#include "llvm/Analysis/Passes.h"
#include "llvm/ADT/SCCIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
-#include <set>
+#include <list>
using namespace llvm;
#define DEBUG_TYPE "globalsmodref-aa"
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);
+
namespace {
-/// FunctionRecord - One instance of this structure is stored for every
-/// function in the program. Later, the entries for these functions are
-/// removed if the function is found to call an external function (in which
-/// case we know nothing about it.
-struct FunctionRecord {
- /// GlobalInfo - Maintain mod/ref info for all of the globals without
- /// addresses taken that are read or written (transitively) by this
- /// function.
- std::map<const GlobalValue *, unsigned> GlobalInfo;
-
- /// MayReadAnyGlobal - May read global variables, but it is not known which.
- bool MayReadAnyGlobal;
-
- unsigned getInfoForGlobal(const GlobalValue *GV) const {
- unsigned Effect = MayReadAnyGlobal ? AliasAnalysis::Ref : 0;
- std::map<const GlobalValue *, unsigned>::const_iterator I =
- GlobalInfo.find(GV);
- if (I != GlobalInfo.end())
- Effect |= I->second;
- return Effect;
+/// 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 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);
}
- /// FunctionEffect - Capture whether or not this function reads or writes to
- /// ANY memory. If not, we can do a lot of aggressive analysis on it.
- unsigned FunctionEffect;
+ /// 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);
+ }
- FunctionRecord() : MayReadAnyGlobal(false), FunctionEffect(0) {}
+ /// 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;
};
/// GlobalsModRef - The actual analysis pass.
class GlobalsModRef : public ModulePass, public AliasAnalysis {
- /// NonAddressTakenGlobals - The globals that do not have their addresses
- /// taken.
- std::set<const GlobalValue *> NonAddressTakenGlobals;
+ /// The globals that do not have their addresses taken.
+ SmallPtrSet<const GlobalValue *, 8> NonAddressTakenGlobals;
/// IndirectGlobals - The memory pointed to by this global is known to be
/// 'owned' by the global.
- std::set<const GlobalValue *> IndirectGlobals;
+ SmallPtrSet<const GlobalValue *, 8> IndirectGlobals;
/// AllocsForIndirectGlobals - If an instruction allocates memory for an
/// indirect global, this map indicates which one.
- std::map<const Value *, const GlobalValue *> AllocsForIndirectGlobals;
+ DenseMap<const Value *, const GlobalValue *> AllocsForIndirectGlobals;
+
+ /// For each function, keep track of what globals are modified or read.
+ DenseMap<const Function *, FunctionInfo> FunctionInfos;
+
+ /// Handle to clear this analysis on deletion of values.
+ struct DeletionCallbackHandle final : CallbackVH {
+ GlobalsModRef &GMR;
+ std::list<DeletionCallbackHandle>::iterator I;
+
+ DeletionCallbackHandle(GlobalsModRef &GMR, Value *V)
+ : CallbackVH(V), GMR(GMR) {}
+
+ void deleted() override {
+ Value *V = getValPtr();
+ if (auto *F = dyn_cast<Function>(V))
+ GMR.FunctionInfos.erase(F);
+
+ if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
+ if (GMR.NonAddressTakenGlobals.erase(GV)) {
+ // This global might be an indirect global. If so, remove it and
+ // remove any AllocRelatedValues for it.
+ if (GMR.IndirectGlobals.erase(GV)) {
+ // Remove any entries in AllocsForIndirectGlobals for this global.
+ for (auto I = GMR.AllocsForIndirectGlobals.begin(),
+ E = GMR.AllocsForIndirectGlobals.end();
+ I != E; ++I)
+ if (I->second == GV)
+ GMR.AllocsForIndirectGlobals.erase(I);
+ }
- /// FunctionInfo - For each function, keep track of what globals are
- /// modified or read.
- std::map<const Function *, FunctionRecord> FunctionInfo;
+ // Scan the function info we have collected and remove this global
+ // from all of them.
+ for (auto &FIPair : GMR.FunctionInfos)
+ FIPair.second.eraseModRefInfoForGlobal(*GV);
+ }
+ }
+
+ // If this is an allocation related to an indirect global, remove it.
+ GMR.AllocsForIndirectGlobals.erase(V);
+
+ // And clear out the handle.
+ setValPtr(nullptr);
+ GMR.Handles.erase(I);
+ // This object is now destroyed!
+ }
+ };
+
+ /// List of callbacks for globals being tracked by this analysis. Note that
+ /// these objects are quite large, but we only anticipate having one per
+ /// global tracked by this analysis. There are numerous optimizations we
+ /// could perform to the memory utilization here if this becomes a problem.
+ std::list<DeletionCallbackHandle> Handles;
public:
static char ID;
AU.setPreservesAll(); // Does not transform code
}
+ /// getAdjustedAnalysisPointer - This method is used when a pass implements
+ /// an analysis interface through multiple inheritance. If needed, it
+ /// should override this to adjust the this pointer as needed for the
+ /// specified pass info.
+ void *getAdjustedAnalysisPointer(AnalysisID PI) override {
+ if (PI == &AliasAnalysis::ID)
+ return (AliasAnalysis *)this;
+ return this;
+ }
+
//------------------------------------------------
// Implement the AliasAnalysis API
//
AliasResult alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) override;
- ModRefResult getModRefInfo(ImmutableCallSite CS,
- const MemoryLocation &Loc) override;
- ModRefResult getModRefInfo(ImmutableCallSite CS1,
- ImmutableCallSite CS2) override {
+ ModRefInfo getModRefInfo(ImmutableCallSite CS,
+ const MemoryLocation &Loc) override;
+ ModRefInfo getModRefInfo(ImmutableCallSite CS1,
+ ImmutableCallSite CS2) override {
return AliasAnalysis::getModRefInfo(CS1, CS2);
}
/// getModRefBehavior - Return the behavior of the specified function if
/// called from the specified call site. The call site may be null in which
/// case the most generic behavior of this function should be returned.
- ModRefBehavior getModRefBehavior(const Function *F) override {
- ModRefBehavior Min = UnknownModRefBehavior;
-
- if (FunctionRecord *FR = getFunctionInfo(F)) {
- if (FR->FunctionEffect == 0)
- Min = DoesNotAccessMemory;
- else if ((FR->FunctionEffect & Mod) == 0)
- Min = OnlyReadsMemory;
+ FunctionModRefBehavior getModRefBehavior(const Function *F) override {
+ 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 ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
+ return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
}
/// getModRefBehavior - Return the behavior of the specified function if
/// called from the specified call site. The call site may be null in which
/// case the most generic behavior of this function should be returned.
- ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
- ModRefBehavior Min = UnknownModRefBehavior;
+ FunctionModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
+ FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
if (const Function *F = CS.getCalledFunction())
- if (FunctionRecord *FR = getFunctionInfo(F)) {
- if (FR->FunctionEffect == 0)
- Min = DoesNotAccessMemory;
- else if ((FR->FunctionEffect & Mod) == 0)
- Min = OnlyReadsMemory;
+ if (FunctionInfo *FI = getFunctionInfo(F)) {
+ if (FI->getModRefInfo() == MRI_NoModRef)
+ Min = FMRB_DoesNotAccessMemory;
+ else if ((FI->getModRefInfo() & MRI_Mod) == 0)
+ Min = FMRB_OnlyReadsMemory;
}
- return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
- }
-
- void deleteValue(Value *V) override;
- void addEscapingUse(Use &U) override;
-
- /// getAdjustedAnalysisPointer - This method is used when a pass implements
- /// an analysis interface through multiple inheritance. If needed, it
- /// should override this to adjust the this pointer as needed for the
- /// specified pass info.
- void *getAdjustedAnalysisPointer(AnalysisID PI) override {
- if (PI == &AliasAnalysis::ID)
- return (AliasAnalysis *)this;
- return this;
+ return FunctionModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
}
private:
- /// getFunctionInfo - Return the function info for the function, or null if
- /// we don't have anything useful to say about it.
- FunctionRecord *getFunctionInfo(const Function *F) {
- std::map<const Function *, FunctionRecord>::iterator I =
- FunctionInfo.find(F);
- if (I != FunctionInfo.end())
+ /// Returns the function info for the function, or null if we don't have
+ /// anything useful to say about it.
+ FunctionInfo *getFunctionInfo(const Function *F) {
+ auto I = FunctionInfos.find(F);
+ if (I != FunctionInfos.end())
return &I->second;
return nullptr;
}
void AnalyzeGlobals(Module &M);
void AnalyzeCallGraph(CallGraph &CG, Module &M);
- bool AnalyzeUsesOfPointer(Value *V, std::vector<Function *> &Readers,
- std::vector<Function *> &Writers,
+ bool AnalyzeUsesOfPointer(Value *V,
+ SmallPtrSetImpl<Function *> *Readers = nullptr,
+ SmallPtrSetImpl<Function *> *Writers = nullptr,
GlobalValue *OkayStoreDest = nullptr);
bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
};
/// (really, their address passed to something nontrivial), record this fact,
/// and record the functions that they are used directly in.
void GlobalsModRef::AnalyzeGlobals(Module &M) {
- std::vector<Function *> Readers, Writers;
+ SmallPtrSet<Function *, 64> TrackedFunctions;
for (Function &F : M)
- if (F.hasLocalLinkage()) {
- if (!AnalyzeUsesOfPointer(&F, Readers, Writers)) {
+ 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;
}
- Readers.clear();
- Writers.clear();
- }
+ SmallPtrSet<Function *, 64> Readers, Writers;
for (GlobalVariable &GV : M.globals())
if (GV.hasLocalLinkage()) {
- if (!AnalyzeUsesOfPointer(&GV, Readers, Writers)) {
+ 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)
- FunctionInfo[Reader].GlobalInfo[&GV] |= Ref;
+ 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)
- FunctionInfo[Writer].GlobalInfo[&GV] |= Mod;
+ 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 OkayStoreDest is non-null, stores into this global are allowed.
bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V,
- std::vector<Function *> &Readers,
- std::vector<Function *> &Writers,
+ 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)) {
- Readers.push_back(LI->getParent()->getParent());
+ if (Readers)
+ Readers->insert(LI->getParent()->getParent());
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (V == SI->getOperand(1)) {
- Writers.push_back(SI->getParent()->getParent());
+ if (Writers)
+ Writers->insert(SI->getParent()->getParent());
} else if (SI->getOperand(1) != OkayStoreDest) {
return true; // Storing the pointer
}
// passing into the function.
if (!CS.isCallee(&U)) {
// Detect calls to free.
- if (isFreeCall(I, TLI))
- Writers.push_back(CS->getParent()->getParent());
- else
+ if (isFreeCall(I, TLI)) {
+ if (Writers)
+ Writers->insert(CS->getParent()->getParent());
+ } else {
return true; // Argument of an unknown call.
+ }
}
} else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
// 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.
- std::vector<Function *> ReadersWriters;
- if (AnalyzeUsesOfPointer(LI, ReadersWriters, ReadersWriters))
+ 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)) {
// 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.
- std::vector<Function *> ReadersWriters;
- if (AnalyzeUsesOfPointer(Ptr, ReadersWriters, ReadersWriters, GV))
+ if (AnalyzeUsesOfPointer(Ptr, /*Readers*/ nullptr, /*Writers*/ nullptr,
+ GV))
return false; // Loaded pointer escapes.
// Remember that this allocation is related to the indirect global.
// 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;
}
// Calls externally - can't say anything useful. Remove any existing
// function records (may have been created when scanning globals).
for (auto *Node : SCC)
- FunctionInfo.erase(Node->getFunction());
+ FunctionInfos.erase(Node->getFunction());
continue;
}
- FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
-
+ FunctionInfo &FI = FunctionInfos[SCC[0]->getFunction()];
bool KnowNothing = false;
- unsigned FunctionEffect = 0;
// Collect the mod/ref properties due to called functions. We only compute
// one mod-ref set.
if (F->doesNotAccessMemory()) {
// Can't do better than that!
} else if (F->onlyReadsMemory()) {
- FunctionEffect |= Ref;
+ 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.
- FR.MayReadAnyGlobal = true;
+ FI.setMayReadAnyGlobal();
} else {
- FunctionEffect |= ModRef;
+ 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();
for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
CI != E && !KnowNothing; ++CI)
if (Function *Callee = CI->second->getFunction()) {
- if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
+ if (FunctionInfo *CalleeFI = getFunctionInfo(Callee)) {
// Propagate function effect up.
- FunctionEffect |= CalleeFR->FunctionEffect;
-
- // Incorporate callee's effects on globals into our info.
- for (const auto &G : CalleeFR->GlobalInfo)
- FR.GlobalInfo[G.first] |= G.second;
- FR.MayReadAnyGlobal |= CalleeFR->MayReadAnyGlobal;
+ FI.addFunctionInfo(*CalleeFI);
} else {
// Can't say anything about it. However, if it is inside our SCC,
// then nothing needs to be done.
}
// If we can't say anything useful about this SCC, remove all SCC functions
- // from the FunctionInfo map.
+ // from the FunctionInfos map.
if (KnowNothing) {
for (auto *Node : SCC)
- FunctionInfo.erase(Node->getFunction());
+ FunctionInfos.erase(Node->getFunction());
continue;
}
// Scan the function bodies for explicit loads or stores.
- for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;
- ++i)
- for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
- E = inst_end(SCC[i]->getFunction());
- II != E && FunctionEffect != ModRef; ++II)
- if (LoadInst *LI = dyn_cast<LoadInst>(&*II)) {
- FunctionEffect |= Ref;
- if (LI->isVolatile())
- // Volatile loads may have side-effects, so mark them as writing
- // memory (for example, a flag inside the processor).
- FunctionEffect |= Mod;
- } else if (StoreInst *SI = dyn_cast<StoreInst>(&*II)) {
- FunctionEffect |= Mod;
- if (SI->isVolatile())
- // Treat volatile stores as reading memory somewhere.
- FunctionEffect |= Ref;
- } else if (isAllocationFn(&*II, TLI) || isFreeCall(&*II, TLI)) {
- FunctionEffect |= ModRef;
- } else if (IntrinsicInst *Intrinsic = dyn_cast<IntrinsicInst>(&*II)) {
- // The callgraph doesn't include intrinsic calls.
- Function *Callee = Intrinsic->getCalledFunction();
- ModRefBehavior Behaviour = AliasAnalysis::getModRefBehavior(Callee);
- FunctionEffect |= (Behaviour & ModRef);
+ for (auto *Node : SCC) {
+ if (FI.getModRefInfo() == MRI_ModRef)
+ break; // The mod/ref lattice saturates here.
+ for (Instruction &I : inst_range(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 =
+ AliasAnalysis::getModRefBehavior(Callee);
+ FI.addModRefInfo(ModRefInfo(Behaviour & MRI_ModRef));
+ }
+ }
+ continue;
}
- if ((FunctionEffect & Mod) == 0)
+ // 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 (FunctionEffect == 0)
+ if (FI.getModRefInfo() == MRI_NoModRef)
++NumNoMemFunctions;
- FR.FunctionEffect = FunctionEffect;
// Finally, now that we know the full effect on this SCC, clone the
// information to each function in the SCC.
for (unsigned i = 1, e = SCC.size(); i != e; ++i)
- FunctionInfo[SCC[i]->getFunction()] = FR;
+ FunctionInfos[SCC[i]->getFunction()] = FI;
}
}
GV2 = nullptr;
// If the two pointers are derived from two different non-addr-taken
- // globals, or if one is and the other isn't, we know these can't alias.
- if ((GV1 || GV2) && GV1 != GV2)
+ // 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;
+
+ // 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.
+ if ((GV1 || GV2) && GV1 != GV2) {
+ const Value *UV = GV1 ? UV2 : UV1;
+
+ // 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.
+ if (isa<Argument>(UV) || isa<CallInst>(UV) || isa<InvokeInst>(UV)) {
+ // Arguments to functions or returns from functions are inherently
+ // escaping, so we can immediately classify those as not aliasing any
+ // non-addr-taken globals.
+ return NoAlias;
+ } else if (auto *LI = dyn_cast<LoadInst>(UV)) {
+ // A pointer loaded from a global would have been captured, and we know
+ // that GV is non-addr-taken, so no alias.
+ if (isa<GlobalValue>(LI->getPointerOperand()))
+ 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 also be from an allocation for the indirect global. If
// so, also handle them.
- if (AllocsForIndirectGlobals.count(UV1))
- GV1 = AllocsForIndirectGlobals[UV1];
- if (AllocsForIndirectGlobals.count(UV2))
- GV2 = AllocsForIndirectGlobals[UV2];
+ 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 either pointer is based on an
- // indirect global and if they are not both based on the same indirect global,
- // they cannot alias.
- if ((GV1 || GV2) && GV1 != GV2)
+ // 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 AliasAnalysis::alias(LocA, LocB);
}
-AliasAnalysis::ModRefResult
-GlobalsModRef::getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc) {
- unsigned Known = ModRef;
+ModRefInfo GlobalsModRef::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 (GV->hasLocalLinkage())
if (const Function *F = CS.getCalledFunction())
if (NonAddressTakenGlobals.count(GV))
- if (const FunctionRecord *FR = getFunctionInfo(F))
- Known = FR->getInfoForGlobal(GV);
-
- if (Known == NoModRef)
- return NoModRef; // No need to query other mod/ref analyses
- return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, Loc));
-}
-
-//===----------------------------------------------------------------------===//
-// Methods to update the analysis as a result of the client transformation.
-//
-void GlobalsModRef::deleteValue(Value *V) {
- if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
- if (NonAddressTakenGlobals.erase(GV)) {
- // This global might be an indirect global. If so, remove it and remove
- // any AllocRelatedValues for it.
- if (IndirectGlobals.erase(GV)) {
- // Remove any entries in AllocsForIndirectGlobals for this global.
- for (std::map<const Value *, const GlobalValue *>::iterator
- I = AllocsForIndirectGlobals.begin(),
- E = AllocsForIndirectGlobals.end();
- I != E;) {
- if (I->second == GV) {
- AllocsForIndirectGlobals.erase(I++);
- } else {
- ++I;
- }
- }
- }
- }
- }
-
- // Otherwise, if this is an allocation related to an indirect global, remove
- // it.
- AllocsForIndirectGlobals.erase(V);
-
- AliasAnalysis::deleteValue(V);
-}
-
-void GlobalsModRef::addEscapingUse(Use &U) {
- // For the purposes of this analysis, it is conservatively correct to treat
- // a newly escaping value equivalently to a deleted one. We could perhaps
- // be more precise by processing the new use and attempting to update our
- // saved analysis results to accommodate it.
- deleteValue(U);
+ if (const FunctionInfo *FI = getFunctionInfo(F))
+ Known = FI->getModRefInfoForGlobal(*GV);
- AliasAnalysis::addEscapingUse(U);
+ if (Known == MRI_NoModRef)
+ return MRI_NoModRef; // No need to query other mod/ref analyses
+ return ModRefInfo(Known & AliasAnalysis::getModRefInfo(CS, Loc));
}
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