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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 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;
};
}
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.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, 1ULL << 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;
}
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