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

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//===-- LowerBitSets.cpp - Bitset lowering pass ---------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass lowers bitset metadata and calls to the llvm.bitset.test intrinsic.
// See http://llvm.org/docs/LangRef.html#bitsets for more information.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/LowerBitSets.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Triple.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalObject.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
#define DEBUG_TYPE "lowerbitsets"
STATISTIC(ByteArraySizeBits, "Byte array size in bits");
STATISTIC(ByteArraySizeBytes, "Byte array size in bytes");
STATISTIC(NumByteArraysCreated, "Number of byte arrays created");
STATISTIC(NumBitSetCallsLowered, "Number of bitset calls lowered");
STATISTIC(NumBitSetDisjointSets, "Number of disjoint sets of bitsets");
static cl::opt<bool> AvoidReuse(
"lowerbitsets-avoid-reuse",
cl::desc("Try to avoid reuse of byte array addresses using aliases"),
cl::Hidden, cl::init(true));
bool BitSetInfo::containsGlobalOffset(uint64_t Offset) const {
if (Offset < ByteOffset)
return false;
if ((Offset - ByteOffset) % (uint64_t(1) << AlignLog2) != 0)
return false;
uint64_t BitOffset = (Offset - ByteOffset) >> AlignLog2;
if (BitOffset >= BitSize)
return false;
return Bits.count(BitOffset);
}
bool BitSetInfo::containsValue(
const DataLayout &DL,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout, Value *V,
uint64_t COffset) const {
if (auto GV = dyn_cast<GlobalObject>(V)) {
auto I = GlobalLayout.find(GV);
if (I == GlobalLayout.end())
return false;
return containsGlobalOffset(I->second + COffset);
}
if (auto GEP = dyn_cast<GEPOperator>(V)) {
APInt APOffset(DL.getPointerSizeInBits(0), 0);
bool Result = GEP->accumulateConstantOffset(DL, APOffset);
if (!Result)
return false;
COffset += APOffset.getZExtValue();
return containsValue(DL, GlobalLayout, GEP->getPointerOperand(),
COffset);
}
if (auto Op = dyn_cast<Operator>(V)) {
if (Op->getOpcode() == Instruction::BitCast)
return containsValue(DL, GlobalLayout, Op->getOperand(0), COffset);
if (Op->getOpcode() == Instruction::Select)
return containsValue(DL, GlobalLayout, Op->getOperand(1), COffset) &&
containsValue(DL, GlobalLayout, Op->getOperand(2), COffset);
}
return false;
}
void BitSetInfo::print(raw_ostream &OS) const {
OS << "offset " << ByteOffset << " size " << BitSize << " align "
<< (1 << AlignLog2);
if (isAllOnes()) {
OS << " all-ones\n";
return;
}
OS << " { ";
for (uint64_t B : Bits)
OS << B << ' ';
OS << "}\n";
}
BitSetInfo BitSetBuilder::build() {
if (Min > Max)
Min = 0;
// Normalize each offset against the minimum observed offset, and compute
// the bitwise OR of each of the offsets. The number of trailing zeros
// in the mask gives us the log2 of the alignment of all offsets, which
// allows us to compress the bitset by only storing one bit per aligned
// address.
uint64_t Mask = 0;
for (uint64_t &Offset : Offsets) {
Offset -= Min;
Mask |= Offset;
}
BitSetInfo BSI;
BSI.ByteOffset = Min;
BSI.AlignLog2 = 0;
if (Mask != 0)
BSI.AlignLog2 = countTrailingZeros(Mask, ZB_Undefined);
// Build the compressed bitset while normalizing the offsets against the
// computed alignment.
BSI.BitSize = ((Max - Min) >> BSI.AlignLog2) + 1;
for (uint64_t Offset : Offsets) {
Offset >>= BSI.AlignLog2;
BSI.Bits.insert(Offset);
}
return BSI;
}
void GlobalLayoutBuilder::addFragment(const std::set<uint64_t> &F) {
// Create a new fragment to hold the layout for F.
Fragments.emplace_back();
std::vector<uint64_t> &Fragment = Fragments.back();
uint64_t FragmentIndex = Fragments.size() - 1;
for (auto ObjIndex : F) {
uint64_t OldFragmentIndex = FragmentMap[ObjIndex];
if (OldFragmentIndex == 0) {
// We haven't seen this object index before, so just add it to the current
// fragment.
Fragment.push_back(ObjIndex);
} else {
// This index belongs to an existing fragment. Copy the elements of the
// old fragment into this one and clear the old fragment. We don't update
// the fragment map just yet, this ensures that any further references to
// indices from the old fragment in this fragment do not insert any more
// indices.
std::vector<uint64_t> &OldFragment = Fragments[OldFragmentIndex];
Fragment.insert(Fragment.end(), OldFragment.begin(), OldFragment.end());
OldFragment.clear();
}
}
// Update the fragment map to point our object indices to this fragment.
for (uint64_t ObjIndex : Fragment)
FragmentMap[ObjIndex] = FragmentIndex;
}
void ByteArrayBuilder::allocate(const std::set<uint64_t> &Bits,
uint64_t BitSize, uint64_t &AllocByteOffset,
uint8_t &AllocMask) {
// Find the smallest current allocation.
unsigned Bit = 0;
for (unsigned I = 1; I != BitsPerByte; ++I)
if (BitAllocs[I] < BitAllocs[Bit])
Bit = I;
AllocByteOffset = BitAllocs[Bit];
// Add our size to it.
unsigned ReqSize = AllocByteOffset + BitSize;
BitAllocs[Bit] = ReqSize;
if (Bytes.size() < ReqSize)
Bytes.resize(ReqSize);
// Set our bits.
AllocMask = 1 << Bit;
for (uint64_t B : Bits)
Bytes[AllocByteOffset + B] |= AllocMask;
}
namespace {
struct ByteArrayInfo {
std::set<uint64_t> Bits;
uint64_t BitSize;
GlobalVariable *ByteArray;
Constant *Mask;
};
struct LowerBitSets : public ModulePass {
static char ID;
LowerBitSets() : ModulePass(ID) {
initializeLowerBitSetsPass(*PassRegistry::getPassRegistry());
}
Module *M;
bool LinkerSubsectionsViaSymbols;
Triple::ArchType Arch;
Triple::ObjectFormatType ObjectFormat;
IntegerType *Int1Ty;
IntegerType *Int8Ty;
IntegerType *Int32Ty;
Type *Int32PtrTy;
IntegerType *Int64Ty;
IntegerType *IntPtrTy;
// The llvm.bitsets named metadata.
NamedMDNode *BitSetNM;
// Mapping from bitset identifiers to the call sites that test them.
DenseMap<Metadata *, std::vector<CallInst *>> BitSetTestCallSites;
std::vector<ByteArrayInfo> ByteArrayInfos;
BitSetInfo
buildBitSet(Metadata *BitSet,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout);
ByteArrayInfo *createByteArray(BitSetInfo &BSI);
void allocateByteArrays();
Value *createBitSetTest(IRBuilder<> &B, BitSetInfo &BSI, ByteArrayInfo *&BAI,
Value *BitOffset);
void lowerBitSetCalls(ArrayRef<Metadata *> BitSets,
Constant *CombinedGlobalAddr,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout);
Value *
lowerBitSetCall(CallInst *CI, BitSetInfo &BSI, ByteArrayInfo *&BAI,
Constant *CombinedGlobal,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout);
void buildBitSetsFromGlobalVariables(ArrayRef<Metadata *> BitSets,
ArrayRef<GlobalVariable *> Globals);
unsigned getJumpTableEntrySize();
Type *getJumpTableEntryType();
Constant *createJumpTableEntry(GlobalObject *Src, Function *Dest,
unsigned Distance);
void verifyBitSetMDNode(MDNode *Op);
void buildBitSetsFromFunctions(ArrayRef<Metadata *> BitSets,
ArrayRef<Function *> Functions);
void buildBitSetsFromDisjointSet(ArrayRef<Metadata *> BitSets,
ArrayRef<GlobalObject *> Globals);
bool buildBitSets();
bool eraseBitSetMetadata();
bool doInitialization(Module &M) override;
bool runOnModule(Module &M) override;
};
} // anonymous namespace
INITIALIZE_PASS_BEGIN(LowerBitSets, "lowerbitsets",
"Lower bitset metadata", false, false)
INITIALIZE_PASS_END(LowerBitSets, "lowerbitsets",
"Lower bitset metadata", false, false)
char LowerBitSets::ID = 0;
ModulePass *llvm::createLowerBitSetsPass() { return new LowerBitSets; }
bool LowerBitSets::doInitialization(Module &Mod) {
M = &Mod;
const DataLayout &DL = Mod.getDataLayout();
Triple TargetTriple(M->getTargetTriple());
LinkerSubsectionsViaSymbols = TargetTriple.isMacOSX();
Arch = TargetTriple.getArch();
ObjectFormat = TargetTriple.getObjectFormat();
Int1Ty = Type::getInt1Ty(M->getContext());
Int8Ty = Type::getInt8Ty(M->getContext());
Int32Ty = Type::getInt32Ty(M->getContext());
Int32PtrTy = PointerType::getUnqual(Int32Ty);
Int64Ty = Type::getInt64Ty(M->getContext());
IntPtrTy = DL.getIntPtrType(M->getContext(), 0);
BitSetNM = M->getNamedMetadata("llvm.bitsets");
BitSetTestCallSites.clear();
return false;
}
/// Build a bit set for BitSet using the object layouts in
/// GlobalLayout.
BitSetInfo LowerBitSets::buildBitSet(
Metadata *BitSet,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout) {
BitSetBuilder BSB;
// Compute the byte offset of each element of this bitset.
if (BitSetNM) {
for (MDNode *Op : BitSetNM->operands()) {
if (Op->getOperand(0) != BitSet || !Op->getOperand(1))
continue;
Constant *OpConst =
cast<ConstantAsMetadata>(Op->getOperand(1))->getValue();
if (auto GA = dyn_cast<GlobalAlias>(OpConst))
OpConst = GA->getAliasee();
auto OpGlobal = dyn_cast<GlobalObject>(OpConst);
if (!OpGlobal)
continue;
uint64_t Offset =
cast<ConstantInt>(cast<ConstantAsMetadata>(Op->getOperand(2))
->getValue())->getZExtValue();
Offset += GlobalLayout.find(OpGlobal)->second;
BSB.addOffset(Offset);
}
}
return BSB.build();
}
/// Build a test that bit BitOffset mod sizeof(Bits)*8 is set in
/// Bits. This pattern matches to the bt instruction on x86.
static Value *createMaskedBitTest(IRBuilder<> &B, Value *Bits,
Value *BitOffset) {
auto BitsType = cast<IntegerType>(Bits->getType());
unsigned BitWidth = BitsType->getBitWidth();
BitOffset = B.CreateZExtOrTrunc(BitOffset, BitsType);
Value *BitIndex =
B.CreateAnd(BitOffset, ConstantInt::get(BitsType, BitWidth - 1));
Value *BitMask = B.CreateShl(ConstantInt::get(BitsType, 1), BitIndex);
Value *MaskedBits = B.CreateAnd(Bits, BitMask);
return B.CreateICmpNE(MaskedBits, ConstantInt::get(BitsType, 0));
}
ByteArrayInfo *LowerBitSets::createByteArray(BitSetInfo &BSI) {
// Create globals to stand in for byte arrays and masks. These never actually
// get initialized, we RAUW and erase them later in allocateByteArrays() once
// we know the offset and mask to use.
auto ByteArrayGlobal = new GlobalVariable(
*M, Int8Ty, /*isConstant=*/true, GlobalValue::PrivateLinkage, nullptr);
auto MaskGlobal = new GlobalVariable(
*M, Int8Ty, /*isConstant=*/true, GlobalValue::PrivateLinkage, nullptr);
ByteArrayInfos.emplace_back();
ByteArrayInfo *BAI = &ByteArrayInfos.back();
BAI->Bits = BSI.Bits;
BAI->BitSize = BSI.BitSize;
BAI->ByteArray = ByteArrayGlobal;
BAI->Mask = ConstantExpr::getPtrToInt(MaskGlobal, Int8Ty);
return BAI;
}
void LowerBitSets::allocateByteArrays() {
std::stable_sort(ByteArrayInfos.begin(), ByteArrayInfos.end(),
[](const ByteArrayInfo &BAI1, const ByteArrayInfo &BAI2) {
return BAI1.BitSize > BAI2.BitSize;
});
std::vector<uint64_t> ByteArrayOffsets(ByteArrayInfos.size());
ByteArrayBuilder BAB;
for (unsigned I = 0; I != ByteArrayInfos.size(); ++I) {
ByteArrayInfo *BAI = &ByteArrayInfos[I];
uint8_t Mask;
BAB.allocate(BAI->Bits, BAI->BitSize, ByteArrayOffsets[I], Mask);
BAI->Mask->replaceAllUsesWith(ConstantInt::get(Int8Ty, Mask));
cast<GlobalVariable>(BAI->Mask->getOperand(0))->eraseFromParent();
}
Constant *ByteArrayConst = ConstantDataArray::get(M->getContext(), BAB.Bytes);
auto ByteArray =
new GlobalVariable(*M, ByteArrayConst->getType(), /*isConstant=*/true,
GlobalValue::PrivateLinkage, ByteArrayConst);
for (unsigned I = 0; I != ByteArrayInfos.size(); ++I) {
ByteArrayInfo *BAI = &ByteArrayInfos[I];
Constant *Idxs[] = {ConstantInt::get(IntPtrTy, 0),
ConstantInt::get(IntPtrTy, ByteArrayOffsets[I])};
Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(
ByteArrayConst->getType(), ByteArray, Idxs);
// Create an alias instead of RAUW'ing the gep directly. On x86 this ensures
// that the pc-relative displacement is folded into the lea instead of the
// test instruction getting another displacement.
if (LinkerSubsectionsViaSymbols) {
BAI->ByteArray->replaceAllUsesWith(GEP);
} else {
GlobalAlias *Alias = GlobalAlias::create(
Int8Ty, 0, GlobalValue::PrivateLinkage, "bits", GEP, M);
BAI->ByteArray->replaceAllUsesWith(Alias);
}
BAI->ByteArray->eraseFromParent();
}
ByteArraySizeBits = BAB.BitAllocs[0] + BAB.BitAllocs[1] + BAB.BitAllocs[2] +
BAB.BitAllocs[3] + BAB.BitAllocs[4] + BAB.BitAllocs[5] +
BAB.BitAllocs[6] + BAB.BitAllocs[7];
ByteArraySizeBytes = BAB.Bytes.size();
}
/// Build a test that bit BitOffset is set in BSI, where
/// BitSetGlobal is a global containing the bits in BSI.
Value *LowerBitSets::createBitSetTest(IRBuilder<> &B, BitSetInfo &BSI,
ByteArrayInfo *&BAI, Value *BitOffset) {
if (BSI.BitSize <= 64) {
// If the bit set is sufficiently small, we can avoid a load by bit testing
// a constant.
IntegerType *BitsTy;
if (BSI.BitSize <= 32)
BitsTy = Int32Ty;
else
BitsTy = Int64Ty;
uint64_t Bits = 0;
for (auto Bit : BSI.Bits)
Bits |= uint64_t(1) << Bit;
Constant *BitsConst = ConstantInt::get(BitsTy, Bits);
return createMaskedBitTest(B, BitsConst, BitOffset);
} else {
if (!BAI) {
++NumByteArraysCreated;
BAI = createByteArray(BSI);
}
Constant *ByteArray = BAI->ByteArray;
Type *Ty = BAI->ByteArray->getValueType();
if (!LinkerSubsectionsViaSymbols && AvoidReuse) {
// Each use of the byte array uses a different alias. This makes the
// backend less likely to reuse previously computed byte array addresses,
// improving the security of the CFI mechanism based on this pass.
ByteArray = GlobalAlias::create(BAI->ByteArray->getValueType(), 0,
GlobalValue::PrivateLinkage, "bits_use",
ByteArray, M);
}
Value *ByteAddr = B.CreateGEP(Ty, ByteArray, BitOffset);
Value *Byte = B.CreateLoad(ByteAddr);
Value *ByteAndMask = B.CreateAnd(Byte, BAI->Mask);
return B.CreateICmpNE(ByteAndMask, ConstantInt::get(Int8Ty, 0));
}
}
/// Lower a llvm.bitset.test call to its implementation. Returns the value to
/// replace the call with.
Value *LowerBitSets::lowerBitSetCall(
CallInst *CI, BitSetInfo &BSI, ByteArrayInfo *&BAI,
Constant *CombinedGlobalIntAddr,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout) {
Value *Ptr = CI->getArgOperand(0);
const DataLayout &DL = M->getDataLayout();
if (BSI.containsValue(DL, GlobalLayout, Ptr))
return ConstantInt::getTrue(M->getContext());
Constant *OffsetedGlobalAsInt = ConstantExpr::getAdd(
CombinedGlobalIntAddr, ConstantInt::get(IntPtrTy, BSI.ByteOffset));
BasicBlock *InitialBB = CI->getParent();
IRBuilder<> B(CI);
Value *PtrAsInt = B.CreatePtrToInt(Ptr, IntPtrTy);
if (BSI.isSingleOffset())
return B.CreateICmpEQ(PtrAsInt, OffsetedGlobalAsInt);
Value *PtrOffset = B.CreateSub(PtrAsInt, OffsetedGlobalAsInt);
Value *BitOffset;
if (BSI.AlignLog2 == 0) {
BitOffset = PtrOffset;
} else {
// We need to check that the offset both falls within our range and is
// suitably aligned. We can check both properties at the same time by
// performing a right rotate by log2(alignment) followed by an integer
// comparison against the bitset size. The rotate will move the lower
// order bits that need to be zero into the higher order bits of the
// result, causing the comparison to fail if they are nonzero. The rotate
// also conveniently gives us a bit offset to use during the load from
// the bitset.
Value *OffsetSHR =
B.CreateLShr(PtrOffset, ConstantInt::get(IntPtrTy, BSI.AlignLog2));
Value *OffsetSHL = B.CreateShl(
PtrOffset,
ConstantInt::get(IntPtrTy, DL.getPointerSizeInBits(0) - BSI.AlignLog2));
BitOffset = B.CreateOr(OffsetSHR, OffsetSHL);
}
Constant *BitSizeConst = ConstantInt::get(IntPtrTy, BSI.BitSize);
Value *OffsetInRange = B.CreateICmpULT(BitOffset, BitSizeConst);
// If the bit set is all ones, testing against it is unnecessary.
if (BSI.isAllOnes())
return OffsetInRange;
TerminatorInst *Term = SplitBlockAndInsertIfThen(OffsetInRange, CI, false);
IRBuilder<> ThenB(Term);
// Now that we know that the offset is in range and aligned, load the
// appropriate bit from the bitset.
Value *Bit = createBitSetTest(ThenB, BSI, BAI, BitOffset);
// The value we want is 0 if we came directly from the initial block
// (having failed the range or alignment checks), or the loaded bit if
// we came from the block in which we loaded it.
B.SetInsertPoint(CI);
PHINode *P = B.CreatePHI(Int1Ty, 2);
P->addIncoming(ConstantInt::get(Int1Ty, 0), InitialBB);
P->addIncoming(Bit, ThenB.GetInsertBlock());
return P;
}
/// Given a disjoint set of bitsets and globals, layout the globals, build the
/// bit sets and lower the llvm.bitset.test calls.
void LowerBitSets::buildBitSetsFromGlobalVariables(
ArrayRef<Metadata *> BitSets, ArrayRef<GlobalVariable *> Globals) {
// Build a new global with the combined contents of the referenced globals.
// This global is a struct whose even-indexed elements contain the original
// contents of the referenced globals and whose odd-indexed elements contain
// any padding required to align the next element to the next power of 2.
std::vector<Constant *> GlobalInits;
const DataLayout &DL = M->getDataLayout();
for (GlobalVariable *G : Globals) {
GlobalInits.push_back(G->getInitializer());
uint64_t InitSize = DL.getTypeAllocSize(G->getValueType());
// Compute the amount of padding required.
uint64_t Padding = NextPowerOf2(InitSize - 1) - InitSize;
// Cap at 128 was found experimentally to have a good data/instruction
// overhead tradeoff.
if (Padding > 128)
Padding = alignTo(InitSize, 128) - InitSize;
GlobalInits.push_back(
ConstantAggregateZero::get(ArrayType::get(Int8Ty, Padding)));
}
if (!GlobalInits.empty())
GlobalInits.pop_back();
Constant *NewInit = ConstantStruct::getAnon(M->getContext(), GlobalInits);
auto *CombinedGlobal =
new GlobalVariable(*M, NewInit->getType(), /*isConstant=*/true,
GlobalValue::PrivateLinkage, NewInit);
StructType *NewTy = cast<StructType>(NewInit->getType());
const StructLayout *CombinedGlobalLayout = DL.getStructLayout(NewTy);
// Compute the offsets of the original globals within the new global.
DenseMap<GlobalObject *, uint64_t> GlobalLayout;
for (unsigned I = 0; I != Globals.size(); ++I)
// Multiply by 2 to account for padding elements.
GlobalLayout[Globals[I]] = CombinedGlobalLayout->getElementOffset(I * 2);
lowerBitSetCalls(BitSets, CombinedGlobal, GlobalLayout);
// Build aliases pointing to offsets into the combined global for each
// global from which we built the combined global, and replace references
// to the original globals with references to the aliases.
for (unsigned I = 0; I != Globals.size(); ++I) {
// Multiply by 2 to account for padding elements.
Constant *CombinedGlobalIdxs[] = {ConstantInt::get(Int32Ty, 0),
ConstantInt::get(Int32Ty, I * 2)};
Constant *CombinedGlobalElemPtr = ConstantExpr::getGetElementPtr(
NewInit->getType(), CombinedGlobal, CombinedGlobalIdxs);
if (LinkerSubsectionsViaSymbols) {
Globals[I]->replaceAllUsesWith(CombinedGlobalElemPtr);
} else {
assert(Globals[I]->getType()->getAddressSpace() == 0);
GlobalAlias *GAlias = GlobalAlias::create(NewTy->getElementType(I * 2), 0,
Globals[I]->getLinkage(), "",
CombinedGlobalElemPtr, M);
GAlias->setVisibility(Globals[I]->getVisibility());
GAlias->takeName(Globals[I]);
Globals[I]->replaceAllUsesWith(GAlias);
}
Globals[I]->eraseFromParent();
}
}
void LowerBitSets::lowerBitSetCalls(
ArrayRef<Metadata *> BitSets, Constant *CombinedGlobalAddr,
const DenseMap<GlobalObject *, uint64_t> &GlobalLayout) {
Constant *CombinedGlobalIntAddr =
ConstantExpr::getPtrToInt(CombinedGlobalAddr, IntPtrTy);
// For each bitset in this disjoint set...
for (Metadata *BS : BitSets) {
// Build the bitset.
BitSetInfo BSI = buildBitSet(BS, GlobalLayout);
DEBUG({
if (auto BSS = dyn_cast<MDString>(BS))
dbgs() << BSS->getString() << ": ";
else
dbgs() << "<unnamed>: ";
BSI.print(dbgs());
});
ByteArrayInfo *BAI = nullptr;
// Lower each call to llvm.bitset.test for this bitset.
for (CallInst *CI : BitSetTestCallSites[BS]) {
++NumBitSetCallsLowered;
Value *Lowered =
lowerBitSetCall(CI, BSI, BAI, CombinedGlobalIntAddr, GlobalLayout);
CI->replaceAllUsesWith(Lowered);
CI->eraseFromParent();
}
}
}
void LowerBitSets::verifyBitSetMDNode(MDNode *Op) {
if (Op->getNumOperands() != 3)
report_fatal_error(
"All operands of llvm.bitsets metadata must have 3 elements");
if (!Op->getOperand(1))
return;
auto OpConstMD = dyn_cast<ConstantAsMetadata>(Op->getOperand(1));
if (!OpConstMD)
report_fatal_error("Bit set element must be a constant");
auto OpGlobal = dyn_cast<GlobalObject>(OpConstMD->getValue());
if (!OpGlobal)
return;
if (OpGlobal->isThreadLocal())
report_fatal_error("Bit set element may not be thread-local");
if (OpGlobal->hasSection())
report_fatal_error("Bit set element may not have an explicit section");
if (isa<GlobalVariable>(OpGlobal) && OpGlobal->isDeclarationForLinker())
report_fatal_error("Bit set global var element must be a definition");
auto OffsetConstMD = dyn_cast<ConstantAsMetadata>(Op->getOperand(2));
if (!OffsetConstMD)
report_fatal_error("Bit set element offset must be a constant");
auto OffsetInt = dyn_cast<ConstantInt>(OffsetConstMD->getValue());
if (!OffsetInt)
report_fatal_error("Bit set element offset must be an integer constant");
}
static const unsigned kX86JumpTableEntrySize = 8;
unsigned LowerBitSets::getJumpTableEntrySize() {
if (Arch != Triple::x86 && Arch != Triple::x86_64)
report_fatal_error("Unsupported architecture for jump tables");
return kX86JumpTableEntrySize;
}
// Create a constant representing a jump table entry for the target. This
// consists of an instruction sequence containing a relative branch to Dest. The
// constant will be laid out at address Src+(Len*Distance) where Len is the
// target-specific jump table entry size.
Constant *LowerBitSets::createJumpTableEntry(GlobalObject *Src, Function *Dest,
unsigned Distance) {
if (Arch != Triple::x86 && Arch != Triple::x86_64)
report_fatal_error("Unsupported architecture for jump tables");
const unsigned kJmpPCRel32Code = 0xe9;
const unsigned kInt3Code = 0xcc;
ConstantInt *Jmp = ConstantInt::get(Int8Ty, kJmpPCRel32Code);
// Build a constant representing the displacement between the constant's
// address and Dest. This will resolve to a PC32 relocation referring to Dest.
Constant *DestInt = ConstantExpr::getPtrToInt(Dest, IntPtrTy);
Constant *SrcInt = ConstantExpr::getPtrToInt(Src, IntPtrTy);
Constant *Disp = ConstantExpr::getSub(DestInt, SrcInt);
ConstantInt *DispOffset =
ConstantInt::get(IntPtrTy, Distance * kX86JumpTableEntrySize + 5);
Constant *OffsetedDisp = ConstantExpr::getSub(Disp, DispOffset);
OffsetedDisp = ConstantExpr::getTruncOrBitCast(OffsetedDisp, Int32Ty);
ConstantInt *Int3 = ConstantInt::get(Int8Ty, kInt3Code);
Constant *Fields[] = {
Jmp, OffsetedDisp, Int3, Int3, Int3,
};
return ConstantStruct::getAnon(Fields, /*Packed=*/true);
}
Type *LowerBitSets::getJumpTableEntryType() {
if (Arch != Triple::x86 && Arch != Triple::x86_64)
report_fatal_error("Unsupported architecture for jump tables");
return StructType::get(M->getContext(),
{Int8Ty, Int32Ty, Int8Ty, Int8Ty, Int8Ty},
/*Packed=*/true);
}
/// Given a disjoint set of bitsets and functions, build a jump table for the
/// functions, build the bit sets and lower the llvm.bitset.test calls.
void LowerBitSets::buildBitSetsFromFunctions(ArrayRef<Metadata *> BitSets,
ArrayRef<Function *> Functions) {
// Unlike the global bitset builder, the function bitset builder cannot
// re-arrange functions in a particular order and base its calculations on the
// layout of the functions' entry points, as we have no idea how large a
// particular function will end up being (the size could even depend on what
// this pass does!) Instead, we build a jump table, which is a block of code
// consisting of one branch instruction for each of the functions in the bit
// set that branches to the target function, and redirect any taken function
// addresses to the corresponding jump table entry. In the object file's
// symbol table, the symbols for the target functions also refer to the jump
// table entries, so that addresses taken outside the module will pass any
// verification done inside the module.
//
// In more concrete terms, suppose we have three functions f, g, h which are
// members of a single bitset, and a function foo that returns their
// addresses:
//
// f:
// mov 0, %eax
// ret
//
// g:
// mov 1, %eax
// ret
//
// h:
// mov 2, %eax
// ret
//
// foo:
// mov f, %eax
// mov g, %edx
// mov h, %ecx
// ret
//
// To create a jump table for these functions, we instruct the LLVM code
// generator to output a jump table in the .text section. This is done by
// representing the instructions in the jump table as an LLVM constant and
// placing them in a global variable in the .text section. The end result will
// (conceptually) look like this:
//
// f:
// jmp .Ltmp0 ; 5 bytes
// int3 ; 1 byte
// int3 ; 1 byte
// int3 ; 1 byte
//
// g:
// jmp .Ltmp1 ; 5 bytes
// int3 ; 1 byte
// int3 ; 1 byte
// int3 ; 1 byte
//
// h:
// jmp .Ltmp2 ; 5 bytes
// int3 ; 1 byte
// int3 ; 1 byte
// int3 ; 1 byte
//
// .Ltmp0:
// mov 0, %eax
// ret
//
// .Ltmp1:
// mov 1, %eax
// ret
//
// .Ltmp2:
// mov 2, %eax
// ret
//
// foo:
// mov f, %eax
// mov g, %edx
// mov h, %ecx
// ret
//
// Because the addresses of f, g, h are evenly spaced at a power of 2, in the
// normal case the check can be carried out using the same kind of simple
// arithmetic that we normally use for globals.
assert(!Functions.empty());
// Build a simple layout based on the regular layout of jump tables.
DenseMap<GlobalObject *, uint64_t> GlobalLayout;
unsigned EntrySize = getJumpTableEntrySize();
for (unsigned I = 0; I != Functions.size(); ++I)
GlobalLayout[Functions[I]] = I * EntrySize;
// Create a constant to hold the jump table.
ArrayType *JumpTableType =
ArrayType::get(getJumpTableEntryType(), Functions.size());
auto JumpTable = new GlobalVariable(*M, JumpTableType,
/*isConstant=*/true,
GlobalValue::PrivateLinkage, nullptr);
JumpTable->setSection(ObjectFormat == Triple::MachO
? "__TEXT,__text,regular,pure_instructions"
: ".text");
lowerBitSetCalls(BitSets, JumpTable, GlobalLayout);
// Build aliases pointing to offsets into the jump table, and replace
// references to the original functions with references to the aliases.
for (unsigned I = 0; I != Functions.size(); ++I) {
Constant *CombinedGlobalElemPtr = ConstantExpr::getBitCast(
ConstantExpr::getGetElementPtr(
JumpTableType, JumpTable,
ArrayRef<Constant *>{ConstantInt::get(IntPtrTy, 0),
ConstantInt::get(IntPtrTy, I)}),
Functions[I]->getType());
if (LinkerSubsectionsViaSymbols || Functions[I]->isDeclarationForLinker()) {
Functions[I]->replaceAllUsesWith(CombinedGlobalElemPtr);
} else {
assert(Functions[I]->getType()->getAddressSpace() == 0);
GlobalAlias *GAlias = GlobalAlias::create(Functions[I]->getValueType(), 0,
Functions[I]->getLinkage(), "",
CombinedGlobalElemPtr, M);
GAlias->setVisibility(Functions[I]->getVisibility());
GAlias->takeName(Functions[I]);
Functions[I]->replaceAllUsesWith(GAlias);
}
if (!Functions[I]->isDeclarationForLinker())
Functions[I]->setLinkage(GlobalValue::PrivateLinkage);
}
// Build and set the jump table's initializer.
std::vector<Constant *> JumpTableEntries;
for (unsigned I = 0; I != Functions.size(); ++I)
JumpTableEntries.push_back(
createJumpTableEntry(JumpTable, Functions[I], I));
JumpTable->setInitializer(
ConstantArray::get(JumpTableType, JumpTableEntries));
}
void LowerBitSets::buildBitSetsFromDisjointSet(
ArrayRef<Metadata *> BitSets, ArrayRef<GlobalObject *> Globals) {
llvm::DenseMap<Metadata *, uint64_t> BitSetIndices;
llvm::DenseMap<GlobalObject *, uint64_t> GlobalIndices;
for (unsigned I = 0; I != BitSets.size(); ++I)
BitSetIndices[BitSets[I]] = I;
for (unsigned I = 0; I != Globals.size(); ++I)
GlobalIndices[Globals[I]] = I;
// For each bitset, build a set of indices that refer to globals referenced by
// the bitset.
std::vector<std::set<uint64_t>> BitSetMembers(BitSets.size());
if (BitSetNM) {
for (MDNode *Op : BitSetNM->operands()) {
// Op = { bitset name, global, offset }
if (!Op->getOperand(1))
continue;
auto I = BitSetIndices.find(Op->getOperand(0));
if (I == BitSetIndices.end())
continue;
auto OpGlobal = dyn_cast<GlobalObject>(
cast<ConstantAsMetadata>(Op->getOperand(1))->getValue());
if (!OpGlobal)
continue;
BitSetMembers[I->second].insert(GlobalIndices[OpGlobal]);
}
}
// Order the sets of indices by size. The GlobalLayoutBuilder works best
// when given small index sets first.
std::stable_sort(
BitSetMembers.begin(), BitSetMembers.end(),
[](const std::set<uint64_t> &O1, const std::set<uint64_t> &O2) {
return O1.size() < O2.size();
});
// Create a GlobalLayoutBuilder and provide it with index sets as layout
// fragments. The GlobalLayoutBuilder tries to lay out members of fragments as
// close together as possible.
GlobalLayoutBuilder GLB(Globals.size());
for (auto &&MemSet : BitSetMembers)
GLB.addFragment(MemSet);
// Build the bitsets from this disjoint set.
if (Globals.empty() || isa<GlobalVariable>(Globals[0])) {
// Build a vector of global variables with the computed layout.
std::vector<GlobalVariable *> OrderedGVs(Globals.size());
auto OGI = OrderedGVs.begin();
for (auto &&F : GLB.Fragments) {
for (auto &&Offset : F) {
auto GV = dyn_cast<GlobalVariable>(Globals[Offset]);
if (!GV)
report_fatal_error(
"Bit set may not contain both global variables and functions");
*OGI++ = GV;
}
}
buildBitSetsFromGlobalVariables(BitSets, OrderedGVs);
} else {
// Build a vector of functions with the computed layout.
std::vector<Function *> OrderedFns(Globals.size());
auto OFI = OrderedFns.begin();
for (auto &&F : GLB.Fragments) {
for (auto &&Offset : F) {
auto Fn = dyn_cast<Function>(Globals[Offset]);
if (!Fn)
report_fatal_error(
"Bit set may not contain both global variables and functions");
*OFI++ = Fn;
}
}
buildBitSetsFromFunctions(BitSets, OrderedFns);
}
}
/// Lower all bit sets in this module.
bool LowerBitSets::buildBitSets() {
Function *BitSetTestFunc =
M->getFunction(Intrinsic::getName(Intrinsic::bitset_test));
if (!BitSetTestFunc)
return false;
// Equivalence class set containing bitsets and the globals they reference.
// This is used to partition the set of bitsets in the module into disjoint
// sets.
typedef EquivalenceClasses<PointerUnion<GlobalObject *, Metadata *>>
GlobalClassesTy;
GlobalClassesTy GlobalClasses;
// Verify the bitset metadata and build a mapping from bitset identifiers to
// their last observed index in BitSetNM. This will used later to
// deterministically order the list of bitset identifiers.
llvm::DenseMap<Metadata *, unsigned> BitSetIdIndices;
if (BitSetNM) {
for (unsigned I = 0, E = BitSetNM->getNumOperands(); I != E; ++I) {
MDNode *Op = BitSetNM->getOperand(I);
verifyBitSetMDNode(Op);
BitSetIdIndices[Op->getOperand(0)] = I;
}
}
for (const Use &U : BitSetTestFunc->uses()) {
auto CI = cast<CallInst>(U.getUser());
auto BitSetMDVal = dyn_cast<MetadataAsValue>(CI->getArgOperand(1));
if (!BitSetMDVal)
report_fatal_error(
"Second argument of llvm.bitset.test must be metadata");
auto BitSet = BitSetMDVal->getMetadata();
// Add the call site to the list of call sites for this bit set. We also use
// BitSetTestCallSites to keep track of whether we have seen this bit set
// before. If we have, we don't need to re-add the referenced globals to the
// equivalence class.
std::pair<DenseMap<Metadata *, std::vector<CallInst *>>::iterator,
bool> Ins =
BitSetTestCallSites.insert(
std::make_pair(BitSet, std::vector<CallInst *>()));
Ins.first->second.push_back(CI);
if (!Ins.second)
continue;
// Add the bitset to the equivalence class.
GlobalClassesTy::iterator GCI = GlobalClasses.insert(BitSet);
GlobalClassesTy::member_iterator CurSet = GlobalClasses.findLeader(GCI);
if (!BitSetNM)
continue;
// Add the referenced globals to the bitset's equivalence class.
for (MDNode *Op : BitSetNM->operands()) {
if (Op->getOperand(0) != BitSet || !Op->getOperand(1))
continue;
auto OpGlobal = dyn_cast<GlobalObject>(
cast<ConstantAsMetadata>(Op->getOperand(1))->getValue());
if (!OpGlobal)
continue;
CurSet = GlobalClasses.unionSets(
CurSet, GlobalClasses.findLeader(GlobalClasses.insert(OpGlobal)));
}
}
if (GlobalClasses.empty())
return false;
// Build a list of disjoint sets ordered by their maximum BitSetNM index
// for determinism.
std::vector<std::pair<GlobalClassesTy::iterator, unsigned>> Sets;
for (GlobalClassesTy::iterator I = GlobalClasses.begin(),
E = GlobalClasses.end();
I != E; ++I) {
if (!I->isLeader()) continue;
++NumBitSetDisjointSets;
unsigned MaxIndex = 0;
for (GlobalClassesTy::member_iterator MI = GlobalClasses.member_begin(I);
MI != GlobalClasses.member_end(); ++MI) {
if ((*MI).is<Metadata *>())
MaxIndex = std::max(MaxIndex, BitSetIdIndices[MI->get<Metadata *>()]);
}
Sets.emplace_back(I, MaxIndex);
}
std::sort(Sets.begin(), Sets.end(),
[](const std::pair<GlobalClassesTy::iterator, unsigned> &S1,
const std::pair<GlobalClassesTy::iterator, unsigned> &S2) {
return S1.second < S2.second;
});
// For each disjoint set we found...
for (const auto &S : Sets) {
// Build the list of bitsets in this disjoint set.
std::vector<Metadata *> BitSets;
std::vector<GlobalObject *> Globals;
for (GlobalClassesTy::member_iterator MI =
GlobalClasses.member_begin(S.first);
MI != GlobalClasses.member_end(); ++MI) {
if ((*MI).is<Metadata *>())
BitSets.push_back(MI->get<Metadata *>());
else
Globals.push_back(MI->get<GlobalObject *>());
}
// Order bitsets by BitSetNM index for determinism. This ordering is stable
// as there is a one-to-one mapping between metadata and indices.
std::sort(BitSets.begin(), BitSets.end(), [&](Metadata *M1, Metadata *M2) {
return BitSetIdIndices[M1] < BitSetIdIndices[M2];
});
// Lower the bitsets in this disjoint set.
buildBitSetsFromDisjointSet(BitSets, Globals);
}
allocateByteArrays();
return true;
}
bool LowerBitSets::eraseBitSetMetadata() {
if (!BitSetNM)
return false;
M->eraseNamedMetadata(BitSetNM);
return true;
}
bool LowerBitSets::runOnModule(Module &M) {
bool Changed = buildBitSets();
Changed |= eraseBitSetMetadata();
return Changed;
}
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