallPtrSet<const Value *, 8> Visited;
SmallVector<const Value *, 8> Inputs;
Visited.insert(V);
Inputs.push_back(V);
do {
const Value *Input = Inputs.pop_back_val();
if (isa<GlobalValue>(Input) || isa<Argument>(Input) || isa<CallInst>(Input) ||
isa<InvokeInst>(Input))
// Arguments to functions or returns from functions are inherently
// escaping, so we can immediately classify those as not aliasing any
// non-addr-taken globals.
//
// (Transitive) loads from a global are also safe - if this aliased
// another global, its address would escape, so no alias.
continue;
// Recurse through a limited number of selects, loads and PHIs. This is an
// arbitrary depth of 4, lower numbers could be used to fix compile time
// issues if needed, but this is generally expected to be only be important
// for small depths.
if (++Depth > 4)
return false;
if (auto *LI = dyn_cast<LoadInst>(Input)) {
Inputs.push_back(GetUnderlyingObject(LI->getPointerOperand(), DL));
continue;
}
if (auto *SI = dyn_cast<SelectInst>(Input)) {
const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);
if (Visited.insert(LHS).second)
Inputs.push_back(LHS);
if (Visited.insert(RHS).second)
Inputs.push_back(RHS);
continue;
}
if (auto *PN = dyn_cast<PHINode>(Input)) {
for (const Value *Op : PN->incoming_values()) {
Op = GetUnderlyingObject(Op, DL);
if (Visited.insert(Op).second)
Inputs.push_back(Op);
}
continue;
}
return false;
} while (!Inputs.empty());
// All inputs were known to be no-alias.
return true;
}
// There are particular cases where we can conclude no-alias between
// a non-addr-taken global and some other underlying object. Specifically,
// a non-addr-taken global is known to not be escaped from any function. It is
// also incorrect for a transformation to introduce an escape of a global in
// a way that is observable when it was not there previously. One function
// being transformed to introduce an escape which could possibly be observed
// (via loading from a global or the return value for example) within another
// function is never safe. If the observation is made through non-atomic
// operations on different threads, it is a data-race and UB. If the
// observation is well defined, by being observed the transformation would have
// changed program behavior by introducing the observed escape, making it an
// invalid transform.
//
// This property does require that transformations which *temporarily* escape
// a global that was not previously escaped, prior to restoring it, cannot rely
// on the results of GMR::alias. This seems a reasonable restriction, although
// currently there is no way to enforce it. There is also no realistic
// optimization pass that would make this mistake. The closest example is
// a transformation pass which does reg2mem of SSA values but stores them into
// global variables temporarily before restoring the global variable's value.
// This could be useful to expose "benign" races for example. However, it seems
// reasonable to require that a pass which introduces escapes of global
// variables in this way to either not trust AA results while the escape is
// active, or to be forced to operate as a module pass that cannot co-exist
// with an alias analysis such as GMR.
bool GlobalsAAResult::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
const Value *V) {
// In order to know that the underlying object cannot alias the
// non-addr-taken global, we must know that it would have to be an escape.
// Thus if the underlying object is a function argument, a load from
// a global, or the return of a function, it cannot alias. We can also
// recurse through PHI nodes and select nodes provided all of their inputs
// resolve to one of these known-escaping roots.
SmallPtrSet<const Value *, 8> Visited;
SmallVector<const Value *, 8> Inputs;
Visited.insert(V);
Inputs.push_back(V);
int Depth = 0;
do {
const Value *Input = Inputs.pop_back_val();
if (auto *InputGV = dyn_cast<GlobalValue>(Input)) {
// If one input is the very global we're querying against, then we can't
// conclude no-alias.
if (InputGV == GV)
return false;
// Distinct GlobalVariables never alias, unless overriden or zero-sized.
// FIXME: The condition can be refined, but be conservative for now.
auto *GVar = dyn_cast<GlobalVariable>(GV);
auto *InputGVar = dyn_cast<GlobalVariable>(InputGV);
if (GVar && InputGVar &&
!GVar->isDeclaration() && !InputGVar->isDeclaration() &&
!GVar->mayBeOverridden() && !InputGVar->mayBeOverridden()) {
Type *GVType = GVar->getInitializer()->getType();
Type *InputGVType = InputGVar->getInitializer()->getType();
if (GVType->isSized() && InputGVType->isSized() &&
(DL.getTypeAllocSize(GVType) > 0) &&
(DL.getTypeAllocSize(InputGVType) > 0))
continue;
}
// Conservatively return false, even though we could be smarter
// (e.g. look through GlobalAliases).
return false;
}
if (isa<Argument>(Input) || isa<CallInst>(Input) ||
isa<InvokeInst>(Input)) {
// Arguments to functions or returns from functions are inherently
// escaping, so we can immediately classify those as not aliasing any
// non-addr-taken globals.
continue;
}
// Recurse through a limited number of selects, loads and PHIs. This is an
// arbitrary depth of 4, lower numbers could be used to fix compile time
// issues if needed, but this is generally expected to be only be important
// for small depths.
if (++Depth > 4)
return false;
if (auto *LI = dyn_cast<LoadInst>(Input)) {
// A pointer loaded from a global would have been captured, and we know
// that the global is non-escaping, so no alias.
const Value *Ptr = GetUnderlyingObject(LI->getPointerOperand(), DL);
if (isNonEscapingGlobalNoAliasWithLoad(GV, Ptr, Depth, DL))
// The load does not alias with GV.
continue;
// Otherwise, a load could come from anywhere, so bail.
return false;
}
if (auto *SI = dyn_cast<SelectInst>(Input)) {
const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);
if (Visited.insert(LHS).second)
Inputs.push_back(LHS);
if (Visited.insert(RHS).second)
Inputs.push_back(RHS);
continue;
}
if (auto *PN = dyn_cast<PHINode>(Input)) {
for (const Value *Op : PN->incoming_values()) {
Op = GetUnderlyingObject(Op, DL);
if (Visited.insert(Op).second)
Inputs.push_back(Op);
}
continue;
}
// FIXME: It would be good to handle other obvious no-alias cases here, but
// it isn't clear how to do so reasonbly without building a small version
// of BasicAA into this code. We could recurse into AAResultBase::alias
// here but that seems likely to go poorly as we're inside the
// implementation of such a query. Until then, just conservatievly retun
// false.
return false;
} while (!Inputs.empty());
// If all the inputs to V were definitively no-alias, then V is no-alias.
return true;
}
/// alias - If one of the pointers is to a global that we are tracking, and the
/// other is some random pointer, we know there cannot be an alias, because the
/// address of the global isn't taken.
AliasResult GlobalsAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
// Get the base object these pointers point to.
const Value *UV1 = GetUnderlyingObject(LocA.Ptr, DL);
const Value *UV2 = GetUnderlyingObject(LocB.Ptr, DL);
// If either of the underlying values is a global, they may be non-addr-taken
// globals, which we can answer queries about.
const GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
const GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
if (GV1 || GV2) {
// If the global's address is taken, pretend we don't know it's a pointer to
// the global.
if (GV1 && !NonAddressTakenGlobals.count(GV1))
GV1 = nullptr;
if (GV2 && !NonAddressTakenGlobals.count(GV2))
GV2 = nullptr;
// If the two pointers are derived from two different non-addr-taken
// globals we know these can't alias.
if (GV1 && GV2 && GV1 != GV2)
return NoAlias;
// If one is and the other isn't, it isn't strictly safe but we can fake
// this result if necessary for performance. This does not appear to be
// a common problem in practice.
if (EnableUnsafeGlobalsModRefAliasResults)
if ((GV1 || GV2) && GV1 != GV2)
return NoAlias;
// Check for a special case where a non-escaping global can be used to
// conclude no-alias.
if ((GV1 || GV2) && GV1 != GV2) {
const GlobalValue *GV = GV1 ? GV1 : GV2;
const Value *UV = GV1 ? UV2 : UV1;
if (isNonEscapingGlobalNoAlias(GV, UV))
return NoAlias;
}
// Otherwise if they are both derived from the same addr-taken global, we
// can't know the two accesses don't overlap.
}
// These pointers may be based on the memory owned by an indirect global. If
// so, we may be able to handle this. First check to see if the base pointer
// is a direct load from an indirect global.
GV1 = GV2 = nullptr;
if (const LoadInst *LI = dyn_cast<LoadInst>(UV1))
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
if (IndirectGlobals.count(GV))
GV1 = GV;
if (const LoadInst *LI = dyn_cast<LoadInst>(UV2))
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
if (IndirectGlobals.count(GV))
GV2 = GV;
// These pointers may also be from an allocation for the indirect global. If
// so, also handle them.
if (!GV1)
GV1 = AllocsForIndirectGlobals.lookup(UV1);
if (!GV2)
GV2 = AllocsForIndirectGlobals.lookup(UV2);
// Now that we know whether the two pointers are related to indirect globals,
// use this to disambiguate the pointers. If the pointers are based on
// different indirect globals they cannot alias.
if (GV1 && GV2 && GV1 != GV2)
return NoAlias;
// If one is based on an indirect global and the other isn't, it isn't
// strictly safe but we can fake this result if necessary for performance.
// This does not appear to be a common problem in practice.
if (EnableUnsafeGlobalsModRefAliasResults)
if ((GV1 || GV2) && GV1 != GV2)
return NoAlias;
return AAResultBase::alias(LocA, LocB);
}
ModRefInfo GlobalsAAResult::getModRefInfoForArgument(ImmutableCallSite CS,
const GlobalValue *GV) {
if (CS.doesNotAccessMemory())
return MRI_NoModRef;
ModRefInfo ConservativeResult = CS.onlyReadsMemory() ? MRI_Ref : MRI_ModRef;
// Iterate through all the arguments to the called function. If any argument
// is based on GV, return the conservative result.
for (auto &A : CS.args()) {
SmallVector<Value*, 4> Objects;
GetUnderlyingObjects(A, Objects, DL);
// All objects must be identified.
if (!std::all_of(Objects.begin(), Objects.end(), [&GV](const Value *V) {
return isIdentifiedObject(V);
}))
return ConservativeResult;
if (std::find(Objects.begin(), Objects.end(), GV) != Objects.end())
return ConservativeResult;
}
// We identified all objects in the argument list, and none of them were GV.
return MRI_NoModRef;
}
ModRefInfo GlobalsAAResult::getModRefInfo(ImmutableCallSite CS,
const MemoryLocation &Loc) {
unsigned Known = MRI_ModRef;
// If we are asking for mod/ref info of a direct call with a pointer to a
// global we are tracking, return information if we have it.
if (const GlobalValue *GV =
dyn_cast<GlobalValue>(GetUnderlyingObject(Loc.Ptr, DL)))
if (GV->hasLocalLinkage())
if (const Function *F = CS.getCalledFunction())
if (NonAddressTakenGlobals.count(GV))
if (const FunctionInfo *FI = getFunctionInfo(F))
Known = FI->getModRefInfoForGlobal(*GV) |
getModRefInfoForArgument(CS, GV);
if (Known == MRI_NoModRef)
return MRI_NoModRef; // No need to query other mod/ref analyses
return ModRefInfo(Known & AAResultBase::getModRefInfo(CS, Loc));
}
GlobalsAAResult::GlobalsAAResult(const DataLayout &DL,
const TargetLibraryInfo &TLI)
: AAResultBase(TLI), DL(DL) {}
GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg)
: AAResultBase(std::move(Arg)), DL(Arg.DL),
NonAddressTakenGlobals(std::move(Arg.NonAddressTakenGlobals)),
IndirectGlobals(std::move(Arg.IndirectGlobals)),
AllocsForIndirectGlobals(std::move(Arg.AllocsForIndirectGlobals)),
FunctionInfos(std::move(Arg.FunctionInfos)),
Handles(std::move(Arg.Handles)) {
// Update the parent for each DeletionCallbackHandle.
for (auto &H : Handles) {
assert(H.GAR == &Arg);
H.GAR = this;
}
}
/*static*/ GlobalsAAResult
GlobalsAAResult::analyzeModule(Module &M, const TargetLibraryInfo &TLI,
CallGraph &CG) {
GlobalsAAResult Result(M.getDataLayout(), TLI);
// Discover which functions aren't recursive, to feed into AnalyzeGlobals.
Result.CollectSCCMembership(CG);
// Find non-addr taken globals.
Result.AnalyzeGlobals(M);
// Propagate on CG.
Result.AnalyzeCallGraph(CG, M);
return Result;
}
GlobalsAAResult GlobalsAA::run(Module &M, AnalysisManager<Module> *AM) {
return GlobalsAAResult::analyzeModule(M,
AM->getResult<TargetLibraryAnalysis>(M),
AM->getResult<CallGraphAnalysis>(M));
}
char GlobalsAA::PassID;
char GlobalsAAWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(GlobalsAAWrapperPass, "globals-aa",
"Globals Alias Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(GlobalsAAWrapperPass, "globals-aa",
"Globals Alias Analysis", false, true)
ModulePass *llvm::createGlobalsAAWrapperPass() {
return new GlobalsAAWrapperPass();
}
GlobalsAAWrapperPass::GlobalsAAWrapperPass() : ModulePass(ID) {
initializeGlobalsAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
bool GlobalsAAWrapperPass::runOnModule(Module &M) {
Result.reset(new GlobalsAAResult(GlobalsAAResult::analyzeModule(
M, getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
getAnalysis<CallGraphWrapperPass>().getCallGraph())));
return false;
}
bool GlobalsAAWrapperPass::doFinalization(Module &M) {
Result.reset();
return false;
}
void GlobalsAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<CallGraphWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
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