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Diffstat (limited to 'llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp')
| -rw-r--r-- | llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp | 724 |
1 files changed, 724 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp new file mode 100644 index 00000000000..92f119b2ba7 --- /dev/null +++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp @@ -0,0 +1,724 @@ +//===- WholeProgramDevirt.cpp - Whole program virtual call optimization ---===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This pass implements whole program optimization of virtual calls in cases +// where we know (via bitset information) that the list of callee is fixed. This +// includes the following: +// - Single implementation devirtualization: if a virtual call has a single +// possible callee, replace all calls with a direct call to that callee. +// - Virtual constant propagation: if the virtual function's return type is an +// integer <=64 bits and all possible callees are readnone, for each class and +// each list of constant arguments: evaluate the function, store the return +// value alongside the virtual table, and rewrite each virtual call as a load +// from the virtual table. +// - Uniform return value optimization: if the conditions for virtual constant +// propagation hold and each function returns the same constant value, replace +// each virtual call with that constant. +// - Unique return value optimization for i1 return values: if the conditions +// for virtual constant propagation hold and a single vtable's function +// returns 0, or a single vtable's function returns 1, replace each virtual +// call with a comparison of the vptr against that vtable's address. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/IPO/WholeProgramDevirt.h" +#include "llvm/Transforms/IPO.h" +#include "llvm/ADT/DenseSet.h" +#include "llvm/ADT/MapVector.h" +#include "llvm/IR/CallSite.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DataLayout.h" +#include "llvm/IR/IRBuilder.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/Intrinsics.h" +#include "llvm/IR/Module.h" +#include "llvm/Pass.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Utils/Evaluator.h" +#include "llvm/Transforms/Utils/Local.h" + +#include <set> + +using namespace llvm; +using namespace wholeprogramdevirt; + +#define DEBUG_TYPE "wholeprogramdevirt" + +// Find the minimum offset that we may store a value of size Size bits at. If +// IsAfter is set, look for an offset before the object, otherwise look for an +// offset after the object. +uint64_t +wholeprogramdevirt::findLowestOffset(ArrayRef<VirtualCallTarget> Targets, + bool IsAfter, uint64_t Size) { + // Find a minimum offset taking into account only vtable sizes. + uint64_t MinByte = 0; + for (const VirtualCallTarget &Target : Targets) { + if (IsAfter) + MinByte = std::max(MinByte, Target.minAfterBytes()); + else + MinByte = std::max(MinByte, Target.minBeforeBytes()); + } + + // Build a vector of arrays of bytes covering, for each target, a slice of the + // used region (see AccumBitVector::BytesUsed in + // llvm/Transforms/IPO/WholeProgramDevirt.h) starting at MinByte. Effectively, + // this aligns the used regions to start at MinByte. + // + // In this example, A, B and C are vtables, # is a byte already allocated for + // a virtual function pointer, AAAA... (etc.) are the used regions for the + // vtables and Offset(X) is the value computed for the Offset variable below + // for X. + // + // Offset(A) + // | | + // |MinByte + // A: ################AAAAAAAA|AAAAAAAA + // B: ########BBBBBBBBBBBBBBBB|BBBB + // C: ########################|CCCCCCCCCCCCCCCC + // | Offset(B) | + // + // This code produces the slices of A, B and C that appear after the divider + // at MinByte. + std::vector<ArrayRef<uint8_t>> Used; + for (const VirtualCallTarget &Target : Targets) { + ArrayRef<uint8_t> VTUsed = IsAfter ? Target.BS->Bits->After.BytesUsed + : Target.BS->Bits->Before.BytesUsed; + uint64_t Offset = IsAfter ? MinByte - Target.minAfterBytes() + : MinByte - Target.minBeforeBytes(); + + // Disregard used regions that are smaller than Offset. These are + // effectively all-free regions that do not need to be checked. + if (VTUsed.size() > Offset) + Used.push_back(VTUsed.slice(Offset)); + } + + if (Size == 1) { + // Find a free bit in each member of Used. + for (unsigned I = 0;; ++I) { + uint8_t BitsUsed = 0; + for (auto &&B : Used) + if (I < B.size()) + BitsUsed |= B[I]; + if (BitsUsed != 0xff) + return (MinByte + I) * 8 + + countTrailingZeros(uint8_t(~BitsUsed), ZB_Undefined); + } + } else { + // Find a free (Size/8) byte region in each member of Used. + // FIXME: see if alignment helps. + for (unsigned I = 0;; ++I) { + for (auto &&B : Used) { + unsigned Byte = 0; + while ((I + Byte) < B.size() && Byte < (Size / 8)) { + if (B[I + Byte]) + goto NextI; + ++Byte; + } + } + return (MinByte + I) * 8; + NextI:; + } + } +} + +void wholeprogramdevirt::setBeforeReturnValues( + MutableArrayRef<VirtualCallTarget> Targets, uint64_t AllocBefore, + unsigned BitWidth, int64_t &OffsetByte, uint64_t &OffsetBit) { + if (BitWidth == 1) + OffsetByte = -(AllocBefore / 8 + 1); + else + OffsetByte = -((AllocBefore + 7) / 8 + (BitWidth + 7) / 8); + OffsetBit = AllocBefore % 8; + + for (VirtualCallTarget &Target : Targets) { + if (BitWidth == 1) + Target.setBeforeBit(AllocBefore); + else + Target.setBeforeBytes(AllocBefore, (BitWidth + 7) / 8); + } +} + +void wholeprogramdevirt::setAfterReturnValues( + MutableArrayRef<VirtualCallTarget> Targets, uint64_t AllocAfter, + unsigned BitWidth, int64_t &OffsetByte, uint64_t &OffsetBit) { + if (BitWidth == 1) + OffsetByte = AllocAfter / 8; + else + OffsetByte = (AllocAfter + 7) / 8; + OffsetBit = AllocAfter % 8; + + for (VirtualCallTarget &Target : Targets) { + if (BitWidth == 1) + Target.setAfterBit(AllocAfter); + else + Target.setAfterBytes(AllocAfter, (BitWidth + 7) / 8); + } +} + +VirtualCallTarget::VirtualCallTarget(Function *Fn, const BitSetInfo *BS) + : Fn(Fn), BS(BS), + IsBigEndian(Fn->getParent()->getDataLayout().isBigEndian()) {} + +namespace { + +// A slot in a set of virtual tables. The BitSetID identifies the set of virtual +// tables, and the ByteOffset is the offset in bytes from the address point to +// the virtual function pointer. +struct VTableSlot { + Metadata *BitSetID; + uint64_t ByteOffset; +}; + +} + +template <> struct DenseMapInfo<VTableSlot> { + static VTableSlot getEmptyKey() { + return {DenseMapInfo<Metadata *>::getEmptyKey(), + DenseMapInfo<uint64_t>::getEmptyKey()}; + } + static VTableSlot getTombstoneKey() { + return {DenseMapInfo<Metadata *>::getTombstoneKey(), + DenseMapInfo<uint64_t>::getTombstoneKey()}; + } + static unsigned getHashValue(const VTableSlot &I) { + return DenseMapInfo<Metadata *>::getHashValue(I.BitSetID) ^ + DenseMapInfo<uint64_t>::getHashValue(I.ByteOffset); + } + static bool isEqual(const VTableSlot &LHS, + const VTableSlot &RHS) { + return LHS.BitSetID == RHS.BitSetID && LHS.ByteOffset == RHS.ByteOffset; + } +}; + +namespace { + +// A virtual call site. VTable is the loaded virtual table pointer, and CS is +// the indirect virtual call. +struct VirtualCallSite { + Value *VTable; + CallSite CS; + + void replaceAndErase(Value *New) { + CS->replaceAllUsesWith(New); + if (auto II = dyn_cast<InvokeInst>(CS.getInstruction())) { + BranchInst::Create(II->getNormalDest(), CS.getInstruction()); + II->getUnwindDest()->removePredecessor(II->getParent()); + } + CS->eraseFromParent(); + } +}; + +struct DevirtModule { + Module &M; + IntegerType *Int8Ty; + PointerType *Int8PtrTy; + IntegerType *Int32Ty; + + MapVector<VTableSlot, std::vector<VirtualCallSite>> CallSlots; + + DevirtModule(Module &M) + : M(M), Int8Ty(Type::getInt8Ty(M.getContext())), + Int8PtrTy(Type::getInt8PtrTy(M.getContext())), + Int32Ty(Type::getInt32Ty(M.getContext())) {} + void findLoadCallsAtConstantOffset(Metadata *BitSet, Value *Ptr, + uint64_t Offset, Value *VTable); + void findCallsAtConstantOffset(Metadata *BitSet, Value *Ptr, uint64_t Offset, + Value *VTable); + + void buildBitSets(std::vector<VTableBits> &Bits, + DenseMap<Metadata *, std::set<BitSetInfo>> &BitSets); + bool tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot, + const std::set<BitSetInfo> &BitSetInfos, + uint64_t ByteOffset); + bool trySingleImplDevirt(ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites); + bool tryEvaluateFunctionsWithArgs( + MutableArrayRef<VirtualCallTarget> TargetsForSlot, + ArrayRef<ConstantInt *> Args); + bool tryUniformRetValOpt(IntegerType *RetType, + ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites); + bool tryUniqueRetValOpt(unsigned BitWidth, + ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites); + bool tryVirtualConstProp(MutableArrayRef<VirtualCallTarget> TargetsForSlot, + ArrayRef<VirtualCallSite> CallSites); + + void rebuildGlobal(VTableBits &B); + + bool run(); +}; + +struct WholeProgramDevirt : public ModulePass { + static char ID; + WholeProgramDevirt() : ModulePass(ID) { + initializeWholeProgramDevirtPass(*PassRegistry::getPassRegistry()); + } + bool runOnModule(Module &M) { return DevirtModule(M).run(); } +}; + +} // anonymous namespace + +INITIALIZE_PASS(WholeProgramDevirt, "wholeprogramdevirt", + "Whole program devirtualization", false, false) +char WholeProgramDevirt::ID = 0; + +ModulePass *llvm::createWholeProgramDevirtPass() { + return new WholeProgramDevirt; +} + +// Search for virtual calls that call FPtr and add them to CallSlots. +void DevirtModule::findCallsAtConstantOffset(Metadata *BitSet, Value *FPtr, + uint64_t Offset, Value *VTable) { + for (const Use &U : FPtr->uses()) { + Value *User = U.getUser(); + if (isa<BitCastInst>(User)) { + findCallsAtConstantOffset(BitSet, User, Offset, VTable); + } else if (auto CI = dyn_cast<CallInst>(User)) { + CallSlots[{BitSet, Offset}].push_back({VTable, CI}); + } else if (auto II = dyn_cast<InvokeInst>(User)) { + CallSlots[{BitSet, Offset}].push_back({VTable, II}); + } + } +} + +// Search for virtual calls that load from VPtr and add them to CallSlots. +void DevirtModule::findLoadCallsAtConstantOffset(Metadata *BitSet, Value *VPtr, + uint64_t Offset, + Value *VTable) { + for (const Use &U : VPtr->uses()) { + Value *User = U.getUser(); + if (isa<BitCastInst>(User)) { + findLoadCallsAtConstantOffset(BitSet, User, Offset, VTable); + } else if (isa<LoadInst>(User)) { + findCallsAtConstantOffset(BitSet, User, Offset, VTable); + } else if (auto GEP = dyn_cast<GetElementPtrInst>(User)) { + // Take into account the GEP offset. + if (VPtr == GEP->getPointerOperand() && GEP->hasAllConstantIndices()) { + SmallVector<Value *, 8> Indices(GEP->op_begin() + 1, GEP->op_end()); + uint64_t GEPOffset = M.getDataLayout().getIndexedOffsetInType( + GEP->getSourceElementType(), Indices); + findLoadCallsAtConstantOffset(BitSet, User, Offset + GEPOffset, VTable); + } + } + } +} + +void DevirtModule::buildBitSets( + std::vector<VTableBits> &Bits, + DenseMap<Metadata *, std::set<BitSetInfo>> &BitSets) { + NamedMDNode *BitSetNM = M.getNamedMetadata("llvm.bitsets"); + if (!BitSetNM) + return; + + DenseMap<GlobalVariable *, VTableBits *> GVToBits; + Bits.reserve(BitSetNM->getNumOperands()); + for (auto Op : BitSetNM->operands()) { + auto OpConstMD = dyn_cast_or_null<ConstantAsMetadata>(Op->getOperand(1)); + if (!OpConstMD) + continue; + auto BitSetID = Op->getOperand(0).get(); + + Constant *OpConst = OpConstMD->getValue(); + if (auto GA = dyn_cast<GlobalAlias>(OpConst)) + OpConst = GA->getAliasee(); + auto OpGlobal = dyn_cast<GlobalVariable>(OpConst); + if (!OpGlobal) + continue; + + uint64_t Offset = + cast<ConstantInt>( + cast<ConstantAsMetadata>(Op->getOperand(2))->getValue()) + ->getZExtValue(); + + VTableBits *&BitsPtr = GVToBits[OpGlobal]; + if (!BitsPtr) { + Bits.emplace_back(); + Bits.back().GV = OpGlobal; + Bits.back().ObjectSize = M.getDataLayout().getTypeAllocSize( + OpGlobal->getInitializer()->getType()); + BitsPtr = &Bits.back(); + } + BitSets[BitSetID].insert({BitsPtr, Offset}); + } +} + +bool DevirtModule::tryFindVirtualCallTargets( + std::vector<VirtualCallTarget> &TargetsForSlot, + const std::set<BitSetInfo> &BitSetInfos, uint64_t ByteOffset) { + for (const BitSetInfo &BS : BitSetInfos) { + if (!BS.Bits->GV->isConstant()) + return false; + + auto Init = dyn_cast<ConstantArray>(BS.Bits->GV->getInitializer()); + if (!Init) + return false; + ArrayType *VTableTy = Init->getType(); + + uint64_t ElemSize = + M.getDataLayout().getTypeAllocSize(VTableTy->getElementType()); + uint64_t GlobalSlotOffset = BS.Offset + ByteOffset; + if (GlobalSlotOffset % ElemSize != 0) + return false; + + unsigned Op = GlobalSlotOffset / ElemSize; + if (Op >= Init->getNumOperands()) + return false; + + auto Fn = dyn_cast<Function>(Init->getOperand(Op)->stripPointerCasts()); + if (!Fn) + return false; + + // We can disregard __cxa_pure_virtual as a possible call target, as + // calls to pure virtuals are UB. + if (Fn->getName() == "__cxa_pure_virtual") + continue; + + TargetsForSlot.push_back({Fn, &BS}); + } + + // Give up if we couldn't find any targets. + return !TargetsForSlot.empty(); +} + +bool DevirtModule::trySingleImplDevirt( + ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites) { + // See if the program contains a single implementation of this virtual + // function. + Function *TheFn = TargetsForSlot[0].Fn; + for (auto &&Target : TargetsForSlot) + if (TheFn != Target.Fn) + return false; + + // If so, update each call site to call that implementation directly. + for (auto &&VCallSite : CallSites) { + VCallSite.CS.setCalledFunction(ConstantExpr::getBitCast( + TheFn, VCallSite.CS.getCalledValue()->getType())); + } + return true; +} + +bool DevirtModule::tryEvaluateFunctionsWithArgs( + MutableArrayRef<VirtualCallTarget> TargetsForSlot, + ArrayRef<ConstantInt *> Args) { + // Evaluate each function and store the result in each target's RetVal + // field. + for (VirtualCallTarget &Target : TargetsForSlot) { + if (Target.Fn->arg_size() != Args.size() + 1) + return false; + for (unsigned I = 0; I != Args.size(); ++I) + if (Target.Fn->getFunctionType()->getParamType(I + 1) != + Args[I]->getType()) + return false; + + Evaluator Eval(M.getDataLayout(), nullptr); + SmallVector<Constant *, 2> EvalArgs; + EvalArgs.push_back( + Constant::getNullValue(Target.Fn->getFunctionType()->getParamType(0))); + EvalArgs.insert(EvalArgs.end(), Args.begin(), Args.end()); + Constant *RetVal; + if (!Eval.EvaluateFunction(Target.Fn, RetVal, EvalArgs) || + !isa<ConstantInt>(RetVal)) + return false; + Target.RetVal = cast<ConstantInt>(RetVal)->getZExtValue(); + } + return true; +} + +bool DevirtModule::tryUniformRetValOpt( + IntegerType *RetType, ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites) { + // Uniform return value optimization. If all functions return the same + // constant, replace all calls with that constant. + uint64_t TheRetVal = TargetsForSlot[0].RetVal; + for (const VirtualCallTarget &Target : TargetsForSlot) + if (Target.RetVal != TheRetVal) + return false; + + auto TheRetValConst = ConstantInt::get(RetType, TheRetVal); + for (auto Call : CallSites) + Call.replaceAndErase(TheRetValConst); + return true; +} + +bool DevirtModule::tryUniqueRetValOpt( + unsigned BitWidth, ArrayRef<VirtualCallTarget> TargetsForSlot, + MutableArrayRef<VirtualCallSite> CallSites) { + // IsOne controls whether we look for a 0 or a 1. + auto tryUniqueRetValOptFor = [&](bool IsOne) { + const BitSetInfo *UniqueBitSet = 0; + for (const VirtualCallTarget &Target : TargetsForSlot) { + if (Target.RetVal == IsOne ? 1 : 0) { + if (UniqueBitSet) + return false; + UniqueBitSet = Target.BS; + } + } + + // We should have found a unique bit set or bailed out by now. We already + // checked for a uniform return value in tryUniformRetValOpt. + assert(UniqueBitSet); + + // Replace each call with the comparison. + for (auto &&Call : CallSites) { + IRBuilder<> B(Call.CS.getInstruction()); + Value *OneAddr = B.CreateBitCast(UniqueBitSet->Bits->GV, Int8PtrTy); + OneAddr = B.CreateConstGEP1_64(OneAddr, UniqueBitSet->Offset); + Value *Cmp = B.CreateICmp(IsOne ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE, + Call.VTable, OneAddr); + Call.replaceAndErase(Cmp); + } + return true; + }; + + if (BitWidth == 1) { + if (tryUniqueRetValOptFor(true)) + return true; + if (tryUniqueRetValOptFor(false)) + return true; + } + return false; +} + +bool DevirtModule::tryVirtualConstProp( + MutableArrayRef<VirtualCallTarget> TargetsForSlot, + ArrayRef<VirtualCallSite> CallSites) { + // This only works if the function returns an integer. + auto RetType = dyn_cast<IntegerType>(TargetsForSlot[0].Fn->getReturnType()); + if (!RetType) + return false; + unsigned BitWidth = RetType->getBitWidth(); + if (BitWidth > 64) + return false; + + // Make sure that each function does not access memory, takes at least one + // argument, does not use its first argument (which we assume is 'this'), + // and has the same return type. + for (VirtualCallTarget &Target : TargetsForSlot) { + if (!Target.Fn->doesNotAccessMemory() || Target.Fn->arg_empty() || + !Target.Fn->arg_begin()->use_empty() || + Target.Fn->getReturnType() != RetType) + return false; + } + + // Group call sites by the list of constant arguments they pass. + // The comparator ensures deterministic ordering. + struct ByAPIntValue { + bool operator()(const std::vector<ConstantInt *> &A, + const std::vector<ConstantInt *> &B) const { + return std::lexicographical_compare( + A.begin(), A.end(), B.begin(), B.end(), + [](ConstantInt *AI, ConstantInt *BI) { + return AI->getValue().ult(BI->getValue()); + }); + } + }; + std::map<std::vector<ConstantInt *>, std::vector<VirtualCallSite>, + ByAPIntValue> + VCallSitesByConstantArg; + for (auto &&VCallSite : CallSites) { + std::vector<ConstantInt *> Args; + if (VCallSite.CS.getType() != RetType) + continue; + for (auto &&Arg : + make_range(VCallSite.CS.arg_begin() + 1, VCallSite.CS.arg_end())) { + if (!isa<ConstantInt>(Arg)) + break; + Args.push_back(cast<ConstantInt>(&Arg)); + } + if (Args.size() + 1 != VCallSite.CS.arg_size()) + continue; + + VCallSitesByConstantArg[Args].push_back(VCallSite); + } + + for (auto &&CSByConstantArg : VCallSitesByConstantArg) { + if (!tryEvaluateFunctionsWithArgs(TargetsForSlot, CSByConstantArg.first)) + continue; + + if (tryUniformRetValOpt(RetType, TargetsForSlot, CSByConstantArg.second)) + continue; + + if (tryUniqueRetValOpt(BitWidth, TargetsForSlot, CSByConstantArg.second)) + continue; + + // Find an allocation offset in bits in all vtables in the bitset. + uint64_t AllocBefore = + findLowestOffset(TargetsForSlot, /*IsAfter=*/false, BitWidth); + uint64_t AllocAfter = + findLowestOffset(TargetsForSlot, /*IsAfter=*/true, BitWidth); + + // Calculate the total amount of padding needed to store a value at both + // ends of the object. + uint64_t TotalPaddingBefore = 0, TotalPaddingAfter = 0; + for (auto &&Target : TargetsForSlot) { + TotalPaddingBefore += std::max<int64_t>( + (AllocBefore + 7) / 8 - Target.allocatedBeforeBytes() - 1, 0); + TotalPaddingAfter += std::max<int64_t>( + (AllocAfter + 7) / 8 - Target.allocatedAfterBytes() - 1, 0); + } + + // If the amount of padding is too large, give up. + // FIXME: do something smarter here. + if (std::min(TotalPaddingBefore, TotalPaddingAfter) > 128) + continue; + + // Calculate the offset to the value as a (possibly negative) byte offset + // and (if applicable) a bit offset, and store the values in the targets. + int64_t OffsetByte; + uint64_t OffsetBit; + if (TotalPaddingBefore <= TotalPaddingAfter) + setBeforeReturnValues(TargetsForSlot, AllocBefore, BitWidth, OffsetByte, + OffsetBit); + else + setAfterReturnValues(TargetsForSlot, AllocAfter, BitWidth, OffsetByte, + OffsetBit); + + // Rewrite each call to a load from OffsetByte/OffsetBit. + for (auto Call : CSByConstantArg.second) { + IRBuilder<> B(Call.CS.getInstruction()); + Value *Addr = B.CreateConstGEP1_64(Call.VTable, OffsetByte); + if (BitWidth == 1) { + Value *Bits = B.CreateLoad(Addr); + Value *Bit = ConstantInt::get(Int8Ty, 1 << OffsetBit); + Value *BitsAndBit = B.CreateAnd(Bits, Bit); + auto IsBitSet = B.CreateICmpNE(BitsAndBit, ConstantInt::get(Int8Ty, 0)); + Call.replaceAndErase(IsBitSet); + } else { + Value *ValAddr = B.CreateBitCast(Addr, RetType->getPointerTo()); + Value *Val = B.CreateLoad(RetType, ValAddr); + Call.replaceAndErase(Val); + } + } + } + return true; +} + +void DevirtModule::rebuildGlobal(VTableBits &B) { + if (B.Before.Bytes.empty() && B.After.Bytes.empty()) + return; + + // Align each byte array to pointer width. + unsigned PointerSize = M.getDataLayout().getPointerSize(); + B.Before.Bytes.resize(alignTo(B.Before.Bytes.size(), PointerSize)); + B.After.Bytes.resize(alignTo(B.After.Bytes.size(), PointerSize)); + + // Before was stored in reverse order; flip it now. + for (size_t I = 0, Size = B.Before.Bytes.size(); I != Size / 2; ++I) + std::swap(B.Before.Bytes[I], B.Before.Bytes[Size - 1 - I]); + + // Build an anonymous global containing the before bytes, followed by the + // original initializer, followed by the after bytes. + auto NewInit = ConstantStruct::getAnon( + {ConstantDataArray::get(M.getContext(), B.Before.Bytes), + B.GV->getInitializer(), + ConstantDataArray::get(M.getContext(), B.After.Bytes)}); + auto NewGV = + new GlobalVariable(M, NewInit->getType(), B.GV->isConstant(), + GlobalVariable::PrivateLinkage, NewInit, "", B.GV); + NewGV->setSection(B.GV->getSection()); + NewGV->setComdat(B.GV->getComdat()); + + // Build an alias named after the original global, pointing at the second + // element (the original initializer). + auto Alias = GlobalAlias::create( + B.GV->getInitializer()->getType(), 0, B.GV->getLinkage(), "", + ConstantExpr::getGetElementPtr( + NewInit->getType(), NewGV, + ArrayRef<Constant *>{ConstantInt::get(Int32Ty, 0), + ConstantInt::get(Int32Ty, 1)}), + &M); + Alias->setVisibility(B.GV->getVisibility()); + Alias->takeName(B.GV); + + B.GV->replaceAllUsesWith(Alias); + B.GV->eraseFromParent(); +} + +bool DevirtModule::run() { + Function *BitSetTestFunc = + M.getFunction(Intrinsic::getName(Intrinsic::bitset_test)); + if (!BitSetTestFunc || BitSetTestFunc->use_empty()) + return false; + + Function *AssumeFunc = M.getFunction(Intrinsic::getName(Intrinsic::assume)); + if (!AssumeFunc || AssumeFunc->use_empty()) + return false; + + // Find all virtual calls via a virtual table pointer %p under an assumption + // of the form llvm.assume(llvm.bitset.test(%p, %md)). This indicates that %p + // points to a vtable in the bitset %md. Group calls by (bitset, offset) pair + // (effectively the identity of the virtual function) and store to CallSlots. + DenseSet<Value *> SeenPtrs; + for (auto I = BitSetTestFunc->use_begin(), E = BitSetTestFunc->use_end(); + I != E;) { + auto CI = dyn_cast<CallInst>(I->getUser()); + ++I; + if (!CI) + continue; + + // Find llvm.assume intrinsics for this llvm.bitset.test call. + SmallVector<CallInst *, 1> Assumes; + for (const Use &CIU : CI->uses()) { + auto AssumeCI = dyn_cast<CallInst>(CIU.getUser()); + if (AssumeCI && AssumeCI->getCalledValue() == AssumeFunc) + Assumes.push_back(AssumeCI); + } + + // If we found any, search for virtual calls based on %p and add them to + // CallSlots. + if (!Assumes.empty()) { + Metadata *BitSet = + cast<MetadataAsValue>(CI->getArgOperand(1))->getMetadata(); + Value *Ptr = CI->getArgOperand(0)->stripPointerCasts(); + if (SeenPtrs.insert(Ptr).second) + findLoadCallsAtConstantOffset(BitSet, Ptr, 0, CI->getArgOperand(0)); + } + + // We no longer need the assumes or the bitset test. + for (auto Assume : Assumes) + Assume->eraseFromParent(); + // We can't use RecursivelyDeleteTriviallyDeadInstructions here because we + // may use the vtable argument later. + if (CI->use_empty()) + CI->eraseFromParent(); + } + + // Rebuild llvm.bitsets metadata into a map for easy lookup. + std::vector<VTableBits> Bits; + DenseMap<Metadata *, std::set<BitSetInfo>> BitSets; + buildBitSets(Bits, BitSets); + if (BitSets.empty()) + return true; + + // For each (bitset, offset) pair: + bool DidVirtualConstProp = false; + for (auto &S : CallSlots) { + // Search each of the vtables in the bitset for the virtual function + // implementation at offset S.first.ByteOffset, and add to TargetsForSlot. + std::vector<VirtualCallTarget> TargetsForSlot; + if (!tryFindVirtualCallTargets(TargetsForSlot, BitSets[S.first.BitSetID], + S.first.ByteOffset)) + continue; + + if (trySingleImplDevirt(TargetsForSlot, S.second)) + continue; + + DidVirtualConstProp |= tryVirtualConstProp(TargetsForSlot, S.second); + } + + // Rebuild each global we touched as part of virtual constant propagation to + // include the before and after bytes. + if (DidVirtualConstProp) + for (VTableBits &B : Bits) + rebuildGlobal(B); + + return true; +} |
