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Writer.cpp
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//===- Writer.cpp ---------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "Config.h"
#include "InputChunks.h"
#include "InputElement.h"
#include "MapFile.h"
#include "OutputSections.h"
#include "OutputSegment.h"
#include "Relocations.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "WriterUtils.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/BinaryFormat/Wasm.h"
#include "llvm/BinaryFormat/WasmTraits.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/Parallel.h"
#include <cstdarg>
#include <map>
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
namespace lld {
namespace wasm {
static constexpr int stackAlignment = 16;
namespace {
// The writer writes a SymbolTable result to a file.
class Writer {
public:
void run();
private:
void openFile();
bool needsPassiveInitialization(const OutputSegment *segment);
bool hasPassiveInitializedSegments();
void createSyntheticInitFunctions();
void createInitMemoryFunction();
void createStartFunction();
void createApplyDataRelocationsFunction();
void createApplyGlobalRelocationsFunction();
void createCallCtorsFunction();
void createInitTLSFunction();
void createCommandExportWrappers();
void createCommandExportWrapper(uint32_t functionIndex, DefinedFunction *f);
void assignIndexes();
void populateSymtab();
void populateProducers();
void populateTargetFeatures();
void calculateInitFunctions();
void calculateImports();
void calculateExports();
void calculateCustomSections();
void calculateTypes();
void createOutputSegments();
OutputSegment *createOutputSegment(StringRef name);
void combineOutputSegments();
void layoutMemory();
void createHeader();
void addSection(OutputSection *sec);
void addSections();
void createCustomSections();
void createSyntheticSections();
void createSyntheticSectionsPostLayout();
void finalizeSections();
// Custom sections
void createRelocSections();
void writeHeader();
void writeSections();
uint64_t fileSize = 0;
std::vector<WasmInitEntry> initFunctions;
llvm::StringMap<std::vector<InputChunk *>> customSectionMapping;
// Stable storage for command export wrapper function name strings.
std::list<std::string> commandExportWrapperNames;
// Elements that are used to construct the final output
std::string header;
std::vector<OutputSection *> outputSections;
std::unique_ptr<FileOutputBuffer> buffer;
std::vector<OutputSegment *> segments;
llvm::SmallDenseMap<StringRef, OutputSegment *> segmentMap;
};
} // anonymous namespace
void Writer::calculateCustomSections() {
log("calculateCustomSections");
bool stripDebug = config->stripDebug || config->stripAll;
for (ObjFile *file : symtab->objectFiles) {
for (InputChunk *section : file->customSections) {
// Exclude COMDAT sections that are not selected for inclusion
if (section->discarded)
continue;
StringRef name = section->getName();
// These custom sections are known the linker and synthesized rather than
// blindly copied.
if (name == "linking" || name == "name" || name == "producers" ||
name == "target_features" || name.startswith("reloc."))
continue;
// These custom sections are generated by `clang -fembed-bitcode`.
// These are used by the rust toolchain to ship LTO data along with
// compiled object code, but they don't want this included in the linker
// output.
if (name == ".llvmbc" || name == ".llvmcmd")
continue;
// Strip debug section in that option was specified.
if (stripDebug && name.startswith(".debug_"))
continue;
// Otherwise include custom sections by default and concatenate their
// contents.
customSectionMapping[name].push_back(section);
}
}
}
void Writer::createCustomSections() {
log("createCustomSections");
for (auto &pair : customSectionMapping) {
StringRef name = pair.first();
LLVM_DEBUG(dbgs() << "createCustomSection: " << name << "\n");
OutputSection *sec = make<CustomSection>(std::string(name), pair.second);
if (config->relocatable || config->emitRelocs) {
auto *sym = make<OutputSectionSymbol>(sec);
out.linkingSec->addToSymtab(sym);
sec->sectionSym = sym;
}
addSection(sec);
}
}
// Create relocations sections in the final output.
// These are only created when relocatable output is requested.
void Writer::createRelocSections() {
log("createRelocSections");
// Don't use iterator here since we are adding to OutputSection
size_t origSize = outputSections.size();
for (size_t i = 0; i < origSize; i++) {
LLVM_DEBUG(dbgs() << "check section " << i << "\n");
OutputSection *sec = outputSections[i];
// Count the number of needed sections.
uint32_t count = sec->getNumRelocations();
if (!count)
continue;
StringRef name;
if (sec->type == WASM_SEC_DATA)
name = "reloc.DATA";
else if (sec->type == WASM_SEC_CODE)
name = "reloc.CODE";
else if (sec->type == WASM_SEC_CUSTOM)
name = saver.save("reloc." + sec->name);
else
llvm_unreachable(
"relocations only supported for code, data, or custom sections");
addSection(make<RelocSection>(name, sec));
}
}
void Writer::populateProducers() {
for (ObjFile *file : symtab->objectFiles) {
const WasmProducerInfo &info = file->getWasmObj()->getProducerInfo();
out.producersSec->addInfo(info);
}
}
void Writer::writeHeader() {
memcpy(buffer->getBufferStart(), header.data(), header.size());
}
void Writer::writeSections() {
uint8_t *buf = buffer->getBufferStart();
parallelForEach(outputSections, [buf](OutputSection *s) {
assert(s->isNeeded());
s->writeTo(buf);
});
}
static void setGlobalPtr(DefinedGlobal *g, uint64_t memoryPtr) {
g->global->setPointerValue(memoryPtr);
}
// Fix the memory layout of the output binary. This assigns memory offsets
// to each of the input data sections as well as the explicit stack region.
// The default memory layout is as follows, from low to high.
//
// - initialized data (starting at Config->globalBase)
// - BSS data (not currently implemented in llvm)
// - explicit stack (Config->ZStackSize)
// - heap start / unallocated
//
// The --stack-first option means that stack is placed before any static data.
// This can be useful since it means that stack overflow traps immediately
// rather than overwriting global data, but also increases code size since all
// static data loads and stores requires larger offsets.
void Writer::layoutMemory() {
uint64_t memoryPtr = 0;
auto placeStack = [&]() {
if (config->relocatable || config->isPic)
return;
memoryPtr = alignTo(memoryPtr, stackAlignment);
if (config->zStackSize != alignTo(config->zStackSize, stackAlignment))
error("stack size must be " + Twine(stackAlignment) + "-byte aligned");
log("mem: stack size = " + Twine(config->zStackSize));
log("mem: stack base = " + Twine(memoryPtr));
memoryPtr += config->zStackSize;
setGlobalPtr(cast<DefinedGlobal>(WasmSym::stackPointer), memoryPtr);
log("mem: stack top = " + Twine(memoryPtr));
};
if (config->stackFirst) {
placeStack();
} else {
memoryPtr = config->globalBase;
log("mem: global base = " + Twine(config->globalBase));
}
if (WasmSym::globalBase)
WasmSym::globalBase->setVA(memoryPtr);
uint64_t dataStart = memoryPtr;
// Arbitrarily set __dso_handle handle to point to the start of the data
// segments.
if (WasmSym::dsoHandle)
WasmSym::dsoHandle->setVA(dataStart);
out.dylinkSec->memAlign = 0;
for (OutputSegment *seg : segments) {
out.dylinkSec->memAlign = std::max(out.dylinkSec->memAlign, seg->alignment);
memoryPtr = alignTo(memoryPtr, 1ULL << seg->alignment);
seg->startVA = memoryPtr;
log(formatv("mem: {0,-15} offset={1,-8} size={2,-8} align={3}", seg->name,
memoryPtr, seg->size, seg->alignment));
if (!config->relocatable && seg->isTLS()) {
if (config->sharedMemory) {
auto *tlsSize = cast<DefinedGlobal>(WasmSym::tlsSize);
setGlobalPtr(tlsSize, seg->size);
auto *tlsAlign = cast<DefinedGlobal>(WasmSym::tlsAlign);
setGlobalPtr(tlsAlign, int64_t{1} << seg->alignment);
} else {
auto *tlsBase = cast<DefinedGlobal>(WasmSym::tlsBase);
setGlobalPtr(tlsBase, memoryPtr);
}
}
memoryPtr += seg->size;
}
// Make space for the memory initialization flag
if (config->sharedMemory && hasPassiveInitializedSegments()) {
memoryPtr = alignTo(memoryPtr, 4);
WasmSym::initMemoryFlag = symtab->addSyntheticDataSymbol(
"__wasm_init_memory_flag", WASM_SYMBOL_VISIBILITY_HIDDEN);
WasmSym::initMemoryFlag->markLive();
WasmSym::initMemoryFlag->setVA(memoryPtr);
log(formatv("mem: {0,-15} offset={1,-8} size={2,-8} align={3}",
"__wasm_init_memory_flag", memoryPtr, 4, 4));
memoryPtr += 4;
}
if (WasmSym::dataEnd)
WasmSym::dataEnd->setVA(memoryPtr);
uint64_t staticDataSize = memoryPtr - dataStart;
log("mem: static data = " + Twine(staticDataSize));
if (config->isPic)
out.dylinkSec->memSize = staticDataSize;
if (!config->stackFirst)
placeStack();
if (WasmSym::heapBase) {
// Set `__heap_base` to directly follow the end of the stack or global data.
// The fact that this comes last means that a malloc/brk implementation
// can grow the heap at runtime.
log("mem: heap base = " + Twine(memoryPtr));
WasmSym::heapBase->setVA(memoryPtr);
}
uint64_t maxMemorySetting = 1ULL
<< (config->is64.getValueOr(false) ? 48 : 32);
if (config->initialMemory != 0) {
if (config->initialMemory != alignTo(config->initialMemory, WasmPageSize))
error("initial memory must be " + Twine(WasmPageSize) + "-byte aligned");
if (memoryPtr > config->initialMemory)
error("initial memory too small, " + Twine(memoryPtr) + " bytes needed");
if (config->initialMemory > maxMemorySetting)
error("initial memory too large, cannot be greater than " +
Twine(maxMemorySetting));
memoryPtr = config->initialMemory;
}
out.memorySec->numMemoryPages =
alignTo(memoryPtr, WasmPageSize) / WasmPageSize;
log("mem: total pages = " + Twine(out.memorySec->numMemoryPages));
if (config->maxMemory != 0) {
if (config->maxMemory != alignTo(config->maxMemory, WasmPageSize))
error("maximum memory must be " + Twine(WasmPageSize) + "-byte aligned");
if (memoryPtr > config->maxMemory)
error("maximum memory too small, " + Twine(memoryPtr) + " bytes needed");
if (config->maxMemory > maxMemorySetting)
error("maximum memory too large, cannot be greater than " +
Twine(maxMemorySetting));
}
// Check max if explicitly supplied or required by shared memory
if (config->maxMemory != 0 || config->sharedMemory) {
uint64_t max = config->maxMemory;
if (max == 0) {
// If no maxMemory config was supplied but we are building with
// shared memory, we need to pick a sensible upper limit.
if (config->isPic)
max = maxMemorySetting;
else
max = alignTo(memoryPtr, WasmPageSize);
}
out.memorySec->maxMemoryPages = max / WasmPageSize;
log("mem: max pages = " + Twine(out.memorySec->maxMemoryPages));
}
}
void Writer::addSection(OutputSection *sec) {
if (!sec->isNeeded())
return;
log("addSection: " + toString(*sec));
sec->sectionIndex = outputSections.size();
outputSections.push_back(sec);
}
// If a section name is valid as a C identifier (which is rare because of
// the leading '.'), linkers are expected to define __start_<secname> and
// __stop_<secname> symbols. They are at beginning and end of the section,
// respectively. This is not requested by the ELF standard, but GNU ld and
// gold provide the feature, and used by many programs.
static void addStartStopSymbols(const OutputSegment *seg) {
StringRef name = seg->name;
if (!isValidCIdentifier(name))
return;
LLVM_DEBUG(dbgs() << "addStartStopSymbols: " << name << "\n");
uint64_t start = seg->startVA;
uint64_t stop = start + seg->size;
symtab->addOptionalDataSymbol(saver.save("__start_" + name), start);
symtab->addOptionalDataSymbol(saver.save("__stop_" + name), stop);
}
void Writer::addSections() {
addSection(out.dylinkSec);
addSection(out.typeSec);
addSection(out.importSec);
addSection(out.functionSec);
addSection(out.tableSec);
addSection(out.memorySec);
addSection(out.tagSec);
addSection(out.globalSec);
addSection(out.exportSec);
addSection(out.startSec);
addSection(out.elemSec);
addSection(out.dataCountSec);
addSection(make<CodeSection>(out.functionSec->inputFunctions));
addSection(make<DataSection>(segments));
createCustomSections();
addSection(out.linkingSec);
if (config->emitRelocs || config->relocatable) {
createRelocSections();
}
addSection(out.nameSec);
addSection(out.producersSec);
addSection(out.targetFeaturesSec);
}
void Writer::finalizeSections() {
for (OutputSection *s : outputSections) {
s->setOffset(fileSize);
s->finalizeContents();
fileSize += s->getSize();
}
}
void Writer::populateTargetFeatures() {
StringMap<std::string> used;
StringMap<std::string> required;
StringMap<std::string> disallowed;
SmallSet<std::string, 8> &allowed = out.targetFeaturesSec->features;
bool tlsUsed = false;
// Only infer used features if user did not specify features
bool inferFeatures = !config->features.hasValue();
if (!inferFeatures) {
auto &explicitFeatures = config->features.getValue();
allowed.insert(explicitFeatures.begin(), explicitFeatures.end());
if (!config->checkFeatures)
return;
}
// Find the sets of used, required, and disallowed features
for (ObjFile *file : symtab->objectFiles) {
StringRef fileName(file->getName());
for (auto &feature : file->getWasmObj()->getTargetFeatures()) {
switch (feature.Prefix) {
case WASM_FEATURE_PREFIX_USED:
used.insert({feature.Name, std::string(fileName)});
break;
case WASM_FEATURE_PREFIX_REQUIRED:
used.insert({feature.Name, std::string(fileName)});
required.insert({feature.Name, std::string(fileName)});
break;
case WASM_FEATURE_PREFIX_DISALLOWED:
disallowed.insert({feature.Name, std::string(fileName)});
break;
default:
error("Unrecognized feature policy prefix " +
std::to_string(feature.Prefix));
}
}
// Find TLS data segments
auto isTLS = [](InputChunk *segment) {
return segment->live && segment->isTLS();
};
tlsUsed = tlsUsed ||
std::any_of(file->segments.begin(), file->segments.end(), isTLS);
}
if (inferFeatures)
for (const auto &key : used.keys())
allowed.insert(std::string(key));
if (!config->checkFeatures)
return;
if (!config->relocatable && allowed.count("mutable-globals") == 0) {
for (const Symbol *sym : out.importSec->importedSymbols) {
if (auto *global = dyn_cast<GlobalSymbol>(sym)) {
if (global->getGlobalType()->Mutable) {
error(Twine("mutable global imported but 'mutable-globals' feature "
"not present in inputs: `") +
toString(*sym) + "`. Use --no-check-features to suppress.");
}
}
}
for (const Symbol *sym : out.exportSec->exportedSymbols) {
if (isa<GlobalSymbol>(sym)) {
error(Twine("mutable global exported but 'mutable-globals' feature "
"not present in inputs: `") +
toString(*sym) + "`. Use --no-check-features to suppress.");
}
}
}
if (config->sharedMemory) {
if (disallowed.count("shared-mem"))
error("--shared-memory is disallowed by " + disallowed["shared-mem"] +
" because it was not compiled with 'atomics' or 'bulk-memory' "
"features.");
for (auto feature : {"atomics", "bulk-memory"})
if (!allowed.count(feature))
error(StringRef("'") + feature +
"' feature must be used in order to use shared memory");
}
if (tlsUsed) {
for (auto feature : {"atomics", "bulk-memory"})
if (!allowed.count(feature))
error(StringRef("'") + feature +
"' feature must be used in order to use thread-local storage");
}
// Validate that used features are allowed in output
if (!inferFeatures) {
for (auto &feature : used.keys()) {
if (!allowed.count(std::string(feature)))
error(Twine("Target feature '") + feature + "' used by " +
used[feature] + " is not allowed.");
}
}
// Validate the required and disallowed constraints for each file
for (ObjFile *file : symtab->objectFiles) {
StringRef fileName(file->getName());
SmallSet<std::string, 8> objectFeatures;
for (auto &feature : file->getWasmObj()->getTargetFeatures()) {
if (feature.Prefix == WASM_FEATURE_PREFIX_DISALLOWED)
continue;
objectFeatures.insert(feature.Name);
if (disallowed.count(feature.Name))
error(Twine("Target feature '") + feature.Name + "' used in " +
fileName + " is disallowed by " + disallowed[feature.Name] +
". Use --no-check-features to suppress.");
}
for (auto &feature : required.keys()) {
if (!objectFeatures.count(std::string(feature)))
error(Twine("Missing target feature '") + feature + "' in " + fileName +
", required by " + required[feature] +
". Use --no-check-features to suppress.");
}
}
}
static bool shouldImport(Symbol *sym) {
if (!sym->isUndefined())
return false;
if (sym->isWeak() && !config->relocatable && !config->isPic)
return false;
if (!sym->isLive())
return false;
if (!sym->isUsedInRegularObj)
return false;
// We don't generate imports for data symbols. They however can be imported
// as GOT entries.
if (isa<DataSymbol>(sym))
return false;
if (config->isPic || config->relocatable || config->importUndefined)
return true;
if (config->allowUndefinedSymbols.count(sym->getName()) != 0)
return true;
if (auto *g = dyn_cast<UndefinedGlobal>(sym))
return g->importName.hasValue();
if (auto *f = dyn_cast<UndefinedFunction>(sym))
return f->importName.hasValue();
if (auto *t = dyn_cast<UndefinedTable>(sym))
return t->importName.hasValue();
return false;
}
void Writer::calculateImports() {
// Some inputs require that the indirect function table be assigned to table
// number 0, so if it is present and is an import, allocate it before any
// other tables.
if (WasmSym::indirectFunctionTable &&
shouldImport(WasmSym::indirectFunctionTable))
out.importSec->addImport(WasmSym::indirectFunctionTable);
for (Symbol *sym : symtab->getSymbols()) {
if (!shouldImport(sym))
continue;
if (sym == WasmSym::indirectFunctionTable)
continue;
LLVM_DEBUG(dbgs() << "import: " << sym->getName() << "\n");
out.importSec->addImport(sym);
}
}
void Writer::calculateExports() {
if (config->relocatable)
return;
if (!config->relocatable && !config->importMemory)
out.exportSec->exports.push_back(
WasmExport{"memory", WASM_EXTERNAL_MEMORY, 0});
unsigned globalIndex =
out.importSec->getNumImportedGlobals() + out.globalSec->numGlobals();
for (Symbol *sym : symtab->getSymbols()) {
if (!sym->isExported())
continue;
if (!sym->isLive())
continue;
StringRef name = sym->getName();
WasmExport export_;
if (auto *f = dyn_cast<DefinedFunction>(sym)) {
if (Optional<StringRef> exportName = f->function->getExportName()) {
name = *exportName;
}
export_ = {name, WASM_EXTERNAL_FUNCTION, f->getFunctionIndex()};
} else if (auto *g = dyn_cast<DefinedGlobal>(sym)) {
if (g->getGlobalType()->Mutable && !g->getFile() && !g->forceExport) {
// Avoid exporting mutable globals are linker synthesized (e.g.
// __stack_pointer or __tls_base) unless they are explicitly exported
// from the command line.
// Without this check `--export-all` would cause any program using the
// stack pointer to export a mutable global even if none of the input
// files were built with the `mutable-globals` feature.
continue;
}
export_ = {name, WASM_EXTERNAL_GLOBAL, g->getGlobalIndex()};
} else if (auto *t = dyn_cast<DefinedTag>(sym)) {
export_ = {name, WASM_EXTERNAL_TAG, t->getTagIndex()};
} else if (auto *d = dyn_cast<DefinedData>(sym)) {
if (d->segment && d->segment->isTLS()) {
// We can't currenly export TLS data symbols.
if (sym->isExportedExplicit())
error("TLS symbols cannot yet be exported: `" + toString(*sym) + "`");
continue;
}
out.globalSec->dataAddressGlobals.push_back(d);
export_ = {name, WASM_EXTERNAL_GLOBAL, globalIndex++};
} else {
auto *t = cast<DefinedTable>(sym);
export_ = {name, WASM_EXTERNAL_TABLE, t->getTableNumber()};
}
LLVM_DEBUG(dbgs() << "Export: " << name << "\n");
out.exportSec->exports.push_back(export_);
out.exportSec->exportedSymbols.push_back(sym);
}
}
void Writer::populateSymtab() {
if (!config->relocatable && !config->emitRelocs)
return;
for (Symbol *sym : symtab->getSymbols())
if (sym->isUsedInRegularObj && sym->isLive())
out.linkingSec->addToSymtab(sym);
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "Local symtab entries: " << file->getName() << "\n");
for (Symbol *sym : file->getSymbols())
if (sym->isLocal() && !isa<SectionSymbol>(sym) && sym->isLive())
out.linkingSec->addToSymtab(sym);
}
}
void Writer::calculateTypes() {
// The output type section is the union of the following sets:
// 1. Any signature used in the TYPE relocation
// 2. The signatures of all imported functions
// 3. The signatures of all defined functions
// 4. The signatures of all imported tags
// 5. The signatures of all defined tags
for (ObjFile *file : symtab->objectFiles) {
ArrayRef<WasmSignature> types = file->getWasmObj()->types();
for (uint32_t i = 0; i < types.size(); i++)
if (file->typeIsUsed[i])
file->typeMap[i] = out.typeSec->registerType(types[i]);
}
for (const Symbol *sym : out.importSec->importedSymbols) {
if (auto *f = dyn_cast<FunctionSymbol>(sym))
out.typeSec->registerType(*f->signature);
else if (auto *t = dyn_cast<TagSymbol>(sym))
out.typeSec->registerType(*t->signature);
}
for (const InputFunction *f : out.functionSec->inputFunctions)
out.typeSec->registerType(f->signature);
for (const InputTag *t : out.tagSec->inputTags)
out.typeSec->registerType(t->signature);
}
// In a command-style link, create a wrapper for each exported symbol
// which calls the constructors and destructors.
void Writer::createCommandExportWrappers() {
// This logic doesn't currently support Emscripten-style PIC mode.
assert(!config->isPic);
// If there are no ctors and there's no libc `__wasm_call_dtors` to
// call, don't wrap the exports.
if (initFunctions.empty() && WasmSym::callDtors == NULL)
return;
std::vector<DefinedFunction *> toWrap;
for (Symbol *sym : symtab->getSymbols())
if (sym->isExported())
if (auto *f = dyn_cast<DefinedFunction>(sym))
toWrap.push_back(f);
for (auto *f : toWrap) {
auto funcNameStr = (f->getName() + ".command_export").str();
commandExportWrapperNames.push_back(funcNameStr);
const std::string &funcName = commandExportWrapperNames.back();
auto func = make<SyntheticFunction>(*f->getSignature(), funcName);
if (f->function->getExportName().hasValue())
func->setExportName(f->function->getExportName()->str());
else
func->setExportName(f->getName().str());
DefinedFunction *def =
symtab->addSyntheticFunction(funcName, f->flags, func);
def->markLive();
def->flags |= WASM_SYMBOL_EXPORTED;
def->flags &= ~WASM_SYMBOL_VISIBILITY_HIDDEN;
def->forceExport = f->forceExport;
f->flags |= WASM_SYMBOL_VISIBILITY_HIDDEN;
f->flags &= ~WASM_SYMBOL_EXPORTED;
f->forceExport = false;
out.functionSec->addFunction(func);
createCommandExportWrapper(f->getFunctionIndex(), def);
}
}
static void finalizeIndirectFunctionTable() {
if (!WasmSym::indirectFunctionTable)
return;
if (shouldImport(WasmSym::indirectFunctionTable) &&
!WasmSym::indirectFunctionTable->hasTableNumber()) {
// Processing -Bsymbolic relocations resulted in a late requirement that the
// indirect function table be present, and we are running in --import-table
// mode. Add the table now to the imports section. Otherwise it will be
// added to the tables section later in assignIndexes.
out.importSec->addImport(WasmSym::indirectFunctionTable);
}
uint32_t tableSize = config->tableBase + out.elemSec->numEntries();
WasmLimits limits = {0, tableSize, 0};
if (WasmSym::indirectFunctionTable->isDefined() && !config->growableTable) {
limits.Flags |= WASM_LIMITS_FLAG_HAS_MAX;
limits.Maximum = limits.Minimum;
}
WasmSym::indirectFunctionTable->setLimits(limits);
}
static void scanRelocations() {
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "scanRelocations: " << file->getName() << "\n");
for (InputChunk *chunk : file->functions)
scanRelocations(chunk);
for (InputChunk *chunk : file->segments)
scanRelocations(chunk);
for (auto &p : file->customSections)
scanRelocations(p);
}
}
void Writer::assignIndexes() {
// Seal the import section, since other index spaces such as function and
// global are effected by the number of imports.
out.importSec->seal();
for (InputFunction *func : symtab->syntheticFunctions)
out.functionSec->addFunction(func);
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "Functions: " << file->getName() << "\n");
for (InputFunction *func : file->functions)
out.functionSec->addFunction(func);
}
for (InputGlobal *global : symtab->syntheticGlobals)
out.globalSec->addGlobal(global);
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "Globals: " << file->getName() << "\n");
for (InputGlobal *global : file->globals)
out.globalSec->addGlobal(global);
}
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "Tags: " << file->getName() << "\n");
for (InputTag *tag : file->tags)
out.tagSec->addTag(tag);
}
for (ObjFile *file : symtab->objectFiles) {
LLVM_DEBUG(dbgs() << "Tables: " << file->getName() << "\n");
for (InputTable *table : file->tables)
out.tableSec->addTable(table);
}
for (InputTable *table : symtab->syntheticTables)
out.tableSec->addTable(table);
out.globalSec->assignIndexes();
out.tableSec->assignIndexes();
}
static StringRef getOutputDataSegmentName(const InputChunk &seg) {
// We always merge .tbss and .tdata into a single TLS segment so all TLS
// symbols are be relative to single __tls_base.
if (seg.isTLS())
return ".tdata";
StringRef name = seg.getName();
if (!config->mergeDataSegments)
return name;
if (name.startswith(".text."))
return ".text";
if (name.startswith(".data."))
return ".data";
if (name.startswith(".bss."))
return ".bss";
if (name.startswith(".rodata."))
return ".rodata";
return name;
}
OutputSegment *Writer::createOutputSegment(StringRef name) {
LLVM_DEBUG(dbgs() << "new segment: " << name << "\n");
OutputSegment *s = make<OutputSegment>(name);
if (config->sharedMemory)
s->initFlags = WASM_DATA_SEGMENT_IS_PASSIVE;
// Exported memories are guaranteed to be zero-initialized, so no need
// to emit data segments for bss sections.
// TODO: consider initializing bss sections with memory.fill
// instructions when memory is imported and bulk-memory is available.
if (!config->importMemory && !config->relocatable && name.startswith(".bss"))
s->isBss = true;
segments.push_back(s);
return s;
}
void Writer::createOutputSegments() {
for (ObjFile *file : symtab->objectFiles) {
for (InputChunk *segment : file->segments) {
if (!segment->live)
continue;
StringRef name = getOutputDataSegmentName(*segment);
OutputSegment *s = nullptr;
// When running in relocatable mode we can't merge segments that are part
// of comdat groups since the ultimate linker needs to be able exclude or
// include them individually.
if (config->relocatable && !segment->getComdatName().empty()) {
s = createOutputSegment(name);
} else {
if (segmentMap.count(name) == 0)
segmentMap[name] = createOutputSegment(name);
s = segmentMap[name];
}
s->addInputSegment(segment);
}
}
// Sort segments by type, placing .bss last
std::stable_sort(segments.begin(), segments.end(),
[](const OutputSegment *a, const OutputSegment *b) {
auto order = [](StringRef name) {
return StringSwitch<int>(name)
.StartsWith(".tdata", 0)
.StartsWith(".rodata", 1)
.StartsWith(".data", 2)
.StartsWith(".bss", 4)
.Default(3);
};
return order(a->name) < order(b->name);
});
for (size_t i = 0; i < segments.size(); ++i)
segments[i]->index = i;
// Merge MergeInputSections into a single MergeSyntheticSection.
LLVM_DEBUG(dbgs() << "-- finalize input semgments\n");
for (OutputSegment *seg : segments)
seg->finalizeInputSegments();
}
void Writer::combineOutputSegments() {
// With PIC code we currently only support a single active data segment since
// we only have a single __memory_base to use as our base address. This pass
// combines all data segments into a single .data segment.
// This restructions can be relaxed once we have extended constant
// expressions available:
// https://github.com/WebAssembly/extended-const
assert(config->isPic && !config->sharedMemory);
if (segments.size() <= 1)
return;
OutputSegment *combined = make<OutputSegment>(".data");
combined->startVA = segments[0]->startVA;
for (OutputSegment *s : segments) {
bool first = true;
for (InputChunk *inSeg : s->inputSegments) {
if (first)
inSeg->alignment = std::max(inSeg->alignment, s->alignment);
first = false;
#ifndef NDEBUG
uint64_t oldVA = inSeg->getVA();
#endif
combined->addInputSegment(inSeg);
#ifndef NDEBUG
uint64_t newVA = inSeg->getVA();
LLVM_DEBUG(dbgs() << "added input segment. name=" << inSeg->getName()
<< " oldVA=" << oldVA << " newVA=" << newVA << "\n");
assert(oldVA == newVA);
#endif
}
}
segments = {combined};
}
static void createFunction(DefinedFunction *func, StringRef bodyContent) {
std::string functionBody;
{
raw_string_ostream os(functionBody);
writeUleb128(os, bodyContent.size(), "function size");
os << bodyContent;
}
ArrayRef<uint8_t> body = arrayRefFromStringRef(saver.save(functionBody));
cast<SyntheticFunction>(func->function)->setBody(body);
}
bool Writer::needsPassiveInitialization(const OutputSegment *segment) {
return segment->initFlags & WASM_DATA_SEGMENT_IS_PASSIVE &&
!segment->isTLS() && !segment->isBss;
}
bool Writer::hasPassiveInitializedSegments() {
return std::find_if(segments.begin(), segments.end(),
[this](const OutputSegment *s) {
return this->needsPassiveInitialization(s);
}) != segments.end();
}
void Writer::createSyntheticInitFunctions() {
if (config->relocatable)
return;
static WasmSignature nullSignature = {{}, {}};
// Passive segments are used to avoid memory being reinitialized on each
// thread's instantiation. These passive segments are initialized and
// dropped in __wasm_init_memory, which is registered as the start function
if (config->sharedMemory && hasPassiveInitializedSegments()) {
WasmSym::initMemory = symtab->addSyntheticFunction(
"__wasm_init_memory", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_init_memory"));
WasmSym::initMemory->markLive();
}
if (config->isPic) {
// For PIC code we create synthetic functions that apply relocations.
// These get called from __wasm_call_ctors before the user-level
// constructors.
WasmSym::applyDataRelocs = symtab->addSyntheticFunction(
"__wasm_apply_data_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
WasmSym::applyDataRelocs->markLive();
if (out.globalSec->needsRelocations()) {
WasmSym::applyGlobalRelocs = symtab->addSyntheticFunction(
"__wasm_apply_global_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_apply_global_relocs"));
WasmSym::applyGlobalRelocs->markLive();
}
}
if (WasmSym::applyGlobalRelocs && WasmSym::initMemory) {
WasmSym::startFunction = symtab->addSyntheticFunction(
"__wasm_start", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_start"));
WasmSym::startFunction->markLive();
}
}
void Writer::createInitMemoryFunction() {
LLVM_DEBUG(dbgs() << "createInitMemoryFunction\n");
assert(WasmSym::initMemory);
assert(WasmSym::initMemoryFlag);
assert(hasPassiveInitializedSegments());
uint64_t flagAddress = WasmSym::initMemoryFlag->getVA();
bool is64 = config->is64.getValueOr(false);