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llvm-project/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

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//===- 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 !type metadata) that the list of callees 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/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/TypeMetadataUtils.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/PassRegistry.h"
#include "llvm/PassSupport.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/Evaluator.h"
#include <algorithm>
#include <cstddef>
#include <map>
#include <set>
#include <string>
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.TM->Bits->After.BytesUsed
: Target.TM->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 TypeMemberInfo *TM)
: Fn(Fn), TM(TM),
IsBigEndian(Fn->getParent()->getDataLayout().isBigEndian()), WasDevirt(false) {}
namespace {
// A slot in a set of virtual tables. The TypeID 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 *TypeID;
uint64_t ByteOffset;
};
} // end anonymous namespace
namespace llvm {
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.TypeID) ^
DenseMapInfo<uint64_t>::getHashValue(I.ByteOffset);
}
static bool isEqual(const VTableSlot &LHS,
const VTableSlot &RHS) {
return LHS.TypeID == RHS.TypeID && LHS.ByteOffset == RHS.ByteOffset;
}
};
} // end namespace llvm
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;
// If non-null, this field points to the associated unsafe use count stored in
// the DevirtModule::NumUnsafeUsesForTypeTest map below. See the description
// of that field for details.
unsigned *NumUnsafeUses;
void emitRemark(const Twine &OptName, const Twine &TargetName) {
Function *F = CS.getCaller();
emitOptimizationRemark(
F->getContext(), DEBUG_TYPE, *F,
CS.getInstruction()->getDebugLoc(),
OptName + ": devirtualized a call to " + TargetName);
}
void replaceAndErase(const Twine &OptName, const Twine &TargetName,
bool RemarksEnabled, Value *New) {
if (RemarksEnabled)
emitRemark(OptName, TargetName);
CS->replaceAllUsesWith(New);
if (auto II = dyn_cast<InvokeInst>(CS.getInstruction())) {
BranchInst::Create(II->getNormalDest(), CS.getInstruction());
II->getUnwindDest()->removePredecessor(II->getParent());
}
CS->eraseFromParent();
// This use is no longer unsafe.
if (NumUnsafeUses)
--*NumUnsafeUses;
}
};
struct DevirtModule {
Module &M;
IntegerType *Int8Ty;
PointerType *Int8PtrTy;
IntegerType *Int32Ty;
bool RemarksEnabled;
MapVector<VTableSlot, std::vector<VirtualCallSite>> CallSlots;
// This map keeps track of the number of "unsafe" uses of a loaded function
// pointer. The key is the associated llvm.type.test intrinsic call generated
// by this pass. An unsafe use is one that calls the loaded function pointer
// directly. Every time we eliminate an unsafe use (for example, by
// devirtualizing it or by applying virtual constant propagation), we
// decrement the value stored in this map. If a value reaches zero, we can
// eliminate the type check by RAUWing the associated llvm.type.test call with
// true.
std::map<CallInst *, unsigned> NumUnsafeUsesForTypeTest;
DevirtModule(Module &M)
: M(M), Int8Ty(Type::getInt8Ty(M.getContext())),
Int8PtrTy(Type::getInt8PtrTy(M.getContext())),
Int32Ty(Type::getInt32Ty(M.getContext())),
RemarksEnabled(areRemarksEnabled()) {}
bool areRemarksEnabled();
void scanTypeTestUsers(Function *TypeTestFunc, Function *AssumeFunc);
void scanTypeCheckedLoadUsers(Function *TypeCheckedLoadFunc);
void buildTypeIdentifierMap(
std::vector<VTableBits> &Bits,
DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap);
Constant *getPointerAtOffset(Constant *I, uint64_t Offset);
bool
tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot,
const std::set<TypeMemberInfo> &TypeMemberInfos,
uint64_t ByteOffset);
bool trySingleImplDevirt(MutableArrayRef<VirtualCallTarget> TargetsForSlot,
MutableArrayRef<VirtualCallSite> CallSites);
bool tryEvaluateFunctionsWithArgs(
MutableArrayRef<VirtualCallTarget> TargetsForSlot,
ArrayRef<ConstantInt *> Args);
bool tryUniformRetValOpt(IntegerType *RetType,
MutableArrayRef<VirtualCallTarget> TargetsForSlot,
MutableArrayRef<VirtualCallSite> CallSites);
bool tryUniqueRetValOpt(unsigned BitWidth,
MutableArrayRef<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) override {
if (skipModule(M))
return false;
return DevirtModule(M).run();
}
};
} // end anonymous namespace
INITIALIZE_PASS(WholeProgramDevirt, "wholeprogramdevirt",
"Whole program devirtualization", false, false)
char WholeProgramDevirt::ID = 0;
ModulePass *llvm::createWholeProgramDevirtPass() {
return new WholeProgramDevirt;
}
PreservedAnalyses WholeProgramDevirtPass::run(Module &M,
ModuleAnalysisManager &) {
if (!DevirtModule(M).run())
return PreservedAnalyses::all();
return PreservedAnalyses::none();
}
void DevirtModule::buildTypeIdentifierMap(
std::vector<VTableBits> &Bits,
DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap) {
DenseMap<GlobalVariable *, VTableBits *> GVToBits;
Bits.reserve(M.getGlobalList().size());
SmallVector<MDNode *, 2> Types;
for (GlobalVariable &GV : M.globals()) {
Types.clear();
GV.getMetadata(LLVMContext::MD_type, Types);
if (Types.empty())
continue;
VTableBits *&BitsPtr = GVToBits[&GV];
if (!BitsPtr) {
Bits.emplace_back();
Bits.back().GV = &GV;
Bits.back().ObjectSize =
M.getDataLayout().getTypeAllocSize(GV.getInitializer()->getType());
BitsPtr = &Bits.back();
}
for (MDNode *Type : Types) {
auto TypeID = Type->getOperand(1).get();
uint64_t Offset =
cast<ConstantInt>(
cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
->getZExtValue();
TypeIdMap[TypeID].insert({BitsPtr, Offset});
}
}
}
Constant *DevirtModule::getPointerAtOffset(Constant *I, uint64_t Offset) {
if (I->getType()->isPointerTy()) {
if (Offset == 0)
return I;
return nullptr;
}
const DataLayout &DL = M.getDataLayout();
if (auto *C = dyn_cast<ConstantStruct>(I)) {
const StructLayout *SL = DL.getStructLayout(C->getType());
if (Offset >= SL->getSizeInBytes())
return nullptr;
unsigned Op = SL->getElementContainingOffset(Offset);
return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
Offset - SL->getElementOffset(Op));
}
if (auto *C = dyn_cast<ConstantArray>(I)) {
ArrayType *VTableTy = C->getType();
uint64_t ElemSize = DL.getTypeAllocSize(VTableTy->getElementType());
unsigned Op = Offset / ElemSize;
if (Op >= C->getNumOperands())
return nullptr;
return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
Offset % ElemSize);
}
return nullptr;
}
bool DevirtModule::tryFindVirtualCallTargets(
std::vector<VirtualCallTarget> &TargetsForSlot,
const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset) {
for (const TypeMemberInfo &TM : TypeMemberInfos) {
if (!TM.Bits->GV->isConstant())
return false;
Constant *Ptr = getPointerAtOffset(TM.Bits->GV->getInitializer(),
TM.Offset + ByteOffset);
if (!Ptr)
return false;
auto Fn = dyn_cast<Function>(Ptr->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, &TM});
}
// Give up if we couldn't find any targets.
return !TargetsForSlot.empty();
}
bool DevirtModule::trySingleImplDevirt(
MutableArrayRef<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 (RemarksEnabled)
TargetsForSlot[0].WasDevirt = true;
// If so, update each call site to call that implementation directly.
for (auto &&VCallSite : CallSites) {
if (RemarksEnabled)
VCallSite.emitRemark("single-impl", TheFn->getName());
VCallSite.CS.setCalledFunction(ConstantExpr::getBitCast(
TheFn, VCallSite.CS.getCalledValue()->getType()));
// This use is no longer unsafe.
if (VCallSite.NumUnsafeUses)
--*VCallSite.NumUnsafeUses;
}
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, MutableArrayRef<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("uniform-ret-val", TargetsForSlot[0].Fn->getName(),
RemarksEnabled, TheRetValConst);
if (RemarksEnabled)
for (auto &&Target : TargetsForSlot)
Target.WasDevirt = true;
return true;
}
bool DevirtModule::tryUniqueRetValOpt(
unsigned BitWidth, MutableArrayRef<VirtualCallTarget> TargetsForSlot,
MutableArrayRef<VirtualCallSite> CallSites) {
// IsOne controls whether we look for a 0 or a 1.
auto tryUniqueRetValOptFor = [&](bool IsOne) {
const TypeMemberInfo *UniqueMember = nullptr;
for (const VirtualCallTarget &Target : TargetsForSlot) {
if (Target.RetVal == (IsOne ? 1 : 0)) {
if (UniqueMember)
return false;
UniqueMember = Target.TM;
}
}
// We should have found a unique member or bailed out by now. We already
// checked for a uniform return value in tryUniformRetValOpt.
assert(UniqueMember);
// Replace each call with the comparison.
for (auto &&Call : CallSites) {
IRBuilder<> B(Call.CS.getInstruction());
Value *OneAddr = B.CreateBitCast(UniqueMember->Bits->GV, Int8PtrTy);
OneAddr = B.CreateConstGEP1_64(OneAddr, UniqueMember->Offset);
Value *Cmp = B.CreateICmp(IsOne ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE,
Call.VTable, OneAddr);
Call.replaceAndErase("unique-ret-val", TargetsForSlot[0].Fn->getName(),
RemarksEnabled, Cmp);
}
// Update devirtualization statistics for targets.
if (RemarksEnabled)
for (auto &&Target : TargetsForSlot)
Target.WasDevirt = true;
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 associated with the
// type.
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);
if (RemarksEnabled)
for (auto &&Target : TargetsForSlot)
Target.WasDevirt = true;
// 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, 1ULL << OffsetBit);
Value *BitsAndBit = B.CreateAnd(Bits, Bit);
auto IsBitSet = B.CreateICmpNE(BitsAndBit, ConstantInt::get(Int8Ty, 0));
Call.replaceAndErase("virtual-const-prop-1-bit",
TargetsForSlot[0].Fn->getName(),
RemarksEnabled, IsBitSet);
} else {
Value *ValAddr = B.CreateBitCast(Addr, RetType->getPointerTo());
Value *Val = B.CreateLoad(RetType, ValAddr);
Call.replaceAndErase("virtual-const-prop",
TargetsForSlot[0].Fn->getName(),
RemarksEnabled, 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());
// Copy the original vtable's metadata to the anonymous global, adjusting
// offsets as required.
NewGV->copyMetadata(B.GV, B.Before.Bytes.size());
// 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::areRemarksEnabled() {
const auto &FL = M.getFunctionList();
if (FL.empty())
return false;
const Function &Fn = FL.front();
auto DI = OptimizationRemark(DEBUG_TYPE, Fn, DebugLoc(), "");
return DI.isEnabled();
}
void DevirtModule::scanTypeTestUsers(Function *TypeTestFunc,
Function *AssumeFunc) {
// Find all virtual calls via a virtual table pointer %p under an assumption
// of the form llvm.assume(llvm.type.test(%p, %md)). This indicates that %p
// points to a member of the type identifier %md. Group calls by (type ID,
// offset) pair (effectively the identity of the virtual function) and store
// to CallSlots.
DenseSet<Value *> SeenPtrs;
for (auto I = TypeTestFunc->use_begin(), E = TypeTestFunc->use_end();
I != E;) {
auto CI = dyn_cast<CallInst>(I->getUser());
++I;
if (!CI)
continue;
// Search for virtual calls based on %p and add them to DevirtCalls.
SmallVector<DevirtCallSite, 1> DevirtCalls;
SmallVector<CallInst *, 1> Assumes;
findDevirtualizableCallsForTypeTest(DevirtCalls, Assumes, CI);
// If we found any, add them to CallSlots. Only do this if we haven't seen
// the vtable pointer before, as it may have been CSE'd with pointers from
// other call sites, and we don't want to process call sites multiple times.
if (!Assumes.empty()) {
Metadata *TypeId =
cast<MetadataAsValue>(CI->getArgOperand(1))->getMetadata();
Value *Ptr = CI->getArgOperand(0)->stripPointerCasts();
if (SeenPtrs.insert(Ptr).second) {
for (DevirtCallSite Call : DevirtCalls) {
CallSlots[{TypeId, Call.Offset}].push_back(
{CI->getArgOperand(0), Call.CS, nullptr});
}
}
}
// We no longer need the assumes or the type 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();
}
}
void DevirtModule::scanTypeCheckedLoadUsers(Function *TypeCheckedLoadFunc) {
Function *TypeTestFunc = Intrinsic::getDeclaration(&M, Intrinsic::type_test);
for (auto I = TypeCheckedLoadFunc->use_begin(),
E = TypeCheckedLoadFunc->use_end();
I != E;) {
auto CI = dyn_cast<CallInst>(I->getUser());
++I;
if (!CI)
continue;
Value *Ptr = CI->getArgOperand(0);
Value *Offset = CI->getArgOperand(1);
Value *TypeIdValue = CI->getArgOperand(2);
Metadata *TypeId = cast<MetadataAsValue>(TypeIdValue)->getMetadata();
SmallVector<DevirtCallSite, 1> DevirtCalls;
SmallVector<Instruction *, 1> LoadedPtrs;
SmallVector<Instruction *, 1> Preds;
bool HasNonCallUses = false;
findDevirtualizableCallsForTypeCheckedLoad(DevirtCalls, LoadedPtrs, Preds,
HasNonCallUses, CI);
// Start by generating "pessimistic" code that explicitly loads the function
// pointer from the vtable and performs the type check. If possible, we will
// eliminate the load and the type check later.
// If possible, only generate the load at the point where it is used.
// This helps avoid unnecessary spills.
IRBuilder<> LoadB(
(LoadedPtrs.size() == 1 && !HasNonCallUses) ? LoadedPtrs[0] : CI);
Value *GEP = LoadB.CreateGEP(Int8Ty, Ptr, Offset);
Value *GEPPtr = LoadB.CreateBitCast(GEP, PointerType::getUnqual(Int8PtrTy));
Value *LoadedValue = LoadB.CreateLoad(Int8PtrTy, GEPPtr);
for (Instruction *LoadedPtr : LoadedPtrs) {
LoadedPtr->replaceAllUsesWith(LoadedValue);
LoadedPtr->eraseFromParent();
}
// Likewise for the type test.
IRBuilder<> CallB((Preds.size() == 1 && !HasNonCallUses) ? Preds[0] : CI);
CallInst *TypeTestCall = CallB.CreateCall(TypeTestFunc, {Ptr, TypeIdValue});
for (Instruction *Pred : Preds) {
Pred->replaceAllUsesWith(TypeTestCall);
Pred->eraseFromParent();
}
// We have already erased any extractvalue instructions that refer to the
// intrinsic call, but the intrinsic may have other non-extractvalue uses
// (although this is unlikely). In that case, explicitly build a pair and
// RAUW it.
if (!CI->use_empty()) {
Value *Pair = UndefValue::get(CI->getType());
IRBuilder<> B(CI);
Pair = B.CreateInsertValue(Pair, LoadedValue, {0});
Pair = B.CreateInsertValue(Pair, TypeTestCall, {1});
CI->replaceAllUsesWith(Pair);
}
// The number of unsafe uses is initially the number of uses.
auto &NumUnsafeUses = NumUnsafeUsesForTypeTest[TypeTestCall];
NumUnsafeUses = DevirtCalls.size();
// If the function pointer has a non-call user, we cannot eliminate the type
// check, as one of those users may eventually call the pointer. Increment
// the unsafe use count to make sure it cannot reach zero.
if (HasNonCallUses)
++NumUnsafeUses;
for (DevirtCallSite Call : DevirtCalls) {
CallSlots[{TypeId, Call.Offset}].push_back(
{Ptr, Call.CS, &NumUnsafeUses});
}
CI->eraseFromParent();
}
}
bool DevirtModule::run() {
Function *TypeTestFunc =
M.getFunction(Intrinsic::getName(Intrinsic::type_test));
Function *TypeCheckedLoadFunc =
M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
Function *AssumeFunc = M.getFunction(Intrinsic::getName(Intrinsic::assume));
if ((!TypeTestFunc || TypeTestFunc->use_empty() || !AssumeFunc ||
AssumeFunc->use_empty()) &&
(!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
return false;
if (TypeTestFunc && AssumeFunc)
scanTypeTestUsers(TypeTestFunc, AssumeFunc);
if (TypeCheckedLoadFunc)
scanTypeCheckedLoadUsers(TypeCheckedLoadFunc);
// Rebuild type metadata into a map for easy lookup.
std::vector<VTableBits> Bits;
DenseMap<Metadata *, std::set<TypeMemberInfo>> TypeIdMap;
buildTypeIdentifierMap(Bits, TypeIdMap);
if (TypeIdMap.empty())
return true;
// For each (type, offset) pair:
bool DidVirtualConstProp = false;
std::map<std::string, Function*> DevirtTargets;
for (auto &S : CallSlots) {
// Search each of the members of the type identifier for the virtual
// function implementation at offset S.first.ByteOffset, and add to
// TargetsForSlot.
std::vector<VirtualCallTarget> TargetsForSlot;
if (!tryFindVirtualCallTargets(TargetsForSlot, TypeIdMap[S.first.TypeID],
S.first.ByteOffset))
continue;
if (!trySingleImplDevirt(TargetsForSlot, S.second) &&
tryVirtualConstProp(TargetsForSlot, S.second))
DidVirtualConstProp = true;
// Collect functions devirtualized at least for one call site for stats.
if (RemarksEnabled)
for (const auto &T : TargetsForSlot)
if (T.WasDevirt)
DevirtTargets[T.Fn->getName()] = T.Fn;
}
if (RemarksEnabled) {
// Generate remarks for each devirtualized function.
for (const auto &DT : DevirtTargets) {
Function *F = DT.second;
DISubprogram *SP = F->getSubprogram();
DebugLoc DL = SP ? DebugLoc::get(SP->getScopeLine(), 0, SP) : DebugLoc();
emitOptimizationRemark(F->getContext(), DEBUG_TYPE, *F, DL,
Twine("devirtualized ") + F->getName());
}
}
// If we were able to eliminate all unsafe uses for a type checked load,
// eliminate the type test by replacing it with true.
if (TypeCheckedLoadFunc) {
auto True = ConstantInt::getTrue(M.getContext());
for (auto &&U : NumUnsafeUsesForTypeTest) {
if (U.second == 0) {
U.first->replaceAllUsesWith(True);
U.first->eraseFromParent();
}
}
}
// 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;
}
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