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llvm-mirror/lib/MC/WasmObjectWriter.cpp
Sam Clegg 98d7dfcbb9 [WebAssembly] Use a separate wasm data segment for each global symbol 
This is stepping stone towards honoring -fdata-sections
and letting the assembler decide how many wasm data
segments to create.

Differential Revision: https://reviews.llvm.org/D37834

llvm-svn: 313313
2017-09-14 23:07:53 +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/MCAsmInfo.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/MCObjectFileInfo.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 {
// 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;
uint32_t Offset;
SmallVector<char, 4> Data;
};
// A wasm import to be written into the import section.
struct WasmImport {
StringRef ModuleName;
StringRef FieldName;
unsigned Kind;
int32_t Type;
};
// A wasm function to be written into the function section.
struct WasmFunction {
int32_t Type;
const MCSymbolWasm *Sym;
};
// A wasm export to be written into the export section.
struct WasmExport {
StringRef FieldName;
unsigned Kind;
uint32_t Index;
};
// A wasm global to be written into the global section.
struct WasmGlobal {
wasm::ValType Type;
bool IsMutable;
bool HasImport;
uint64_t InitialValue;
uint32_t ImportIndex;
};
// 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;
}
#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 {
/// Helper struct for containing some precomputed information on symbols.
struct WasmSymbolData {
const MCSymbolWasm *Symbol;
StringRef Name;
// Support lexicographic sorting.
bool operator<(const WasmSymbolData &RHS) const { return Name < RHS.Name; }
};
/// 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> IndirectSymbolIndices;
// Maps function/global symbols to the function/global index space.
DenseMap<const MCSymbolWasm *, uint32_t> SymbolIndices;
DenseMap<WasmFunctionType, int32_t, WasmFunctionTypeDenseMapInfo>
FunctionTypeIndices;
SmallVector<WasmFunctionType, 4> FunctionTypes;
SmallVector<WasmGlobal, 4> Globals;
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(MCWasmObjectTargetWriter *MOTW, raw_pwrite_stream &OS)
: MCObjectWriter(OS, /*IsLittleEndian=*/true), TargetObjectWriter(MOTW) {}
private:
~WasmObjectWriter() override;
void reset() override {
CodeRelocations.clear();
DataRelocations.clear();
TypeIndices.clear();
SymbolIndices.clear();
IndirectSymbolIndices.clear();
FunctionTypeIndices.clear();
FunctionTypes.clear();
Globals.clear();
MCObjectWriter::reset();
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) {
encodeSLEB128(int32_t(Ty), getStream());
}
void writeTypeSection(const SmallVector<WasmFunctionType, 4> &FunctionTypes);
void writeImportSection(const SmallVector<WasmImport, 4> &Imports);
void writeFunctionSection(const SmallVector<WasmFunction, 4> &Functions);
void writeTableSection(uint32_t NumElements);
void writeMemorySection(uint32_t DataSize);
void writeGlobalSection();
void writeExportSection(const SmallVector<WasmExport, 4> &Exports);
void writeElemSection(const SmallVector<uint32_t, 4> &TableElems);
void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
const SmallVector<WasmFunction, 4> &Functions);
void writeDataSection(const SmallVector<WasmDataSegment, 4> &Segments);
void writeNameSection(const SmallVector<WasmFunction, 4> &Functions,
const SmallVector<WasmImport, 4> &Imports,
uint32_t NumFuncImports);
void writeCodeRelocSection();
void writeDataRelocSection();
void writeLinkingMetaDataSection(uint32_t DataSize, uint32_t DataAlignment,
ArrayRef<StringRef> WeakSymbols,
bool HasStackPointer,
uint32_t StackPointerGlobal);
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() {}
// Return the padding size to write a 32-bit value into a 5-byte ULEB128.
static unsigned PaddingFor5ByteULEB128(uint32_t X) {
return X == 0 ? 4 : (4u - (31u - countLeadingZeros(X)) / 7u);
}
// Return the padding size to write a 32-bit value into a 5-byte SLEB128.
static unsigned PaddingFor5ByteSLEB128(int32_t X) {
return 5 - getSLEB128Size(X);
}
// 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");
encodeULEB128(SectionId, getStream());
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");
unsigned Padding = PaddingFor5ByteULEB128(Size);
// 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, Padding);
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();
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");
if (FixupSection.hasInstructions())
CodeRelocations.push_back(Rec);
else
DataRelocations.push_back(Rec);
}
// 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 Padding = PaddingFor5ByteULEB128(X);
unsigned SizeLen = encodeULEB128(X, Buffer, Padding);
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 Padding = PaddingFor5ByteSLEB128(X);
unsigned SizeLen = encodeSLEB128(X, Buffer, Padding);
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);
}
// Compute a value to write into the code at the location covered
// by RelEntry. This value isn't used by the static linker, since
// we have addends; it just serves to make the code more readable
// and to make standalone wasm modules directly usable.
uint32_t
WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
const MCSymbolWasm *Sym = RelEntry.Symbol;
// For undefined symbols, use a hopefully invalid value.
if (!Sym->isDefined(/*SetUsed=*/false))
return UINT32_MAX;
uint32_t GlobalIndex = SymbolIndices[Sym];
const WasmGlobal& Global = Globals[GlobalIndex - NumGlobalImports];
uint64_t Address = Global.InitialValue + RelEntry.Addend;
// Ignore overflow. LLVM allows address arithmetic to silently wrap.
uint32_t Value = Address;
return Value;
}
uint32_t WasmObjectWriter::getRelocationIndexValue(
const WasmRelocationEntry &RelEntry) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
if (!IndirectSymbolIndices.count(RelEntry.Symbol))
report_fatal_error("symbol not found table index space: " +
RelEntry.Symbol->getName());
return IndirectSymbolIndices[RelEntry.Symbol];
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
if (!SymbolIndices.count(RelEntry.Symbol))
report_fatal_error("symbol not found function/global index space: " +
RelEntry.Symbol->getName());
return SymbolIndices[RelEntry.Symbol];
case 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];
default:
llvm_unreachable("invalid relocation type");
}
}
// 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");
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB:
case wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB: {
uint32_t Index = getRelocationIndexValue(RelEntry);
WritePatchableSLEB(Stream, Index, Offset);
break;
}
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32: {
uint32_t Index = getRelocationIndexValue(RelEntry);
WriteI32(Stream, Index, Offset);
break;
}
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB: {
uint32_t Value = getProvisionalValue(RelEntry);
WritePatchableSLEB(Stream, Value, Offset);
break;
}
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB: {
uint32_t Value = getProvisionalValue(RelEntry);
WritePatchableLEB(Stream, Value, Offset);
break;
}
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32: {
uint32_t Value = getProvisionalValue(RelEntry);
WriteI32(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);
encodeULEB128(RelEntry.Type, Stream);
encodeULEB128(Offset, Stream);
encodeULEB128(Index, Stream);
if (RelEntry.hasAddend())
encodeSLEB128(RelEntry.Addend, Stream);
}
}
void WasmObjectWriter::writeTypeSection(
const SmallVector<WasmFunctionType, 4> &FunctionTypes) {
if (FunctionTypes.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_TYPE);
encodeULEB128(FunctionTypes.size(), getStream());
for (const WasmFunctionType &FuncTy : FunctionTypes) {
encodeSLEB128(wasm::WASM_TYPE_FUNC, getStream());
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(
const SmallVector<WasmImport, 4> &Imports) {
if (Imports.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_IMPORT);
encodeULEB128(Imports.size(), getStream());
for (const WasmImport &Import : Imports) {
writeString(Import.ModuleName);
writeString(Import.FieldName);
encodeULEB128(Import.Kind, getStream());
switch (Import.Kind) {
case wasm::WASM_EXTERNAL_FUNCTION:
encodeULEB128(Import.Type, getStream());
break;
case wasm::WASM_EXTERNAL_GLOBAL:
encodeSLEB128(int32_t(Import.Type), getStream());
encodeULEB128(0, getStream()); // mutability
break;
default:
llvm_unreachable("unsupported import kind");
}
}
endSection(Section);
}
void WasmObjectWriter::writeFunctionSection(
const SmallVector<WasmFunction, 4> &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::writeTableSection(uint32_t NumElements) {
// 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.
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_TABLE);
encodeULEB128(1, getStream()); // The number of tables.
// Fixed to 1 for now.
encodeSLEB128(wasm::WASM_TYPE_ANYFUNC, getStream()); // Type of table
encodeULEB128(0, getStream()); // flags
encodeULEB128(NumElements, getStream()); // initial
endSection(Section);
}
void WasmObjectWriter::writeMemorySection(uint32_t DataSize) {
// For now, always emit the memory section, 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.
SectionBookkeeping Section;
uint32_t NumPages = (DataSize + wasm::WasmPageSize - 1) / wasm::WasmPageSize;
startSection(Section, wasm::WASM_SEC_MEMORY);
encodeULEB128(1, getStream()); // number of memory spaces
encodeULEB128(0, getStream()); // flags
encodeULEB128(NumPages, getStream()); // initial
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(Global.Type);
write8(Global.IsMutable);
if (Global.HasImport) {
assert(Global.InitialValue == 0);
write8(wasm::WASM_OPCODE_GET_GLOBAL);
encodeULEB128(Global.ImportIndex, getStream());
} else {
assert(Global.ImportIndex == 0);
write8(wasm::WASM_OPCODE_I32_CONST);
encodeSLEB128(Global.InitialValue, getStream()); // offset
}
write8(wasm::WASM_OPCODE_END);
}
endSection(Section);
}
void WasmObjectWriter::writeExportSection(
const SmallVector<WasmExport, 4> &Exports) {
if (Exports.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_EXPORT);
encodeULEB128(Exports.size(), getStream());
for (const WasmExport &Export : Exports) {
writeString(Export.FieldName);
encodeSLEB128(Export.Kind, getStream());
encodeULEB128(Export.Index, getStream());
}
endSection(Section);
}
void WasmObjectWriter::writeElemSection(
const SmallVector<uint32_t, 4> &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(0, 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,
const SmallVector<WasmFunction, 4> &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(
const SmallVector<WasmDataSegment, 4> &Segments) {
if (Segments.empty())
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_DATA);
encodeULEB128(Segments.size(), getStream()); // count
for (const WasmDataSegment & Segment : Segments) {
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::writeNameSection(
const SmallVector<WasmFunction, 4> &Functions,
const SmallVector<WasmImport, 4> &Imports,
unsigned NumFuncImports) {
uint32_t TotalFunctions = NumFuncImports + Functions.size();
if (TotalFunctions == 0)
return;
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CUSTOM, "name");
SectionBookkeeping SubSection;
startSection(SubSection, wasm::WASM_NAMES_FUNCTION);
encodeULEB128(TotalFunctions, getStream());
uint32_t Index = 0;
for (const WasmImport &Import : Imports) {
if (Import.Kind == wasm::WASM_EXTERNAL_FUNCTION) {
encodeULEB128(Index, getStream());
writeString(Import.FieldName);
++Index;
}
}
for (const WasmFunction &Func : Functions) {
encodeULEB128(Index, getStream());
writeString(Func.Sym->getName());
++Index;
}
endSection(SubSection);
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(
uint32_t DataSize, uint32_t DataAlignment, ArrayRef<StringRef> WeakSymbols,
bool HasStackPointer, uint32_t StackPointerGlobal) {
SectionBookkeeping Section;
startSection(Section, wasm::WASM_SEC_CUSTOM, "linking");
SectionBookkeeping SubSection;
if (HasStackPointer) {
startSection(SubSection, wasm::WASM_STACK_POINTER);
encodeULEB128(StackPointerGlobal, getStream()); // id
endSection(SubSection);
}
if (WeakSymbols.size() != 0) {
startSection(SubSection, wasm::WASM_SYMBOL_INFO);
encodeULEB128(WeakSymbols.size(), getStream());
for (const StringRef Export: WeakSymbols) {
writeString(Export);
encodeULEB128(wasm::WASM_SYMBOL_FLAG_WEAK, getStream());
}
endSection(SubSection);
}
if (DataSize > 0) {
startSection(SubSection, wasm::WASM_DATA_SIZE);
encodeULEB128(DataSize, getStream());
endSection(SubSection);
startSection(SubSection, wasm::WASM_DATA_ALIGNMENT);
encodeULEB128(DataAlignment, 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;
if (Symbol.isVariable()) {
const MCExpr *Expr = Symbol.getVariableValue();
auto *Inner = cast<MCSymbolRefExpr>(Expr);
const auto *ResolvedSym = cast<MCSymbolWasm>(&Inner->getSymbol());
F.Returns = ResolvedSym->getReturns();
F.Params = ResolvedSym->getParams();
} else {
F.Returns = Symbol.getReturns();
F.Params = Symbol.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();
wasm::ValType PtrType = is64Bit() ? wasm::ValType::I64 : wasm::ValType::I32;
// Collect information from the available symbols.
SmallVector<WasmFunction, 4> Functions;
SmallVector<uint32_t, 4> TableElems;
SmallVector<WasmImport, 4> Imports;
SmallVector<WasmExport, 4> Exports;
SmallVector<StringRef, 4> WeakSymbols;
SmallPtrSet<const MCSymbolWasm *, 4> IsAddressTaken;
unsigned NumFuncImports = 0;
SmallVector<WasmDataSegment, 4> DataSegments;
uint32_t DataAlignment = 1;
uint32_t StackPointerGlobal = 0;
uint32_t DataSize = 0;
bool HasStackPointer = false;
// Populate the IsAddressTaken set.
for (const WasmRelocationEntry &RelEntry : CodeRelocations) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
IsAddressTaken.insert(RelEntry.Symbol);
break;
default:
break;
}
}
for (const WasmRelocationEntry &RelEntry : DataRelocations) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
IsAddressTaken.insert(RelEntry.Symbol);
break;
default:
break;
}
}
// Populate FunctionTypeIndices and Imports.
for (const MCSymbol &S : Asm.symbols()) {
const auto &WS = static_cast<const MCSymbolWasm &>(S);
if (WS.isTemporary())
continue;
if (WS.isFunction())
registerFunctionType(WS);
// If the symbol is not defined in this translation unit, import it.
if (!WS.isDefined(/*SetUsed=*/false) || WS.isVariable()) {
WasmImport Import;
Import.ModuleName = WS.getModuleName();
Import.FieldName = WS.getName();
if (WS.isFunction()) {
Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
Import.Type = getFunctionType(WS);
SymbolIndices[&WS] = NumFuncImports;
++NumFuncImports;
} else {
Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
Import.Type = int32_t(PtrType);
SymbolIndices[&WS] = NumGlobalImports;
++NumGlobalImports;
}
Imports.push_back(Import);
}
}
// In the special .global_variables section, we've encoded global
// variables used by the function. Translate them into the Globals
// list.
MCSectionWasm *GlobalVars = Ctx.getWasmSection(".global_variables", 0, 0);
if (!GlobalVars->getFragmentList().empty()) {
if (GlobalVars->getFragmentList().size() != 1)
report_fatal_error("only one .global_variables fragment supported");
const MCFragment &Frag = *GlobalVars->begin();
if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
report_fatal_error("only data supported in .global_variables");
const auto &DataFrag = cast<MCDataFragment>(Frag);
if (!DataFrag.getFixups().empty())
report_fatal_error("fixups not supported in .global_variables");
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; ) {
WasmGlobal G;
if (end - p < 3)
report_fatal_error("truncated global variable encoding");
G.Type = wasm::ValType(int8_t(*p++));
G.IsMutable = bool(*p++);
G.HasImport = bool(*p++);
if (G.HasImport) {
G.InitialValue = 0;
WasmImport Import;
Import.ModuleName = (const char *)p;
const uint8_t *nul = (const uint8_t *)memchr(p, '\0', end - p);
if (!nul)
report_fatal_error("global module name must be nul-terminated");
p = nul + 1;
nul = (const uint8_t *)memchr(p, '\0', end - p);
if (!nul)
report_fatal_error("global base name must be nul-terminated");
Import.FieldName = (const char *)p;
p = nul + 1;
Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
Import.Type = int32_t(G.Type);
G.ImportIndex = NumGlobalImports;
++NumGlobalImports;
Imports.push_back(Import);
} else {
unsigned n;
G.InitialValue = decodeSLEB128(p, &n);
G.ImportIndex = 0;
if ((ptrdiff_t)n > end - p)
report_fatal_error("global initial value must be valid SLEB128");
p += n;
}
Globals.push_back(G);
}
}
// In the special .stack_pointer section, we've encoded the stack pointer
// index.
MCSectionWasm *StackPtr = Ctx.getWasmSection(".stack_pointer", 0, 0);
if (!StackPtr->getFragmentList().empty()) {
if (StackPtr->getFragmentList().size() != 1)
report_fatal_error("only one .stack_pointer fragment supported");
const MCFragment &Frag = *StackPtr->begin();
if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
report_fatal_error("only data supported in .stack_pointer");
const auto &DataFrag = cast<MCDataFragment>(Frag);
if (!DataFrag.getFixups().empty())
report_fatal_error("fixups not supported in .stack_pointer");
const SmallVectorImpl<char> &Contents = DataFrag.getContents();
if (Contents.size() != 4)
report_fatal_error("only one entry supported in .stack_pointer");
HasStackPointer = true;
StackPointerGlobal = NumGlobalImports + *(const int32_t *)Contents.data();
}
// Handle regular defined and undefined 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()
<< " isVariable=" << WS.isVariable() << "\n");
if (WS.isWeak())
WeakSymbols.push_back(WS.getName());
if (WS.isVariable())
continue;
unsigned Index;
if (WS.isFunction()) {
if (WS.isDefined(/*SetUsed=*/false)) {
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. Take the next available index.
Index = NumFuncImports + Functions.size();
// Prepare the function.
WasmFunction Func;
Func.Type = getFunctionType(WS);
Func.Sym = &WS;
SymbolIndices[&WS] = Index;
Functions.push_back(Func);
} else {
// An import; the index was assigned above.
Index = SymbolIndices.find(&WS)->second;
}
DEBUG(dbgs() << " -> function index: " << Index << "\n");
// If needed, prepare the function to be called indirectly.
if (IsAddressTaken.count(&WS) != 0) {
IndirectSymbolIndices[&WS] = TableElems.size();
DEBUG(dbgs() << " -> adding to table: " << TableElems.size() << "\n");
TableElems.push_back(Index);
}
} else {
if (WS.isTemporary() && !WS.getSize())
continue;
if (!WS.isDefined(/*SetUsed=*/false))
continue;
if (WS.getOffset() != 0)
report_fatal_error("data sections must contain one variable each: " +
WS.getName());
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());
if (uint64_t(Size) != Layout.getSectionFileSize(&DataSection))
report_fatal_error("data sections must contain at most one variable");
DataAlignment = std::max(DataAlignment, DataSection.getAlignment());
DataSegments.emplace_back();
WasmDataSegment &Segment = DataSegments.back();
DataSize = alignTo(DataSize, DataSection.getAlignment());
Segment.Offset = DataSize;
Segment.Section = &DataSection;
// For each global, prepare a corresponding wasm global holding its
// address. For externals these will also be named exports.
Index = NumGlobalImports + Globals.size();
WasmGlobal Global;
Global.Type = PtrType;
Global.IsMutable = false;
Global.HasImport = false;
Global.InitialValue = DataSize;
Global.ImportIndex = 0;
SymbolIndices[&WS] = Index;
DEBUG(dbgs() << " -> global index: " << Index << "\n");
Globals.push_back(Global);
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(Segment.Data.size(), Align->getAlignment()),
Segment.Data.size() + Align->getMaxBytesToEmit());
Segment.Data.resize(Size, Value);
} else if (auto *Fill = dyn_cast<MCFillFragment>(&Frag)) {
Segment.Data.insert(Segment.Data.end(), Fill->getSize(), Fill->getValue());
} else {
const auto &DataFrag = cast<MCDataFragment>(Frag);
const SmallVectorImpl<char> &Contents = DataFrag.getContents();
Segment.Data.insert(Segment.Data.end(), Contents.begin(),
Contents.end());
}
}
DataSize += Segment.Data.size();
}
// If the symbol is visible outside this translation unit, export it.
if ((WS.isExternal() && WS.isDefined(/*SetUsed=*/false))) {
WasmExport Export;
Export.FieldName = WS.getName();
Export.Index = Index;
if (WS.isFunction())
Export.Kind = wasm::WASM_EXTERNAL_FUNCTION;
else
Export.Kind = wasm::WASM_EXTERNAL_GLOBAL;
DEBUG(dbgs() << " -> export " << Exports.size() << "\n");
Exports.push_back(Export);
}
}
// Handle weak aliases. 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(/*SetUsed=*/false));
const auto &WS = static_cast<const MCSymbolWasm &>(S);
// Find the target symbol of this weak alias and export that index
const MCExpr *Expr = WS.getVariableValue();
auto *Inner = cast<MCSymbolRefExpr>(Expr);
const auto *ResolvedSym = cast<MCSymbolWasm>(&Inner->getSymbol());
DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym << "'\n");
assert(SymbolIndices.count(ResolvedSym) > 0);
uint32_t Index = SymbolIndices.find(ResolvedSym)->second;
DEBUG(dbgs() << " -> index:" << Index << "\n");
WasmExport Export;
Export.FieldName = WS.getName();
Export.Index = Index;
if (WS.isFunction())
Export.Kind = wasm::WASM_EXTERNAL_FUNCTION;
else
Export.Kind = wasm::WASM_EXTERNAL_GLOBAL;
DEBUG(dbgs() << " -> export " << Exports.size() << "\n");
Exports.push_back(Export);
}
// Add types for indirect function calls.
for (const WasmRelocationEntry &Fixup : CodeRelocations) {
if (Fixup.Type != wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB)
continue;
registerFunctionType(*Fixup.Symbol);
}
// Write out the Wasm header.
writeHeader(Asm);
writeTypeSection(FunctionTypes);
writeImportSection(Imports);
writeFunctionSection(Functions);
writeTableSection(TableElems.size());
writeMemorySection(DataSize);
writeGlobalSection();
writeExportSection(Exports);
// TODO: Start Section
writeElemSection(TableElems);
writeCodeSection(Asm, Layout, Functions);
writeDataSection(DataSegments);
writeNameSection(Functions, Imports, NumFuncImports);
writeCodeRelocSection();
writeDataRelocSection();
writeLinkingMetaDataSection(DataSize, DataAlignment, WeakSymbols, HasStackPointer, StackPointerGlobal);
// TODO: Translate the .comment section to the output.
// TODO: Translate debug sections to the output.
}
MCObjectWriter *llvm::createWasmObjectWriter(MCWasmObjectTargetWriter *MOTW,
raw_pwrite_stream &OS) {
return new WasmObjectWriter(MOTW, OS);
}
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