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llvm-mirror/lib/MC/WasmObjectWriter.cpp
Nicholas Wilson b8ac3c4437 [WebAssembly] Reorder reloc sections to come between symtab and name 
This is required in order to enable relocs to be validated
as they are read in.

Also update tests with new section ordering.

Differential Revision: https://reviews.llvm.org/D43940

llvm-svn: 326694
2018-03-05 12:59:03 +00:00

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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()) {
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);
}
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