| //===- 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 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) >= (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! |
| /// It overlaps with ModRefInfo::Must bit! |
| /// FunctionInfo.getModRefInfo() masks out everything except ModRef so |
| /// this remains correct, but the Must info is lost. |
| enum { MayReadAnyGlobal = 4 }; |
| |
| /// Checks to document the invariants of the bit packing here. |
| static_assert((MayReadAnyGlobal & static_cast<int>(ModRefInfo::MustModRef)) == |
| 0, |
| "ModRef and the MayReadAnyGlobal flag bits overlap."); |
| static_assert(((MayReadAnyGlobal | |
| static_cast<int>(ModRefInfo::MustModRef)) >> |
| 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; |
| } |
| |
| /// This method clears MayReadAnyGlobal bit added by GlobalsAAResult to return |
| /// the corresponding ModRefInfo. It must align in functionality with |
| /// clearMust(). |
| ModRefInfo globalClearMayReadAnyGlobal(int I) const { |
| return ModRefInfo((I & static_cast<int>(ModRefInfo::ModRef)) | |
| static_cast<int>(ModRefInfo::NoModRef)); |
| } |
| |
| /// 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>(setMust(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 = unionModRef(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 = unionModRef(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 (!isModOrRefSet(FI->getModRefInfo())) |
| Min = FMRB_DoesNotAccessMemory; |
| else if (!isModSet(FI->getModRefInfo())) |
| Min = FMRB_OnlyReadsMemory; |
| } |
| |
| return FunctionModRefBehavior(AAResultBase::getModRefBehavior(F) & Min); |
| } |
| |
| FunctionModRefBehavior |
| GlobalsAAResult::getModRefBehavior(ImmutableCallSite CS) { |
| FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior; |
| |
| if (!CS.hasOperandBundles()) |
| if (const Function *F = CS.getCalledFunction()) |
| if (FunctionInfo *FI = getFunctionInfo(F)) { |
| if (!isModOrRefSet(FI->getModRefInfo())) |
| Min = FMRB_DoesNotAccessMemory; |
| else if (!isModSet(FI->getModRefInfo())) |
| 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 *, 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; |
| } |
| |
| 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) { |
| 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.isDataOperand(&U)) { |
| // Detect calls to free. |
| if (CS.isArgOperand(&U) && 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))) |
| 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 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?"); |
| |
| 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; |
| |
| // 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->hasFnAttribute(Attribute::OptimizeNone)) { |
| // 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->isIntrinsic() && !F->onlyAccessesArgMemory()) |
| // 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); |
| // 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 (!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()->hasFnAttribute(Attribute::OptimizeNone)) |
| 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 (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(ModRefInfo::ModRef); |
| } else if (Function *Callee = CS.getCalledFunction()) { |
| // The callgraph doesn't include intrinsic calls. |
| if (Callee->isIntrinsic()) { |
| if (isa<DbgInfoIntrinsic>(I)) |
| // Don't let dbg intrinsics affect alias info. |
| continue; |
| |
| FunctionModRefBehavior Behaviour = |
| AAResultBase::getModRefBehavior(Callee); |
| FI.addModRefInfo(createModRefInfo(Behaviour)); |
| } |
| } |
| continue; |
| } |
| |
| // All non-call instructions we use the primary predicates for whether |
| // thay 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(), 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->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(), 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 ModRefInfo::NoModRef; |
| ModRefInfo ConservativeResult = |
| CS.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 (auto &A : CS.args()) { |
| SmallVector<Value*, 4> Objects; |
| GetUnderlyingObjects(A, Objects, DL); |
| |
| // 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, [&](Value *V) { |
| return this->alias(MemoryLocation(V), MemoryLocation(GV)) == 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(ImmutableCallSite CS, |
| const MemoryLocation &Loc) { |
| 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, DL))) |
| if (GV->hasLocalLinkage()) |
| if (const Function *F = CS.getCalledFunction()) |
| if (NonAddressTakenGlobals.count(GV)) |
| if (const FunctionInfo *FI = getFunctionInfo(F)) |
| Known = unionModRef(FI->getModRefInfoForGlobal(*GV), |
| getModRefInfoForArgument(CS, GV)); |
| |
| if (!isModOrRefSet(Known)) |
| return ModRefInfo::NoModRef; // No need to query other mod/ref analyses |
| return intersectModRef(Known, AAResultBase::getModRefInfo(CS, Loc)); |
| } |
| |
| GlobalsAAResult::GlobalsAAResult(const DataLayout &DL, |
| const TargetLibraryInfo &TLI) |
| : AAResultBase(), DL(DL), TLI(TLI) {} |
| |
| GlobalsAAResult::GlobalsAAResult(GlobalsAAResult &&Arg) |
| : AAResultBase(std::move(Arg)), DL(Arg.DL), TLI(Arg.TLI), |
| 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() {} |
| |
| /*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; |
| } |
| |
| AnalysisKey GlobalsAA::Key; |
| |
| GlobalsAAResult GlobalsAA::run(Module &M, ModuleAnalysisManager &AM) { |
| return GlobalsAAResult::analyzeModule(M, |
| AM.getResult<TargetLibraryAnalysis>(M), |
| AM.getResult<CallGraphAnalysis>(M)); |
| } |
| |
| 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>(); |
| } |
