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llvm-project/llvm/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/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 "llvm/Support/Wasm.h"
#include <vector>
using namespace llvm;
#undef DEBUG_TYPE
#define DEBUG_TYPE "reloc-info"
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;
};
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;
// Fixups for call_indirect type indices.
std::vector<WasmRelocationEntry> TypeIndexFixups;
// Index values to use for fixing up call_indirect type indices.
std::vector<uint32_t> TypeIndexFixupTypes;
// TargetObjectWriter wrappers.
bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
const MCFixup &Fixup, bool IsPCRel) const {
return TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);
}
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:
void reset() override {
MCObjectWriter::reset();
}
~WasmObjectWriter() override;
void writeHeader(const MCAssembler &Asm);
void writeValueType(wasm::ValType Ty) {
encodeSLEB128(int32_t(Ty), getStream());
}
void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
MCValue Target, bool &IsPCRel,
uint64_t &FixedValue) override;
void executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
void writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
};
} // 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");
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) {
encodeULEB128(strlen(Name), getStream());
writeBytes(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");
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,
bool &IsPCRel, uint64_t &FixedValue) {
MCSectionWasm &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;
bool ViaWeakRef = false;
if (SymA && SymA->isVariable()) {
const MCExpr *Expr = SymA->getVariableValue();
if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF) {
SymA = cast<MCSymbolWasm>(&Inner->getSymbol());
ViaWeakRef = true;
}
}
}
// 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) {
if (ViaWeakRef)
llvm_unreachable("weakref used in reloc not yet implemented");
else
SymA->setUsedInReloc();
}
if (RefA) {
if (RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX) {
assert(C == 0);
WasmRelocationEntry Rec(FixupOffset, SymA, C,
wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB,
&FixupSection);
TypeIndexFixups.push_back(Rec);
return;
}
}
unsigned Type = getRelocType(Ctx, Target, Fixup, IsPCRel);
WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
if (FixupSection.hasInstructions())
CodeRelocations.push_back(Rec);
else
DataRelocations.push_back(Rec);
}
namespace {
// 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 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;
};
} // end anonymous namespace
// 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.
static uint32_t ProvisionalValue(const WasmRelocationEntry &RelEntry) {
const MCSymbolWasm *Sym = RelEntry.Symbol;
// For undefined symbols, use a hopefully invalid value.
if (!Sym->isDefined(false))
return UINT32_MAX;
MCSectionWasm &Section =
cast<MCSectionWasm>(RelEntry.Symbol->getSection(false));
uint64_t Address = Section.getSectionOffset() + RelEntry.Addend;
// Ignore overflow. LLVM allows address arithmetic to silently wrap.
uint32_t Value = Address;
return Value;
}
// Apply the portions of the relocation records that we can handle ourselves
// directly.
static void ApplyRelocations(
ArrayRef<WasmRelocationEntry> Relocations,
raw_pwrite_stream &Stream,
DenseMap<const MCSymbolWasm *, uint32_t> &SymbolIndices,
uint64_t ContentsOffset)
{
for (const WasmRelocationEntry &RelEntry : Relocations) {
uint64_t Offset = ContentsOffset +
RelEntry.FixupSection->getSectionOffset() +
RelEntry.Offset;
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB: {
uint32_t Index = SymbolIndices[RelEntry.Symbol];
assert(RelEntry.Addend == 0);
WritePatchableLEB(Stream, Index, Offset);
break;
}
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB: {
uint32_t Index = SymbolIndices[RelEntry.Symbol];
assert(RelEntry.Addend == 0);
WritePatchableSLEB(Stream, Index, Offset);
break;
}
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_SLEB: {
uint32_t Value = ProvisionalValue(RelEntry);
WritePatchableSLEB(Stream, Value, Offset);
break;
}
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_LEB: {
uint32_t Value = ProvisionalValue(RelEntry);
WritePatchableLEB(Stream, Value, Offset);
break;
}
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32: {
uint32_t Index = SymbolIndices[RelEntry.Symbol];
assert(RelEntry.Addend == 0);
WriteI32(Stream, Index, Offset);
break;
}
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_I32: {
uint32_t Value = ProvisionalValue(RelEntry);
WriteI32(Stream, Value, Offset);
break;
}
default:
break;
}
}
}
// Write out the portions of the relocation records that the linker will
// need to handle.
static void
WriteRelocations(ArrayRef<WasmRelocationEntry> Relocations,
raw_pwrite_stream &Stream,
DenseMap<const MCSymbolWasm *, uint32_t> &SymbolIndices,
uint64_t HeaderSize) {
for (const WasmRelocationEntry RelEntry : Relocations) {
encodeULEB128(RelEntry.Type, Stream);
uint64_t Offset = RelEntry.Offset +
RelEntry.FixupSection->getSectionOffset() + HeaderSize;
uint32_t Index = SymbolIndices[RelEntry.Symbol];
int64_t Addend = RelEntry.Addend;
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
encodeULEB128(Offset, Stream);
encodeULEB128(Index, Stream);
assert(Addend == 0 && "addends not supported for functions");
break;
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_LEB:
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_SLEB:
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_I32:
encodeULEB128(Offset, Stream);
encodeULEB128(Index, Stream);
encodeSLEB128(Addend, Stream);
break;
default:
llvm_unreachable("unsupported relocation type");
}
}
}
// Write out the the type relocation records that the linker will
// need to handle.
static void WriteTypeRelocations(
ArrayRef<WasmRelocationEntry> TypeIndexFixups,
ArrayRef<uint32_t> TypeIndexFixupTypes,
raw_pwrite_stream &Stream)
{
for (size_t i = 0, e = TypeIndexFixups.size(); i < e; ++i) {
const WasmRelocationEntry &Fixup = TypeIndexFixups[i];
uint32_t Type = TypeIndexFixupTypes[i];
assert(Fixup.Type == wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB);
assert(Fixup.Addend == 0);
uint64_t Offset = Fixup.Offset +
Fixup.FixupSection->getSectionOffset();
encodeULEB128(Fixup.Type, Stream);
encodeULEB128(Offset, Stream);
encodeULEB128(Type, Stream);
}
}
void WasmObjectWriter::writeObject(MCAssembler &Asm,
const MCAsmLayout &Layout) {
MCContext &Ctx = Asm.getContext();
wasm::ValType PtrType = is64Bit() ? wasm::ValType::I64 : wasm::ValType::I32;
// Collect information from the available symbols.
DenseMap<WasmFunctionType, int32_t, WasmFunctionTypeDenseMapInfo>
FunctionTypeIndices;
SmallVector<WasmFunctionType, 4> FunctionTypes;
SmallVector<WasmFunction, 4> Functions;
SmallVector<uint32_t, 4> TableElems;
SmallVector<WasmGlobal, 4> Globals;
SmallVector<WasmImport, 4> Imports;
SmallVector<WasmExport, 4> Exports;
DenseMap<const MCSymbolWasm *, uint32_t> SymbolIndices;
SmallPtrSet<const MCSymbolWasm *, 4> IsAddressTaken;
unsigned NumFuncImports = 0;
unsigned NumGlobalImports = 0;
SmallVector<char, 0> DataBytes;
uint32_t StackPointerGlobal = 0;
bool HasStackPointer = false;
// Populate the IsAddressTaken set.
for (WasmRelocationEntry RelEntry : CodeRelocations) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_SLEB:
IsAddressTaken.insert(RelEntry.Symbol);
break;
default:
break;
}
}
for (WasmRelocationEntry RelEntry : DataRelocations) {
switch (RelEntry.Type) {
case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
case wasm::R_WEBASSEMBLY_GLOBAL_ADDR_I32:
IsAddressTaken.insert(RelEntry.Symbol);
break;
default:
break;
}
}
// Populate the Imports set.
for (const MCSymbol &S : Asm.symbols()) {
const auto &WS = static_cast<const MCSymbolWasm &>(S);
int32_t Type;
if (WS.isFunction()) {
// Prepare the function's type, if we haven't seen it yet.
WasmFunctionType F;
F.Returns = WS.getReturns();
F.Params = WS.getParams();
auto Pair =
FunctionTypeIndices.insert(std::make_pair(F, FunctionTypes.size()));
if (Pair.second)
FunctionTypes.push_back(F);
Type = Pair.first->second;
} else {
Type = int32_t(PtrType);
}
// If the symbol is not defined in this translation unit, import it.
if (!WS.isTemporary() && !WS.isDefined(/*SetUsed=*/false)) {
WasmImport Import;
Import.ModuleName = WS.getModuleName();
Import.FieldName = WS.getName();
if (WS.isFunction()) {
Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
Import.Type = Type;
SymbolIndices[&WS] = NumFuncImports;
++NumFuncImports;
} else {
Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
Import.Type = Type;
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 MCDataFragment &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 MCDataFragment &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 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);
unsigned Index;
if (WS.isFunction()) {
// Prepare the function's type, if we haven't seen it yet.
WasmFunctionType F;
F.Returns = WS.getReturns();
F.Params = WS.getParams();
auto Pair =
FunctionTypeIndices.insert(std::make_pair(F, FunctionTypes.size()));
if (Pair.second)
FunctionTypes.push_back(F);
int32_t Type = Pair.first->second;
if (WS.isDefined(/*SetUsed=*/false)) {
// A definition. Take the next available index.
Index = NumFuncImports + Functions.size();
// Prepare the function.
WasmFunction Func;
Func.Type = Type;
Func.Sym = &WS;
SymbolIndices[&WS] = Index;
Functions.push_back(Func);
} else {
// An import; the index was assigned above.
Index = SymbolIndices.find(&WS)->second;
}
// If needed, prepare the function to be called indirectly.
if (IsAddressTaken.count(&WS))
TableElems.push_back(Index);
} else {
// For now, ignore temporary non-function symbols.
if (S.isTemporary())
continue;
if (WS.getOffset() != 0)
report_fatal_error("data sections must contain one variable each");
if (!WS.getSize())
report_fatal_error("data symbols must have a size set with .size");
int64_t Size = 0;
if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
report_fatal_error(".size expression must be evaluatable");
if (WS.isDefined(false)) {
MCSectionWasm &DataSection =
static_cast<MCSectionWasm &>(WS.getSection());
if (uint64_t(Size) != Layout.getSectionFileSize(&DataSection))
report_fatal_error("data sections must contain at most one variable");
DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
DataSection.setSectionOffset(DataBytes.size());
for (MCSection::iterator I = DataSection.begin(), E = DataSection.end();
I != E; ++I) {
const MCFragment &Frag = *I;
if (Frag.hasInstructions())
report_fatal_error("only data supported in data sections");
if (const MCAlignFragment *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 (const MCFillFragment *Fill =
dyn_cast<MCFillFragment>(&Frag)) {
DataBytes.insert(DataBytes.end(), Size, Fill->getValue());
} else {
const MCDataFragment &DataFrag = cast<MCDataFragment>(Frag);
const SmallVectorImpl<char> &Contents = DataFrag.getContents();
DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
}
}
// For each external global, prepare a corresponding wasm global
// holding its address.
if (WS.isExternal()) {
Index = NumGlobalImports + Globals.size();
WasmGlobal Global;
Global.Type = PtrType;
Global.IsMutable = false;
Global.HasImport = false;
Global.InitialValue = DataSection.getSectionOffset();
Global.ImportIndex = 0;
SymbolIndices[&WS] = Index;
Globals.push_back(Global);
}
}
}
// If the symbol is visible outside this translation unit, export it.
if (WS.isExternal()) {
assert(WS.isDefined(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;
Exports.push_back(Export);
}
}
// Add types for indirect function calls.
for (const WasmRelocationEntry &Fixup : TypeIndexFixups) {
assert(Fixup.Addend == 0);
assert(Fixup.Type == wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB);
WasmFunctionType F;
F.Returns = Fixup.Symbol->getReturns();
F.Params = Fixup.Symbol->getParams();
auto Pair =
FunctionTypeIndices.insert(std::make_pair(F, FunctionTypes.size()));
if (Pair.second)
FunctionTypes.push_back(F);
TypeIndexFixupTypes.push_back(Pair.first->second);
}
// Write out the Wasm header.
writeHeader(Asm);
SectionBookkeeping Section;
// === Type Section =========================================================
if (!FunctionTypes.empty()) {
startSection(Section, wasm::WASM_SEC_TYPE);
encodeULEB128(FunctionTypes.size(), getStream());
for (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);
}
// === Import Section ========================================================
if (!Imports.empty()) {
startSection(Section, wasm::WASM_SEC_IMPORT);
encodeULEB128(Imports.size(), getStream());
for (const WasmImport &Import : Imports) {
StringRef ModuleName = Import.ModuleName;
encodeULEB128(ModuleName.size(), getStream());
writeBytes(ModuleName);
StringRef FieldName = Import.FieldName;
encodeULEB128(FieldName.size(), getStream());
writeBytes(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);
}
// === Function Section ======================================================
if (!Functions.empty()) {
startSection(Section, wasm::WASM_SEC_FUNCTION);
encodeULEB128(Functions.size(), getStream());
for (const WasmFunction &Func : Functions)
encodeULEB128(Func.Type, getStream());
endSection(Section);
}
// === Table Section =========================================================
// 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.
startSection(Section, wasm::WASM_SEC_TABLE);
// The number of tables, fixed to 1 for now.
encodeULEB128(1, getStream());
encodeSLEB128(wasm::WASM_TYPE_ANYFUNC, getStream());
encodeULEB128(0, getStream()); // flags
encodeULEB128(TableElems.size(), getStream()); // initial
endSection(Section);
// === Memory Section ========================================================
// 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.
uint32_t NumPages =
(DataBytes.size() + 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);
// === Global Section ========================================================
if (!Globals.empty()) {
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);
}
// === Export Section ========================================================
if (!Exports.empty()) {
startSection(Section, wasm::WASM_SEC_EXPORT);
encodeULEB128(Exports.size(), getStream());
for (const WasmExport &Export : Exports) {
encodeULEB128(Export.FieldName.size(), getStream());
writeBytes(Export.FieldName);
encodeSLEB128(Export.Kind, getStream());
encodeULEB128(Export.Index, getStream());
}
endSection(Section);
}
#if 0 // TODO: Start Section
if (HaveStartFunction) {
// === Start Section =========================================================
startSection(Section, wasm::WASM_SEC_START);
encodeSLEB128(StartFunction, getStream());
endSection(Section);
}
#endif
// === Elem Section ==========================================================
if (!TableElems.empty()) {
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);
}
// === Code Section ==========================================================
if (!Functions.empty()) {
startSection(Section, wasm::WASM_SEC_CODE);
encodeULEB128(Functions.size(), getStream());
for (const WasmFunction &Func : Functions) {
MCSectionWasm &FuncSection =
static_cast<MCSectionWasm &>(Func.Sym->getSection());
if (Func.Sym->isVariable())
report_fatal_error("weak symbols not supported yet");
if (Func.Sym->getOffset() != 0)
report_fatal_error("function sections must contain one function each");
if (!Func.Sym->getSize())
report_fatal_error("function symbols must have a size set with .size");
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 the type index fixups for call_indirect etc. instructions.
for (size_t i = 0, e = TypeIndexFixups.size(); i < e; ++i) {
uint32_t Type = TypeIndexFixupTypes[i];
unsigned Padding = PaddingFor5ByteULEB128(Type);
const WasmRelocationEntry &Fixup = TypeIndexFixups[i];
assert(Fixup.Addend == 0);
assert(Fixup.Type == wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB);
uint64_t Offset = Fixup.Offset +
Fixup.FixupSection->getSectionOffset();
uint8_t Buffer[16];
unsigned SizeLen = encodeULEB128(Type, Buffer, Padding);
assert(SizeLen == 5);
getStream().pwrite((char *)Buffer, SizeLen,
Section.ContentsOffset + Offset);
}
// Apply fixups.
ApplyRelocations(CodeRelocations, getStream(), SymbolIndices,
Section.ContentsOffset);
endSection(Section);
}
// === Data Section ==========================================================
uint32_t DataSectionHeaderSize = 0;
if (!DataBytes.empty()) {
startSection(Section, wasm::WASM_SEC_DATA);
encodeULEB128(1, getStream()); // count
encodeULEB128(0, getStream()); // memory index
write8(wasm::WASM_OPCODE_I32_CONST);
encodeSLEB128(0, getStream()); // offset
write8(wasm::WASM_OPCODE_END);
encodeULEB128(DataBytes.size(), getStream()); // size
DataSectionHeaderSize = getStream().tell() - Section.ContentsOffset;
writeBytes(DataBytes); // data
// Apply fixups.
ApplyRelocations(DataRelocations, getStream(), SymbolIndices,
Section.ContentsOffset + DataSectionHeaderSize);
endSection(Section);
}
// === Name Section ==========================================================
uint32_t TotalFunctions = NumFuncImports + Functions.size();
if (TotalFunctions != 0) {
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());
encodeULEB128(Import.FieldName.size(), getStream());
writeBytes(Import.FieldName);
++Index;
}
}
for (const WasmFunction &Func : Functions) {
encodeULEB128(Index, getStream());
encodeULEB128(Func.Sym->getName().size(), getStream());
writeBytes(Func.Sym->getName());
++Index;
}
endSection(SubSection);
endSection(Section);
}
// See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
// for descriptions of the reloc sections.
// === Code Reloc Section ====================================================
if (!CodeRelocations.empty()) {
startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.CODE");
encodeULEB128(wasm::WASM_SEC_CODE, getStream());
encodeULEB128(CodeRelocations.size() + TypeIndexFixups.size(), getStream());
WriteRelocations(CodeRelocations, getStream(), SymbolIndices, 0);
WriteTypeRelocations(TypeIndexFixups, TypeIndexFixupTypes, getStream());
endSection(Section);
}
// === Data Reloc Section ====================================================
if (!DataRelocations.empty()) {
startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.DATA");
encodeULEB128(wasm::WASM_SEC_DATA, getStream());
encodeULEB128(DataRelocations.size(), getStream());
WriteRelocations(DataRelocations, getStream(), SymbolIndices,
DataSectionHeaderSize);
endSection(Section);
}
// === Linking Metadata Section ==============================================
if (HasStackPointer) {
startSection(Section, wasm::WASM_SEC_CUSTOM, "linking");
encodeULEB128(1, getStream()); // count
encodeULEB128(wasm::WASM_STACK_POINTER, getStream()); // type
encodeULEB128(StackPointerGlobal, getStream()); // id
endSection(Section);
}
// 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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