//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis 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
//
//===----------------------------------------------------------------------===//
//
// 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/CallGraph.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/InitializePasses.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 asymmetry 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 alignas(8) AlignedMap {
AlignedMap() = default;
AlignedMap(const AlignedMap &Arg) = default;
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;
}
static constexpr int NumLowBitsAvailable = 3;
static_assert(alignof(AlignedMap) >= (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.
/// FIXME: This assumes ModRefInfo lattice will remain 4 bits!
/// FunctionInfo.getModRefInfo() masks out everything except ModRef so
/// this remains correct.
enum { MayReadAnyGlobal = 4 };
/// Checks to document the invariants of the bit packing here.
static_assert((MayReadAnyGlobal & static_cast<int>(ModRefInfo::ModRef)) == 0,
"ModRef and the MayReadAnyGlobal flag bits overlap.");
static_assert(((MayReadAnyGlobal | static_cast<int>(ModRefInfo::ModRef)) >>
AlignedMapPointerTraits::NumLowBitsAvailable) == 0,
"Insufficient low bits to store our flag and ModRef info.");
public:
FunctionInfo() = default;
~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;
}
/// This method clears MayReadAnyGlobal bit added by GlobalsAAResult to return
/// the corresponding ModRefInfo.
ModRefInfo globalClearMayReadAnyGlobal(int I) const {
return ModRefInfo(I & static_cast<int>(ModRefInfo::ModRef));
}
/// Returns the \c ModRefInfo info for this function.
ModRefInfo getModRefInfo() const {
return globalClearMayReadAnyGlobal(Info.getInt());
}
/// Adds new \c ModRefInfo for this function to its state.
void addModRefInfo(ModRefInfo NewMRI) {
Info.setInt(Info.getInt() | static_cast<int>(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() ? ModRefInfo::Ref : ModRefInfo::NoModRef;
if (AlignedMap *P = Info.getPointer()) {
auto I = P->Map.find(&GV);
if (I != P->Map.end())
GlobalMRI |= I->second;
}
return GlobalMRI;
}
/// Add mod/ref info from another function into ours, saturating towards
/// 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 |= 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!
}
MemoryEffects GlobalsAAResult::getMemoryEffects(const Function *F) {
if (FunctionInfo *FI = getFunctionInfo(F))
return MemoryEffects(FI->getModRefInfo());
return MemoryEffects::unknown();
}
/// 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 *, 32> 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;
} else
UnknownFunctionsWithLocalLinkage = true;
}
SmallPtrSet<Function *, 16> 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, ModRefInfo::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, ModRefInfo::Mod);
}
++NumNonAddrTakenGlobalVars;
// If this global holds a pointer type, see if it is an indirect global.
if (GV.getValueType()->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 ||
Operator::getOpcode(I) == Instruction::AddrSpaceCast) {
if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
return true;
} else if (auto *Call = dyn_cast<CallBase>(I)) {
// Make sure that this is just the function being called, not that it is
// passing into the function.
if (Call->isDataOperand(&U)) {
// Detect calls to free.
if (Call->isArgOperand(&U) &&
getFreedOperand(Call, &GetTLI(*Call->getFunction())) == U) {
if (Writers)
Writers->insert(Call->getParent()->getParent());
} else {
// In general, we return true for unknown calls, but there are
// some simple checks that we can do for functions that
// will never call back into the module.
auto *F = Call->getCalledFunction();
// TODO: we should be able to remove isDeclaration() check
// and let the function body analysis check for captures,
// and collect the mod-ref effects. This information will
// be later propagated via the call graph.
if (!F || !F->isDeclaration())
return true;
// Note that the NoCallback check here is a little bit too
// conservative. If there are no captures of the global
// in the module, then this call may not be a capture even
// if it does not have NoCallback.
if (!Call->hasFnAttr(Attribute::NoCallback) ||
!Call->isArgOperand(&U) ||
!Call->doesNotCapture(Call->getArgOperandNo(&U)))
return true;
// Conservatively, assume the call reads and writes the global.
// We could use memory attributes to make it more precise.
if (Readers)
Readers->insert(Call->getParent()->getParent());
if (Writers)
Writers->insert(Call->getParent()->getParent());
}
}
} else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
return true; // Allow comparison against null.
} else if (Constant *C = dyn_cast<Constant>(I)) {
// Ignore constants which don't have any live uses.
if (isa<GlobalValue>(C) || C->isConstantUsed())
return true;
} 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 store a value known
/// to be obtained via a noalias return function call which have no other use.
/// 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 noalias 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));
if (!isNoAliasCall(Ptr))
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?");
Function *F = SCC[0]->getFunction();
if (!F || !F->isDefinitionExact()) {
// Calls externally or not exact - 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[F];
Handles.emplace_front(*this, F);
Handles.front().I = Handles.begin();
bool KnowNothing = false;
// Intrinsics, like any other synchronizing function, can make effects
// of other threads visible. Without nosync we know nothing really.
// Similarly, if `nocallback` is missing the function, or intrinsic,
// can call into the module arbitrarily. If both are set the function
// has an effect but will not interact with accesses of internal
// globals inside the module. We are conservative here for optnone
// functions, might not be necessary.
auto MaySyncOrCallIntoModule = [](const Function &F) {
return !F.isDeclaration() || !F.hasNoSync() ||
!F.hasFnAttribute(Attribute::NoCallback);
};
// 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) {
if (!F) {
KnowNothing = true;
break;
}
if (F->isDeclaration() || F->hasOptNone()) {
// Try to get mod/ref behaviour from function attributes.
if (F->doesNotAccessMemory()) {
// Can't do better than that!
} else if (F->onlyReadsMemory()) {
FI.addModRefInfo(ModRefInfo::Ref);
if (!F->onlyAccessesArgMemory() && MaySyncOrCallIntoModule(*F))
// 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(ModRefInfo::ModRef);
if (!F->onlyAccessesArgMemory())
FI.setMayReadAnyGlobal();
if (MaySyncOrCallIntoModule(*F)) {
KnowNothing = true;
break;
}
}
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 (!is_contained(SCC, CalleeNode))
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 (isModAndRefSet(FI.getModRefInfo()))
break; // The mod/ref lattice saturates here.
// Don't prove any properties based on the implementation of an optnone
// function. Function attributes were already used as a best approximation
// above.
if (Node->getFunction()->hasOptNone())
continue;
for (Instruction &I : instructions(Node->getFunction())) {
if (isModAndRefSet(FI.getModRefInfo()))
break; // The mod/ref lattice saturates here.
// We handle calls specially because the graph-relevant aspects are
// handled above.
if (isa<CallBase>(&I))
continue;
// All non-call instructions we use the primary predicates for whether
// they read or write memory.
if (I.mayReadFromMemory())
FI.addModRefInfo(ModRefInfo::Ref);
if (I.mayWriteToMemory())
FI.addModRefInfo(ModRefInfo::Mod);
}
}
if (!isModSet(FI.getModRefInfo()))
++NumReadMemFunctions;
if (!isModOrRefSet(FI.getModRefInfo()))
++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()));
continue;
}
if (auto *SI = dyn_cast<SelectInst>(Input)) {
const Value *LHS = getUnderlyingObject(SI->getTrueValue());
const Value *RHS = getUnderlyingObject(SI->getFalseValue());
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);
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->isInterposable() && !InputGVar->isInterposable()) {
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());
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());
const Value *RHS = getUnderlyingObject(SI->getFalseValue());
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);
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 reasonably without building a small version
// of BasicAA into this code.
return false;
} while (!Inputs.empty());
// If all the inputs to V were definitively no-alias, then V is no-alias.
return true;
}
bool GlobalsAAResult::invalidate(Module &, const PreservedAnalyses &PA,
ModuleAnalysisManager::Invalidator &) {
// Check whether the analysis has been explicitly invalidated. Otherwise, it's
// stateless and remains preserved.
auto PAC = PA.getChecker<GlobalsAA>();
return !PAC.preservedWhenStateless();
}
/// 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,
AAQueryInfo &AAQI, const Instruction *) {
// Get the base object these pointers point to.
const Value *UV1 =
getUnderlyingObject(LocA.Ptr->stripPointerCastsForAliasAnalysis());
const Value *UV2 =
getUnderlyingObject(LocB.Ptr->stripPointerCastsForAliasAnalysis());
// 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 AliasResult::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 AliasResult::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 AliasResult::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 AliasResult::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 AliasResult::NoAlias;
return AliasResult::MayAlias;
}
ModRefInfo GlobalsAAResult::getModRefInfoForArgument(const CallBase *Call,
const GlobalValue *GV,
AAQueryInfo &AAQI) {
if (Call->doesNotAccessMemory())
return ModRefInfo::NoModRef;
ModRefInfo ConservativeResult =
Call->onlyReadsMemory() ? ModRefInfo::Ref : ModRefInfo::ModRef;
// Iterate through all the arguments to the called function. If any argument
// is based on GV, return the conservative result.
for (const auto &A : Call->args()) {
SmallVector<const Value*, 4> Objects;
getUnderlyingObjects(A, Objects);
// All objects must be identified.
if (!all_of(Objects, isIdentifiedObject) &&
// Try ::alias to see if all objects are known not to alias GV.
!all_of(Objects, [&](const Value *V) {
return this->alias(MemoryLocation::getBeforeOrAfter(V),
MemoryLocation::getBeforeOrAfter(GV), AAQI,
nullptr) == AliasResult::NoAlias;
}))
return ConservativeResult;
if (is_contained(Objects, GV))
return ConservativeResult;
}
// We identified all objects in the argument list, and none of them were GV.
return ModRefInfo::NoModRef;
}
ModRefInfo GlobalsAAResult::getModRefInfo(const CallBase *Call,
const MemoryLocation &Loc,
AAQueryInfo &AAQI) {
ModRefInfo Known = ModRefInfo::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)))
// If GV is internal to this IR and there is no function with local linkage
// that has had their address taken, keep looking for a tighter ModRefInfo.
if (GV->hasLocalLinkage() && !UnknownFunctionsWithLocalLinkage)
if (const Function *F = Call->getCalledFunction())
if (NonAddressTakenGlobals.count(GV))
if (const FunctionInfo *FI = getFunctionInfo(F))
Known = FI->getModRefInfoForGlobal(*GV) |
getModRefInfoForArgument(Call, GV, AAQI);
return Known;
}
GlobalsAAResult::GlobalsAAResult(
const DataLayout &DL,
std::function<const TargetLibraryInfo &(Function &F)> GetTLI)
: DL(DL), GetTLI(std::move(GetTLI)) {}
GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg)
: AAResultBase(std::move(Arg)), DL(Arg.DL), GetTLI(std::move(Arg.GetTLI)),
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;
}
}
GlobalsAAResult::~GlobalsAAResult() = default;
/*static*/ GlobalsAAResult GlobalsAAResult::analyzeModule(
Module &M, std::function<const TargetLibraryInfo &(Function &F)> GetTLI,
CallGraph &CG) {
GlobalsAAResult Result(M.getDataLayout(), GetTLI);
// 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;
}
AnalysisKey GlobalsAA::Key;
GlobalsAAResult GlobalsAA::run(Module &M, ModuleAnalysisManager &AM) {
FunctionAnalysisManager &FAM =
AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
return FAM.getResult<TargetLibraryAnalysis>(F);
};
return GlobalsAAResult::analyzeModule(M, GetTLI,
AM.getResult<CallGraphAnalysis>(M));
}
PreservedAnalyses RecomputeGlobalsAAPass::run(Module &M,
ModuleAnalysisManager &AM) {
if (auto *G = AM.getCachedResult<GlobalsAA>(M)) {
auto &CG = AM.getResult<CallGraphAnalysis>(M);
G->NonAddressTakenGlobals.clear();
G->UnknownFunctionsWithLocalLinkage = false;
G->IndirectGlobals.clear();
G->AllocsForIndirectGlobals.clear();
G->FunctionInfos.clear();
G->FunctionToSCCMap.clear();
G->Handles.clear();
G->CollectSCCMembership(CG);
G->AnalyzeGlobals(M);
G->AnalyzeCallGraph(CG, M);
}
return PreservedAnalyses::all();
}
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) {
auto GetTLI = [this](Function &F) -> TargetLibraryInfo & {
return this->getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
};
Result.reset(new GlobalsAAResult(GlobalsAAResult::analyzeModule(
M, 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>();
}
