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llvm / llvm / 7abb456200f8f62cb07251c0fa6447942973dbbb / . / lib / MC / WasmObjectWriter.cpp
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//===- lib/MC/WasmObjectWriter.cpp - Wasm File Writer ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Wasm object file writer information.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/BinaryFormat/Wasm.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSectionWasm.h"
#include "llvm/MC/MCSymbolWasm.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/MCWasmObjectWriter.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/StringSaver.h"
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "mc"
namespace {
// Went we ceate the indirect function table we start at 1, so that there is
// and emtpy slot at 0 and therefore calling a null function pointer will trap.
static const uint32_t kInitialTableOffset = 1;
// For patching purposes, we need to remember where each section starts, both
// for patching up the section size field, and for patching up references to
// locations within the section.
struct SectionBookkeeping {
// Where the size of the section is written.
uint64_t SizeOffset;
// Where the contents of the section starts (after the header).
uint64_t ContentsOffset;
};
// The signature of a wasm function, in a struct capable of being used as a
// DenseMap key.
struct WasmFunctionType {
// Support empty and tombstone instances, needed by DenseMap.
enum { Plain, Empty, Tombstone } State;
// The return types of the function.
SmallVector<wasm::ValType, 1> Returns;
// The parameter types of the function.
SmallVector<wasm::ValType, 4> Params;
WasmFunctionType() : State(Plain) {}
bool operator==(const WasmFunctionType &Other) const {
return State == Other.State && Returns == Other.Returns &&
Params == Other.Params;
}
};
// Traits for using WasmFunctionType in a DenseMap.
struct WasmFunctionTypeDenseMapInfo {
static WasmFunctionType getEmptyKey() {
WasmFunctionType FuncTy;
FuncTy.State = WasmFunctionType::Empty;
return FuncTy;
}
static WasmFunctionType getTombstoneKey() {
WasmFunctionType FuncTy;
FuncTy.State = WasmFunctionType::Tombstone;
return FuncTy;
}
static unsigned getHashValue(const WasmFunctionType &FuncTy) {
uintptr_t Value = FuncTy.State;
for (wasm::ValType Ret : FuncTy.Returns)
Value += DenseMapInfo<int32_t>::getHashValue(int32_t(Ret));
for (wasm::ValType Param : FuncTy.Params)
Value += DenseMapInfo<int32_t>::getHashValue(int32_t(Param));
return Value;
}
static bool isEqual(const WasmFunctionType &LHS,
const WasmFunctionType &RHS) {
return LHS == RHS;
}
};
// A wasm data segment. A wasm binary contains only a single data section
// but that can contain many segments, each with their own virtual location
// in memory. Each MCSection data created by llvm is modeled as its own
// wasm data segment.
struct WasmDataSegment {
MCSectionWasm *Section;
StringRef Name;
uint32_t Offset;
uint32_t Alignment;
uint32_t Flags;
SmallVector<char, 4> Data;
};
// A wasm function to be written into the function section.
struct WasmFunction {
int32_t Type;
const MCSymbolWasm *Sym;
};
// A wasm global to be written into the global section.
struct WasmGlobal {
wasm::WasmGlobalType Type;
uint64_t InitialValue;
};
// Information about a single item which is part of a COMDAT. For each data
// segment or function which is in the COMDAT, there is a corresponding
// WasmComdatEntry.
struct WasmComdatEntry {
unsigned Kind;
uint32_t Index;
};
// Information about a single relocation.
struct WasmRelocationEntry {
uint64_t Offset; // Where is the relocation.
const MCSymbolWasm *Symbol; // The symbol to relocate with.
int64_t Addend; // A value to add to the symbol.
unsigned Type; // The type of the relocation.
const MCSectionWasm *FixupSection;// The section the relocation is targeting.
WasmRelocationEntry(uint64_t Offset, const MCSymbolWasm *Symbol,
int64_t Addend, unsigned Type,
const MCSectionWasm *FixupSection)
: Offset(Offset), Symbol(Symbol), Addend(Addend), Type(Type),
FixupSection(FixupSection) {}
bool hasAddend() const {
switch (Type) {
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
return true;
default:
return false;
}
}
void print(raw_ostream &Out) const {
Out << "Off=" << Offset << ", Sym=" << *Symbol << ", Addend=" << Addend
<< ", Type=" << Type
<< ", FixupSection=" << FixupSection->getSectionName();
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void dump() const { print(dbgs()); }
#endif
};
#if !defined(NDEBUG)
raw_ostream &operator<<(raw_ostream &OS, const WasmRelocationEntry &Rel) {
Rel.print(OS);
return OS;
}
#endif
class WasmObjectWriter : public MCObjectWriter {
/// The target specific Wasm writer instance.
std::unique_ptr<MCWasmObjectTargetWriter> TargetObjectWriter;
// Relocations for fixing up references in the code section.
std::vector<WasmRelocationEntry> CodeRelocations;
// Relocations for fixing up references in the data section.
std::vector<WasmRelocationEntry> DataRelocations;
// Index values to use for fixing up call_indirect type indices.
// Maps function symbols to the index of the type of the function
DenseMap<const MCSymbolWasm *, uint32_t> TypeIndices;
// Maps function symbols to the table element index space. Used
// for TABLE_INDEX relocation types (i.e. address taken functions).
DenseMap<const MCSymbolWasm *, uint32_t> TableIndices;
// Maps function/global symbols to the (shared) Symbol index space.
DenseMap<const MCSymbolWasm *, uint32_t> SymbolIndices;
// Maps function/global symbols to the function/global Wasm index space.
DenseMap<const MCSymbolWasm *, uint32_t> WasmIndices;
// Maps data symbols to the Wasm segment and offset/size with the segment.
DenseMap<const MCSymbolWasm *, wasm::WasmDataReference> DataLocations;
DenseMap<WasmFunctionType, int32_t, WasmFunctionTypeDenseMapInfo>
FunctionTypeIndices;
SmallVector<WasmFunctionType, 4> FunctionTypes;
SmallVector<WasmGlobal, 4> Globals;
SmallVector<WasmDataSegment, 4> DataSegments;
unsigned NumFunctionImports = 0;
unsigned NumGlobalImports = 0;
// TargetObjectWriter wrappers.
bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
unsigned getRelocType(const MCValue &Target, const MCFixup &Fixup) const {
return TargetObjectWriter->getRelocType(Target, Fixup);
}
void startSection(SectionBookkeeping &Section, unsigned SectionId,
const char *Name = nullptr);
void endSection(SectionBookkeeping &Section);
public:
WasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
raw_pwrite_stream &OS)
: MCObjectWriter(OS, /*IsLittleEndian=*/true),
TargetObjectWriter(std::move(MOTW)) {}
~WasmObjectWriter() override;
private:
void reset() override {
CodeRelocations.clear();
DataRelocations.clear();
TypeIndices.clear();
SymbolIndices.clear();
WasmIndices.clear();
TableIndices.clear();
DataLocations.clear();
FunctionTypeIndices.clear();
FunctionTypes.clear();
Globals.clear();
DataSegments.clear();
MCObjectWriter::reset();
NumFunctionImports = 0;
NumGlobalImports = 0;
}
void writeHeader(const MCAssembler &Asm);
void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
MCValue Target, uint64_t &FixedValue) override;
void executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
void writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
void writeString(const StringRef Str) {
encodeULEB128(Str.size(), getStream());
writeBytes(Str);
}
void writeValueType(wasm::ValType Ty) {
write8(static_cast<uint8_t>(Ty));
}
void writeTypeSection(ArrayRef<WasmFunctionType> FunctionTypes);
void writeImportSection(ArrayRef<wasm::WasmImport> Imports, uint32_t DataSize,
uint32_t NumElements);
void writeFunctionSection(ArrayRef<WasmFunction> Functions);
void writeGlobalSection();
void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
void writeElemSection(ArrayRef<uint32_t> TableElems);
void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
ArrayRef<WasmFunction> Functions);
void writeDataSection();
void writeCodeRelocSection();
void writeDataRelocSection();
void writeLinkingMetaDataSection(
ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats);
uint32_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
uint64_t ContentsOffset);
void writeRelocations(ArrayRef<WasmRelocationEntry> Relocations);
uint32_t getRelocationIndexValue(const WasmRelocationEntry &RelEntry);
uint32_t getFunctionType(const MCSymbolWasm& Symbol);
uint32_t registerFunctionType(const MCSymbolWasm& Symbol);
};
} // end anonymous namespace
WasmObjectWriter::~WasmObjectWriter() {}
// Write out a section header and a patchable section size field.
void WasmObjectWriter::startSection(SectionBookkeeping &Section,
unsigned SectionId,
const char *Name) {
assert((Name != nullptr) == (SectionId == wasm::WASM_SEC_CUSTOM) &&
"Only custom sections can have names");
DEBUG(dbgs() << "startSection " << SectionId << ": " << Name << "\n");
write8(SectionId);
Section.SizeOffset = getStream().tell();
// The section size. We don't know the size yet, so reserve enough space
// for any 32-bit value; we'll patch it later.
encodeULEB128(UINT32_MAX, getStream());
// The position where the section starts, for measuring its size.
Section.ContentsOffset = getStream().tell();
// Custom sections in wasm also have a string identifier.
if (SectionId == wasm::WASM_SEC_CUSTOM) {
assert(Name);
writeString(StringRef(Name));
}
}
// Now that the section is complete and we know how big it is, patch up the
// section size field at the start of the section.
void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
uint64_t Size = getStream().tell() - Section.ContentsOffset;
if (uint32_t(Size) != Size)
report_fatal_error("section size does not fit in a uint32_t");
DEBUG(dbgs() << "endSection size=" << Size << "\n");
// Write the final section size to the payload_len field, which follows
// the section id byte.
uint8_t Buffer[16];
unsigned SizeLen = encodeULEB128(Size, Buffer, 5);
assert(SizeLen == 5);
getStream().pwrite((char *)Buffer, SizeLen, Section.SizeOffset);
}
// Emit the Wasm header.
void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
writeBytes(StringRef(wasm::WasmMagic, sizeof(wasm::WasmMagic)));
writeLE32(wasm::WasmVersion);
}
void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) {
}
void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup, MCValue Target,
uint64_t &FixedValue) {
MCAsmBackend &Backend = Asm.getBackend();
bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
MCFixupKindInfo::FKF_IsPCRel;
const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());
uint64_t C = Target.getConstant();
uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
MCContext &Ctx = Asm.getContext();
// The .init_array isn't translated as data, so don't do relocations in it.
if (FixupSection.getSectionName().startswith(".init_array"))
return;
// TODO(sbc): Add support for debug sections.
if (FixupSection.getKind().isMetadata())
return;
if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
"Should not have constructed this");
// Let A, B and C being the components of Target and R be the location of
// the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
// If it is pcrel, we want to compute (A - B + C - R).
// In general, Wasm has no relocations for -B. It can only represent (A + C)
// or (A + C - R). If B = R + K and the relocation is not pcrel, we can
// replace B to implement it: (A - R - K + C)
if (IsPCRel) {
Ctx.reportError(
Fixup.getLoc(),
"No relocation available to represent this relative expression");
return;
}
const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
if (SymB.isUndefined()) {
Ctx.reportError(Fixup.getLoc(),
Twine("symbol '") + SymB.getName() +
"' can not be undefined in a subtraction expression");
return;
}
assert(!SymB.isAbsolute() && "Should have been folded");
const MCSection &SecB = SymB.getSection();
if (&SecB != &FixupSection) {
Ctx.reportError(Fixup.getLoc(),
"Cannot represent a difference across sections");
return;
}
uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
uint64_t K = SymBOffset - FixupOffset;
IsPCRel = true;
C -= K;
}
// We either rejected the fixup or folded B into C at this point.
const MCSymbolRefExpr *RefA = Target.getSymA();
const auto *SymA = RefA ? cast<MCSymbolWasm>(&RefA->getSymbol()) : nullptr;
if (SymA && SymA->isVariable()) {
const MCExpr *Expr = SymA->getVariableValue();
const auto *Inner = cast<MCSymbolRefExpr>(Expr);
if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
llvm_unreachable("weakref used in reloc not yet implemented");
}
// Put any constant offset in an addend. Offsets can be negative, and
// LLVM expects wrapping, in contrast to wasm's immediates which can't
// be negative and don't wrap.
FixedValue = 0;
if (SymA)
SymA->setUsedInReloc();
assert(!IsPCRel);
assert(SymA);
unsigned Type = getRelocType(Target, Fixup);
WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
DEBUG(dbgs() << "WasmReloc: " << Rec << "\n");
// Relocation other than R_WEBASSEMBLY_TYPE_INDEX_LEB are currently required
// to be against a named symbol.
// TODO(sbc): Add support for relocations against unnamed temporaries such
// as those generated by llvm's `blockaddress`.
// See: test/MC/WebAssembly/blockaddress.ll
if (SymA->getName().empty() && Type != wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB)
report_fatal_error("relocations against un-named temporaries are not yet "
"supported by wasm");
if (FixupSection.isWasmData())
DataRelocations.push_back(Rec);
else if (FixupSection.getKind().isText())
CodeRelocations.push_back(Rec);
else
llvm_unreachable("unexpected section type");
}
// Write X as an (unsigned) LEB value at offset Offset in Stream, padded
// to allow patching.
static void
WritePatchableLEB(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
uint8_t Buffer[5];
unsigned SizeLen = encodeULEB128(X, Buffer, 5);
assert(SizeLen == 5);
Stream.pwrite((char *)Buffer, SizeLen, Offset);
}
// Write X as an signed LEB value at offset Offset in Stream, padded
// to allow patching.
static void
WritePatchableSLEB(raw_pwrite_stream &Stream, int32_t X, uint64_t Offset) {
uint8_t Buffer[5];
unsigned SizeLen = encodeSLEB128(X, Buffer, 5);
assert(SizeLen == 5);
Stream.pwrite((char *)Buffer, SizeLen, Offset);
}
// Write X as a plain integer value at offset Offset in Stream.
static void WriteI32(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
uint8_t Buffer[4];
support::endian::write32le(Buffer, X);
Stream.pwrite((char *)Buffer, sizeof(Buffer), Offset);
}
static const MCSymbolWasm* ResolveSymbol(const MCSymbolWasm& Symbol) {
if (Symbol.isVariable()) {
const MCExpr *Expr = Symbol.getVariableValue();
auto *Inner = cast<MCSymbolRefExpr>(Expr);
return cast<MCSymbolWasm>(&Inner->getSymbol());
}
return &Symbol;
}
// Compute a value to write into the code at the location covered
// by RelEntry. This value isn't used by the static linker; it just serves
// to make the object format more readable and more likely to be directly
// useable.
uint32_t
WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32: {
// Provisional value is table address of the resolved symbol itself
const MCSymbolWasm *Sym = ResolveSymbol(*RelEntry.Symbol);
assert(Sym->isFunction());
return TableIndices[Sym];
}
case wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB:
// Provisional value is same as the index
return getRelocationIndexValue(RelEntry);
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
// Provisional value is function/global Wasm index
if (!WasmIndices.count(RelEntry.Symbol))
report_fatal_error("symbol not found in wasm index space: " +
RelEntry.Symbol->getName());
return WasmIndices[RelEntry.Symbol];
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB: {
// Provisional value is address of the global
const MCSymbolWasm *Sym = ResolveSymbol(*RelEntry.Symbol);
// For undefined symbols, use zero
if (!Sym->isDefined())
return 0;
const wasm::WasmDataReference &Ref = DataLocations[Sym];
const WasmDataSegment &Segment = DataSegments[Ref.Segment];
// Ignore overflow. LLVM allows address arithmetic to silently wrap.
return Segment.Offset + Ref.Offset + RelEntry.Addend;
}
default:
llvm_unreachable("invalid relocation type");
}
}
static void addData(SmallVectorImpl<char> &DataBytes,
MCSectionWasm &DataSection) {
DEBUG(errs() << "addData: " << DataSection.getSectionName() << "\n");
DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
for (const MCFragment &Frag : DataSection) {
if (Frag.hasInstructions())
report_fatal_error("only data supported in data sections");
if (auto *Align = dyn_cast<MCAlignFragment>(&Frag)) {
if (Align->getValueSize() != 1)
report_fatal_error("only byte values supported for alignment");
// If nops are requested, use zeros, as this is the data section.
uint8_t Value = Align->hasEmitNops() ? 0 : Align->getValue();
uint64_t Size = std::min<uint64_t>(alignTo(DataBytes.size(),
Align->getAlignment()),
DataBytes.size() +
Align->getMaxBytesToEmit());
DataBytes.resize(Size, Value);
} else if (auto *Fill = dyn_cast<MCFillFragment>(&Frag)) {
int64_t Size;
if (!Fill->getSize().evaluateAsAbsolute(Size))
llvm_unreachable("The fill should be an assembler constant");
DataBytes.insert(DataBytes.end(), Size, Fill->getValue());
} else {
const auto &DataFrag = cast<MCDataFragment>(Frag);
const SmallVectorImpl<char> &Contents = DataFrag.getContents();
DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
}
}
DEBUG(dbgs() << "addData -> " << DataBytes.size() << "\n");
}
uint32_t
WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
if (RelEntry.Type == wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB) {
if (!TypeIndices.count(RelEntry.Symbol))
report_fatal_error("symbol not found in type index space: " +
RelEntry.Symbol->getName());
return TypeIndices[RelEntry.Symbol];
}
if (!SymbolIndices.count(RelEntry.Symbol))
report_fatal_error("symbol not found in symbol index space: " +
RelEntry.Symbol->getName());
return SymbolIndices[RelEntry.Symbol];
}
// Apply the portions of the relocation records that we can handle ourselves
// directly.
void WasmObjectWriter::applyRelocations(
ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset) {
raw_pwrite_stream &Stream = getStream();
for (const WasmRelocationEntry &RelEntry : Relocations) {
uint64_t Offset = ContentsOffset +
RelEntry.FixupSection->getSectionOffset() +
RelEntry.Offset;
DEBUG(dbgs() << "applyRelocation: " << RelEntry << "\n");
uint32_t Value = getProvisionalValue(RelEntry);
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB:
case wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB:
WritePatchableLEB(Stream, Value, Offset);
break;
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
WriteI32(Stream, Value, Offset);
break;
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
WritePatchableSLEB(Stream, Value, Offset);
break;
default:
llvm_unreachable("invalid relocation type");
}
}
}
// Write out the portions of the relocation records that the linker will
// need to handle.
void WasmObjectWriter::writeRelocations(
ArrayRef<WasmRelocationEntry> Relocations) {
raw_pwrite_stream &Stream = getStream();
for (const WasmRelocationEntry& RelEntry : Relocations) {
uint64_t Offset = RelEntry.Offset +
RelEntry.FixupSection->getSectionOffset();
uint32_t Index = getRelocationIndexValue(RelEntry);
write8(RelEntry.Type);
encodeULEB128(Offset, Stream);
encodeULEB128(Index, Stream);
if (RelEntry.hasAddend())
encodeSLEB128(RelEntry.Addend, Stream);
}
}
void WasmObjectWriter::writeTypeSection(
ArrayRef<WasmFunctionType> FunctionTypes) {
if (FunctionTypes.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_TYPE);
encodeULEB128(FunctionTypes.size(), getStream());
for (const WasmFunctionType &FuncTy : FunctionTypes) {
write8(wasm::WASM_TYPE_FUNC);
encodeULEB128(FuncTy.Params.size(), getStream());
for (wasm::ValType Ty : FuncTy.Params)
writeValueType(Ty);
encodeULEB128(FuncTy.Returns.size(), getStream());
for (wasm::ValType Ty : FuncTy.Returns)
writeValueType(Ty);
}
endSection(Section);
}
void WasmObjectWriter::writeImportSection(ArrayRef<wasm::WasmImport> Imports,
uint32_t DataSize,
uint32_t NumElements) {
if (Imports.empty())
return;
uint32_t NumPages = (DataSize + wasm::WasmPageSize - 1) / wasm::WasmPageSize;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_IMPORT);
encodeULEB128(Imports.size(), getStream());
for (const wasm::WasmImport &Import : Imports) {
writeString(Import.Module);
writeString(Import.Field);
write8(Import.Kind);
switch (Import.Kind) {
case wasm::WASM_EXTERNAL_FUNCTION:
encodeULEB128(Import.SigIndex, getStream());
break;
case wasm::WASM_EXTERNAL_GLOBAL:
write8(Import.Global.Type);
write8(Import.Global.Mutable ? 1 : 0);
break;
case wasm::WASM_EXTERNAL_MEMORY:
encodeULEB128(0, getStream()); // flags
encodeULEB128(NumPages, getStream()); // initial
break;
case wasm::WASM_EXTERNAL_TABLE:
write8(Import.Table.ElemType);
encodeULEB128(0, getStream()); // flags
encodeULEB128(NumElements, getStream()); // initial
break;
default:
llvm_unreachable("unsupported import kind");
}
}
endSection(Section);
}
void WasmObjectWriter::writeFunctionSection(ArrayRef<WasmFunction> Functions) {
if (Functions.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_FUNCTION);
encodeULEB128(Functions.size(), getStream());
for (const WasmFunction &Func : Functions)
encodeULEB128(Func.Type, getStream());
endSection(Section);
}
void WasmObjectWriter::writeGlobalSection() {
if (Globals.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_GLOBAL);
encodeULEB128(Globals.size(), getStream());
for (const WasmGlobal &Global : Globals) {
writeValueType(static_cast<wasm::ValType>(Global.Type.Type));
write8(Global.Type.Mutable);
write8(wasm::WASM_OPCODE_I32_CONST);
encodeSLEB128(Global.InitialValue, getStream());
write8(wasm::WASM_OPCODE_END);
}
endSection(Section);
}
void WasmObjectWriter::writeExportSection(ArrayRef<wasm::WasmExport> Exports) {
if (Exports.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_EXPORT);
encodeULEB128(Exports.size(), getStream());
for (const wasm::WasmExport &Export : Exports) {
writeString(Export.Name);
write8(Export.Kind);
encodeULEB128(Export.Index, getStream());
}
endSection(Section);
}
void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
if (TableElems.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_ELEM);
encodeULEB128(1, getStream()); // number of "segments"
encodeULEB128(0, getStream()); // the table index
// init expr for starting offset
write8(wasm::WASM_OPCODE_I32_CONST);
encodeSLEB128(kInitialTableOffset, getStream());
write8(wasm::WASM_OPCODE_END);
encodeULEB128(TableElems.size(), getStream());
for (uint32_t Elem : TableElems)
encodeULEB128(Elem, getStream());
endSection(Section);
}
void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
const MCAsmLayout &Layout,
ArrayRef<WasmFunction> Functions) {
if (Functions.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CODE);
encodeULEB128(Functions.size(), getStream());
for (const WasmFunction &Func : Functions) {
auto &FuncSection = static_cast<MCSectionWasm &>(Func.Sym->getSection());
int64_t Size = 0;
if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
report_fatal_error(".size expression must be evaluatable");
encodeULEB128(Size, getStream());
FuncSection.setSectionOffset(getStream().tell() - Section.ContentsOffset);
Asm.writeSectionData(&FuncSection, Layout);
}
// Apply fixups.
applyRelocations(CodeRelocations, Section.ContentsOffset);
endSection(Section);
}
void WasmObjectWriter::writeDataSection() {
if (DataSegments.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_DATA);
encodeULEB128(DataSegments.size(), getStream()); // count
for (const WasmDataSegment &Segment : DataSegments) {
encodeULEB128(0, getStream()); // memory index
write8(wasm::WASM_OPCODE_I32_CONST);
encodeSLEB128(Segment.Offset, getStream()); // offset
write8(wasm::WASM_OPCODE_END);
encodeULEB128(Segment.Data.size(), getStream()); // size
Segment.Section->setSectionOffset(getStream().tell() - Section.ContentsOffset);
writeBytes(Segment.Data); // data
}
// Apply fixups.
applyRelocations(DataRelocations, Section.ContentsOffset);
endSection(Section);
}
void WasmObjectWriter::writeCodeRelocSection() {
// See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
// for descriptions of the reloc sections.
if (CodeRelocations.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.CODE");
encodeULEB128(wasm::WASM_SEC_CODE, getStream());
encodeULEB128(CodeRelocations.size(), getStream());
writeRelocations(CodeRelocations);
endSection(Section);
}
void WasmObjectWriter::writeDataRelocSection() {
// See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
// for descriptions of the reloc sections.
if (DataRelocations.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.DATA");
encodeULEB128(wasm::WASM_SEC_DATA, getStream());
encodeULEB128(DataRelocations.size(), getStream());
writeRelocations(DataRelocations);
endSection(Section);
}
void WasmObjectWriter::writeLinkingMetaDataSection(
ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats) {
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CUSTOM, "linking");
SectionBookkeeping SubSection;
if (SymbolInfos.size() != 0) {
startSection(SubSection, wasm::WASM_SYMBOL_TABLE);
encodeULEB128(SymbolInfos.size(), getStream());
for (const wasm::WasmSymbolInfo &Sym : SymbolInfos) {
encodeULEB128(Sym.Kind, getStream());
encodeULEB128(Sym.Flags, getStream());
switch (Sym.Kind) {
case wasm::WASM_SYMBOL_TYPE_FUNCTION:
case wasm::WASM_SYMBOL_TYPE_GLOBAL:
encodeULEB128(Sym.ElementIndex, getStream());
if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0)
writeString(Sym.Name);
break;
case wasm::WASM_SYMBOL_TYPE_DATA:
writeString(Sym.Name);
if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0) {
encodeULEB128(Sym.DataRef.Segment, getStream());
encodeULEB128(Sym.DataRef.Offset, getStream());
encodeULEB128(Sym.DataRef.Size, getStream());
}
break;
default:
llvm_unreachable("unexpected kind");
}
}
endSection(SubSection);
}
if (DataSegments.size()) {
startSection(SubSection, wasm::WASM_SEGMENT_INFO);
encodeULEB128(DataSegments.size(), getStream());
for (const WasmDataSegment &Segment : DataSegments) {
writeString(Segment.Name);
encodeULEB128(Segment.Alignment, getStream());
encodeULEB128(Segment.Flags, getStream());
}
endSection(SubSection);
}
if (!InitFuncs.empty()) {
startSection(SubSection, wasm::WASM_INIT_FUNCS);
encodeULEB128(InitFuncs.size(), getStream());
for (auto &StartFunc : InitFuncs) {
encodeULEB128(StartFunc.first, getStream()); // priority
encodeULEB128(StartFunc.second, getStream()); // function index
}
endSection(SubSection);
}
if (Comdats.size()) {
startSection(SubSection, wasm::WASM_COMDAT_INFO);
encodeULEB128(Comdats.size(), getStream());
for (const auto &C : Comdats) {
writeString(C.first);
encodeULEB128(0, getStream()); // flags for future use
encodeULEB128(C.second.size(), getStream());
for (const WasmComdatEntry &Entry : C.second) {
encodeULEB128(Entry.Kind, getStream());
encodeULEB128(Entry.Index, getStream());
}
}
endSection(SubSection);
}
endSection(Section);
}
uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm& Symbol) {
assert(Symbol.isFunction());
assert(TypeIndices.count(&Symbol));
return TypeIndices[&Symbol];
}
uint32_t WasmObjectWriter::registerFunctionType(const MCSymbolWasm& Symbol) {
assert(Symbol.isFunction());
WasmFunctionType F;
const MCSymbolWasm* ResolvedSym = ResolveSymbol(Symbol);
F.Returns = ResolvedSym->getReturns();
F.Params = ResolvedSym->getParams();
auto Pair =
FunctionTypeIndices.insert(std::make_pair(F, FunctionTypes.size()));
if (Pair.second)
FunctionTypes.push_back(F);
TypeIndices[&Symbol] = Pair.first->second;
DEBUG(dbgs() << "registerFunctionType: " << Symbol << " new:" << Pair.second << "\n");
DEBUG(dbgs() << " -> type index: " << Pair.first->second << "\n");
return Pair.first->second;
}
void WasmObjectWriter::writeObject(MCAssembler &Asm,
const MCAsmLayout &Layout) {
DEBUG(dbgs() << "WasmObjectWriter::writeObject\n");
MCContext &Ctx = Asm.getContext();
// Collect information from the available symbols.
SmallVector<WasmFunction, 4> Functions;
SmallVector<uint32_t, 4> TableElems;
SmallVector<wasm::WasmImport, 4> Imports;
SmallVector<wasm::WasmExport, 4> Exports;
SmallVector<wasm::WasmSymbolInfo, 4> SymbolInfos;
SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
uint32_t DataSize = 0;
// For now, always emit the memory import, since loads and stores are not
// valid without it. In the future, we could perhaps be more clever and omit
// it if there are no loads or stores.
MCSymbolWasm *MemorySym =
cast<MCSymbolWasm>(Ctx.getOrCreateSymbol("__linear_memory"));
wasm::WasmImport MemImport;
MemImport.Module = MemorySym->getModuleName();
MemImport.Field = MemorySym->getName();
MemImport.Kind = wasm::WASM_EXTERNAL_MEMORY;
Imports.push_back(MemImport);
// For now, always emit the table section, since indirect calls are not
// valid without it. In the future, we could perhaps be more clever and omit
// it if there are no indirect calls.
MCSymbolWasm *TableSym =
cast<MCSymbolWasm>(Ctx.getOrCreateSymbol("__indirect_function_table"));
wasm::WasmImport TableImport;
TableImport.Module = TableSym->getModuleName();
TableImport.Field = TableSym->getName();
TableImport.Kind = wasm::WASM_EXTERNAL_TABLE;
TableImport.Table.ElemType = wasm::WASM_TYPE_ANYFUNC;
Imports.push_back(TableImport);
// Populate FunctionTypeIndices, and Imports and WasmIndices for undefined
// symbols. This must be done before populating WasmIndices for defined
// symbols.
for (const MCSymbol &S : Asm.symbols()) {
const auto &WS = static_cast<const MCSymbolWasm &>(S);
// Register types for all functions, including those with private linkage
// (because wasm always needs a type signature).
if (WS.isFunction())
registerFunctionType(WS);
if (WS.isTemporary())
continue;
// If the symbol is not defined in this translation unit, import it.
if (!WS.isDefined() && !WS.isComdat()) {
if (WS.isFunction()) {
wasm::WasmImport Import;
Import.Module = WS.getModuleName();
Import.Field = WS.getName();
Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
Import.SigIndex = getFunctionType(WS);
Imports.push_back(Import);
WasmIndices[&WS] = NumFunctionImports++;
} else if (WS.isGlobal()) {
if (WS.isWeak())
report_fatal_error("undefined global symbol cannot be weak");
wasm::WasmImport Import;
Import.Module = WS.getModuleName();
Import.Field = WS.getName();
Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
Import.Global = WS.getGlobalType();
Imports.push_back(Import);
WasmIndices[&WS] = NumGlobalImports++;
}
}
}
// Populate DataSegments, which must be done before populating DataLocations.
for (MCSection &Sec : Asm) {
auto &Section = static_cast<MCSectionWasm &>(Sec);
if (!Section.isWasmData())
continue;
// .init_array sections are handled specially elsewhere.
if (cast<MCSectionWasm>(Sec).getSectionName().startswith(".init_array"))
continue;
uint32_t SegmentIndex = DataSegments.size();
DataSize = alignTo(DataSize, Section.getAlignment());
DataSegments.emplace_back();
WasmDataSegment &Segment = DataSegments.back();
Segment.Name = Section.getSectionName();
Segment.Offset = DataSize;
Segment.Section = &Section;
addData(Segment.Data, Section);
Segment.Alignment = Section.getAlignment();
Segment.Flags = 0;
DataSize += Segment.Data.size();
Section.setSegmentIndex(SegmentIndex);
if (const MCSymbolWasm *C = Section.getGroup()) {
Comdats[C->getName()].emplace_back(
WasmComdatEntry{wasm::WASM_COMDAT_DATA, SegmentIndex});
}
}
// Populate WasmIndices and DataLocations for defined symbols.
for (const MCSymbol &S : Asm.symbols()) {
// Ignore unnamed temporary symbols, which aren't ever exported, imported,
// or used in relocations.
if (S.isTemporary() && S.getName().empty())
continue;
const auto &WS = static_cast<const MCSymbolWasm &>(S);
DEBUG(dbgs() << "MCSymbol: '" << S << "'"
<< " isDefined=" << S.isDefined()
<< " isExternal=" << S.isExternal()
<< " isTemporary=" << S.isTemporary()
<< " isFunction=" << WS.isFunction()
<< " isWeak=" << WS.isWeak()
<< " isHidden=" << WS.isHidden()
<< " isVariable=" << WS.isVariable() << "\n");
if (WS.isVariable())
continue;
if (WS.isComdat() && !WS.isDefined())
continue;
if (WS.isFunction()) {
unsigned Index;
if (WS.isDefined()) {
if (WS.getOffset() != 0)
report_fatal_error(
"function sections must contain one function each");
if (WS.getSize() == 0)
report_fatal_error(
"function symbols must have a size set with .size");
// A definition. Write out the function body.
Index = NumFunctionImports + Functions.size();
WasmFunction Func;
Func.Type = getFunctionType(WS);
Func.Sym = &WS;
WasmIndices[&WS] = Index;
Functions.push_back(Func);
auto &Section = static_cast<MCSectionWasm &>(WS.getSection());
if (const MCSymbolWasm *C = Section.getGroup()) {
Comdats[C->getName()].emplace_back(
WasmComdatEntry{wasm::WASM_COMDAT_FUNCTION, Index});
}
} else {
// An import; the index was assigned above.
Index = WasmIndices.find(&WS)->second;
}
DEBUG(dbgs() << " -> function index: " << Index << "\n");
} else if (WS.isData()) {
if (WS.isTemporary() && !WS.getSize())
continue;
if (!WS.isDefined()) {
DEBUG(dbgs() << " -> segment index: -1");
continue;
}
if (!WS.getSize())
report_fatal_error("data symbols must have a size set with .size: " +
WS.getName());
int64_t Size = 0;
if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
report_fatal_error(".size expression must be evaluatable");
auto &DataSection = static_cast<MCSectionWasm &>(WS.getSection());
assert(DataSection.isWasmData());
// For each data symbol, export it in the symtab as a reference to the
// corresponding Wasm data segment.
wasm::WasmDataReference Ref = wasm::WasmDataReference{
DataSection.getSegmentIndex(),
static_cast<uint32_t>(Layout.getSymbolOffset(WS)),
static_cast<uint32_t>(Size)};
DataLocations[&WS] = Ref;
DEBUG(dbgs() << " -> segment index: " << Ref.Segment);
} else {
// A "true" Wasm global (currently just __stack_pointer)
if (WS.isDefined())
report_fatal_error("don't yet support defined globals");
// An import; the index was assigned above
DEBUG(dbgs() << " -> global index: " << WasmIndices.find(&WS)->second
<< "\n");
}
}
// Populate WasmIndices and DataLocations for aliased symbols. We need to
// process these in a separate pass because we need to have processed the
// target of the alias before the alias itself and the symbols are not
// necessarily ordered in this way.
for (const MCSymbol &S : Asm.symbols()) {
if (!S.isVariable())
continue;
assert(S.isDefined());
// Find the target symbol of this weak alias and export that index
const auto &WS = static_cast<const MCSymbolWasm &>(S);
const MCSymbolWasm *ResolvedSym = ResolveSymbol(WS);
DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym << "'\n");
if (WS.isFunction()) {
assert(WasmIndices.count(ResolvedSym) > 0);
uint32_t WasmIndex = WasmIndices.find(ResolvedSym)->second;
WasmIndices[&WS] = WasmIndex;
DEBUG(dbgs() << " -> index:" << WasmIndex << "\n");
} else if (WS.isData()) {
assert(DataLocations.count(ResolvedSym) > 0);
const wasm::WasmDataReference &Ref =
DataLocations.find(ResolvedSym)->second;
DataLocations[&WS] = Ref;
DEBUG(dbgs() << " -> index:" << Ref.Segment << "\n");
} else {
report_fatal_error("don't yet support global aliases");
}
}
// Finally, populate the symbol table itself, in its "natural" order.
for (const MCSymbol &S : Asm.symbols()) {
const auto &WS = static_cast<const MCSymbolWasm &>(S);
if (WS.isTemporary() && WS.getName().empty())
continue;
if (WS.isComdat() && !WS.isDefined())
continue;
if (WS.isTemporary() && WS.isData() && !WS.getSize())
continue;
uint32_t Flags = 0;
if (WS.isWeak())
Flags |= wasm::WASM_SYMBOL_BINDING_WEAK;
if (WS.isHidden())
Flags |= wasm::WASM_SYMBOL_VISIBILITY_HIDDEN;
if (!WS.isExternal() && WS.isDefined())
Flags |= wasm::WASM_SYMBOL_BINDING_LOCAL;
if (WS.isUndefined())
Flags |= wasm::WASM_SYMBOL_UNDEFINED;
wasm::WasmSymbolInfo Info;
Info.Name = WS.getName();
Info.Kind = WS.getType();
Info.Flags = Flags;
if (!WS.isData())
Info.ElementIndex = WasmIndices.find(&WS)->second;
else if (WS.isDefined())
Info.DataRef = DataLocations.find(&WS)->second;
SymbolIndices[&WS] = SymbolInfos.size();
SymbolInfos.emplace_back(Info);
}
{
auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
// Functions referenced by a relocation need to put in the table. This is
// purely to make the object file's provisional values readable, and is
// ignored by the linker, which re-calculates the relocations itself.
if (Rel.Type != wasm::R_WEBASSEMBLY_TABLE_INDEX_I32 &&
Rel.Type != wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB)
return;
assert(Rel.Symbol->isFunction());
const MCSymbolWasm &WS = *ResolveSymbol(*Rel.Symbol);
uint32_t FunctionIndex = WasmIndices.find(&WS)->second;
uint32_t TableIndex = TableElems.size() + kInitialTableOffset;
if (TableIndices.try_emplace(&WS, TableIndex).second) {
DEBUG(dbgs() << " -> adding " << WS.getName()
<< " to table: " << TableIndex << "\n");
TableElems.push_back(FunctionIndex);
registerFunctionType(WS);
}
};
for (const WasmRelocationEntry &RelEntry : CodeRelocations)
HandleReloc(RelEntry);
for (const WasmRelocationEntry &RelEntry : DataRelocations)
HandleReloc(RelEntry);
}
// Translate .init_array section contents into start functions.
for (const MCSection &S : Asm) {
const auto &WS = static_cast<const MCSectionWasm &>(S);
if (WS.getSectionName().startswith(".fini_array"))
report_fatal_error(".fini_array sections are unsupported");
if (!WS.getSectionName().startswith(".init_array"))
continue;
if (WS.getFragmentList().empty())
continue;
if (WS.getFragmentList().size() != 2)
report_fatal_error("only one .init_array section fragment supported");
const MCFragment &AlignFrag = *WS.begin();
if (AlignFrag.getKind() != MCFragment::FT_Align)
report_fatal_error(".init_array section should be aligned");
if (cast<MCAlignFragment>(AlignFrag).getAlignment() != (is64Bit() ? 8 : 4))
report_fatal_error(".init_array section should be aligned for pointers");
const MCFragment &Frag = *std::next(WS.begin());
if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
report_fatal_error("only data supported in .init_array section");
uint16_t Priority = UINT16_MAX;
if (WS.getSectionName().size() != 11) {
if (WS.getSectionName()[11] != '.')
report_fatal_error(".init_array section priority should start with '.'");
if (WS.getSectionName().substr(12).getAsInteger(10, Priority))
report_fatal_error("invalid .init_array section priority");
}
const auto &DataFrag = cast<MCDataFragment>(Frag);
const SmallVectorImpl<char> &Contents = DataFrag.getContents();
for (const uint8_t *p = (const uint8_t *)Contents.data(),
*end = (const uint8_t *)Contents.data() + Contents.size();
p != end; ++p) {
if (*p != 0)
report_fatal_error("non-symbolic data in .init_array section");
}
for (const MCFixup &Fixup : DataFrag.getFixups()) {
assert(Fixup.getKind() == MCFixup::getKindForSize(is64Bit() ? 8 : 4, false));
const MCExpr *Expr = Fixup.getValue();
auto *Sym = dyn_cast<MCSymbolRefExpr>(Expr);
if (!Sym)
report_fatal_error("fixups in .init_array should be symbol references");
if (Sym->getKind() != MCSymbolRefExpr::VK_WebAssembly_FUNCTION)
report_fatal_error("symbols in .init_array should be for functions");
auto I = SymbolIndices.find(cast<MCSymbolWasm>(&Sym->getSymbol()));
if (I == SymbolIndices.end())
report_fatal_error("symbols in .init_array should be defined");
uint32_t Index = I->second;
InitFuncs.push_back(std::make_pair(Priority, Index));
}
}
// Write out the Wasm header.
writeHeader(Asm);
writeTypeSection(FunctionTypes);
writeImportSection(Imports, DataSize, TableElems.size());
writeFunctionSection(Functions);
// Skip the "table" section; we import the table instead.
// Skip the "memory" section; we import the memory instead.
writeGlobalSection();
writeExportSection(Exports);
writeElemSection(TableElems);
writeCodeSection(Asm, Layout, Functions);
writeDataSection();
writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);
writeCodeRelocSection();
writeDataRelocSection();
// TODO: Translate the .comment section to the output.
// TODO: Translate debug sections to the output.
}
std::unique_ptr<MCObjectWriter>
llvm::createWasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
raw_pwrite_stream &OS) {
return llvm::make_unique<WasmObjectWriter>(std::move(MOTW), OS);
}