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llvm-project/lld/wasm/Writer.cpp
Nick Fitzgerald 6018930ef1
[lld][WebAssembly] Support for the custom-page-sizes WebAssembly proposal (#128942) 
This commit adds support for WebAssembly's custom-page-sizes proposal to
`wasm-ld`. An overview of the proposal can be found
[here](https://github.com/WebAssembly/custom-page-sizes/blob/main/proposals/custom-page-sizes/Overview.md).
In a sentence, it allows customizing a Wasm memory's page size, enabling
Wasm to target environments with less than 64KiB of memory (the default
Wasm page size) available for Wasm memories.

This commit contains the following:

* Adds a `--page-size=N` CLI flag to `wasm-ld` for configuring the
linked Wasm binary's linear memory's page size.

* When the page size is configured to a non-default value, then the
final Wasm binary will use the encodings defined in the
custom-page-sizes proposal to declare the linear memory's page size.

* Defines a `__wasm_first_page_end` symbol, whose address points to the
first page in the Wasm linear memory, a.k.a. is the Wasm memory's page
size. This allows writing code that is compatible with any page size,
and doesn't require re-compiling its object code. At the same time,
because it just lowers to a constant rather than a memory access or
something, it enables link-time optimization.

* Adds tests for these new features.

r? @sbc100 

cc @sunfishcode
2025-03-04 09:39:30 -08:00

1895 lines
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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/Arrays.h"
#include "lld/Common/CommonLinkerContext.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.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 "llvm/Support/RandomNumberGenerator.h"
#include "llvm/Support/SHA1.h"
#include "llvm/Support/xxhash.h"
#include <cstdarg>
#include <map>
#include <optional>
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
namespace lld::wasm {
static constexpr int stackAlignment = 16;
static constexpr int heapAlignment = 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 createApplyTLSRelocationsFunction();
void createApplyGlobalTLSRelocationsFunction();
void createCallCtorsFunction();
void createInitTLSFunction();
void createCommandExportWrappers();
void createCommandExportWrapper(uint32_t functionIndex, DefinedFunction *f);
void assignIndexes();
void populateSymtab();
void populateProducers();
void populateTargetFeatures();
// populateTargetFeatures happens early on so some checks are delayed
// until imports and exports are finalized. There are run unstead
// in checkImportExportTargetFeatures
void checkImportExportTargetFeatures();
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();
void writeBuildId();
uint64_t fileSize = 0;
std::vector<WasmInitEntry> initFunctions;
llvm::MapVector<StringRef, 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 = ctx.arg.stripDebug || ctx.arg.stripAll;
for (ObjFile *file : ctx.objectFiles) {
for (InputChunk *section : file->customSections) {
// Exclude COMDAT sections that are not selected for inclusion
if (section->discarded)
continue;
// Ignore empty custom sections. In particular objcopy/strip will
// sometimes replace stripped sections with empty custom sections to
// avoid section re-numbering.
if (section->getSize() == 0)
continue;
StringRef name = section->name;
// These custom sections are known the linker and synthesized rather than
// blindly copied.
if (name == "linking" || name == "name" || name == "producers" ||
name == "target_features" || name.starts_with("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.starts_with(".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 (ctx.arg.relocatable || ctx.arg.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 : ctx.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);
});
}
// Computes a hash value of Data using a given hash function.
// In order to utilize multiple cores, we first split data into 1MB
// chunks, compute a hash for each chunk, and then compute a hash value
// of the hash values.
static void
computeHash(llvm::MutableArrayRef<uint8_t> hashBuf,
llvm::ArrayRef<uint8_t> data,
std::function<void(uint8_t *dest, ArrayRef<uint8_t> arr)> hashFn) {
std::vector<ArrayRef<uint8_t>> chunks = split(data, 1024 * 1024);
std::vector<uint8_t> hashes(chunks.size() * hashBuf.size());
// Compute hash values.
parallelFor(0, chunks.size(), [&](size_t i) {
hashFn(hashes.data() + i * hashBuf.size(), chunks[i]);
});
// Write to the final output buffer.
hashFn(hashBuf.data(), hashes);
}
static void makeUUID(unsigned version, llvm::ArrayRef<uint8_t> fileHash,
llvm::MutableArrayRef<uint8_t> output) {
assert((version == 4 || version == 5) && "Unknown UUID version");
assert(output.size() == 16 && "Wrong size for UUID output");
if (version == 5) {
// Build a valid v5 UUID from a hardcoded (randomly-generated) namespace
// UUID, and the computed hash of the output.
std::array<uint8_t, 16> namespaceUUID{0xA1, 0xFA, 0x48, 0x2D, 0x0E, 0x22,
0x03, 0x8D, 0x33, 0x8B, 0x52, 0x1C,
0xD6, 0xD2, 0x12, 0xB2};
SHA1 sha;
sha.update(namespaceUUID);
sha.update(fileHash);
auto s = sha.final();
std::copy(s.data(), &s.data()[output.size()], output.data());
} else if (version == 4) {
if (auto ec = llvm::getRandomBytes(output.data(), output.size()))
error("entropy source failure: " + ec.message());
}
// Set the UUID version and variant fields.
// The version is the upper nibble of byte 6 (0b0101xxxx or 0b0100xxxx)
output[6] = (static_cast<uint8_t>(version) << 4) | (output[6] & 0xF);
// The variant is DCE 1.1/ISO 11578 (0b10xxxxxx)
output[8] &= 0xBF;
output[8] |= 0x80;
}
void Writer::writeBuildId() {
if (!out.buildIdSec->isNeeded())
return;
if (ctx.arg.buildId == BuildIdKind::Hexstring) {
out.buildIdSec->writeBuildId(ctx.arg.buildIdVector);
return;
}
// Compute a hash of all sections of the output file.
size_t hashSize = out.buildIdSec->hashSize;
std::vector<uint8_t> buildId(hashSize);
llvm::ArrayRef<uint8_t> buf{buffer->getBufferStart(), size_t(fileSize)};
switch (ctx.arg.buildId) {
case BuildIdKind::Fast: {
std::vector<uint8_t> fileHash(8);
computeHash(fileHash, buf, [](uint8_t *dest, ArrayRef<uint8_t> arr) {
support::endian::write64le(dest, xxh3_64bits(arr));
});
makeUUID(5, fileHash, buildId);
break;
}
case BuildIdKind::Sha1:
computeHash(buildId, buf, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
memcpy(dest, SHA1::hash(arr).data(), hashSize);
});
break;
case BuildIdKind::Uuid:
makeUUID(4, {}, buildId);
break;
default:
llvm_unreachable("unknown BuildIdKind");
}
out.buildIdSec->writeBuildId(buildId);
}
static void setGlobalPtr(DefinedGlobal *g, uint64_t memoryPtr) {
LLVM_DEBUG(dbgs() << "setGlobalPtr " << g->getName() << " -> " << memoryPtr << "\n");
g->global->setPointerValue(memoryPtr);
}
static void checkPageAligned(StringRef name, uint64_t value) {
if (value != alignTo(value, ctx.arg.pageSize))
error(name + " must be aligned to the page size (" +
Twine(ctx.arg.pageSize) + " bytes)");
}
// 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 ctx.arg.globalBase)
// - BSS data (not currently implemented in llvm)
// - explicit stack (ctx.arg.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 (ctx.arg.relocatable || ctx.isPic)
return;
memoryPtr = alignTo(memoryPtr, stackAlignment);
if (WasmSym::stackLow)
WasmSym::stackLow->setVA(memoryPtr);
if (ctx.arg.zStackSize != alignTo(ctx.arg.zStackSize, stackAlignment))
error("stack size must be " + Twine(stackAlignment) + "-byte aligned");
log("mem: stack size = " + Twine(ctx.arg.zStackSize));
log("mem: stack base = " + Twine(memoryPtr));
memoryPtr += ctx.arg.zStackSize;
setGlobalPtr(cast<DefinedGlobal>(WasmSym::stackPointer), memoryPtr);
if (WasmSym::stackHigh)
WasmSym::stackHigh->setVA(memoryPtr);
log("mem: stack top = " + Twine(memoryPtr));
};
if (ctx.arg.stackFirst) {
placeStack();
if (ctx.arg.globalBase) {
if (ctx.arg.globalBase < memoryPtr) {
error("--global-base cannot be less than stack size when --stack-first is used");
return;
}
memoryPtr = ctx.arg.globalBase;
}
} else {
memoryPtr = ctx.arg.globalBase;
}
log("mem: global base = " + Twine(memoryPtr));
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 (!ctx.arg.relocatable && seg->isTLS()) {
if (WasmSym::tlsSize) {
auto *tlsSize = cast<DefinedGlobal>(WasmSym::tlsSize);
setGlobalPtr(tlsSize, seg->size);
}
if (WasmSym::tlsAlign) {
auto *tlsAlign = cast<DefinedGlobal>(WasmSym::tlsAlign);
setGlobalPtr(tlsAlign, int64_t{1} << seg->alignment);
}
if (!ctx.arg.sharedMemory && WasmSym::tlsBase) {
auto *tlsBase = cast<DefinedGlobal>(WasmSym::tlsBase);
setGlobalPtr(tlsBase, memoryPtr);
}
}
memoryPtr += seg->size;
}
// Make space for the memory initialization flag
if (ctx.arg.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 (ctx.isPic)
out.dylinkSec->memSize = staticDataSize;
if (!ctx.arg.stackFirst)
placeStack();
if (WasmSym::heapBase) {
// Set `__heap_base` to 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.
// We'll align the heap base here because memory allocators might expect
// __heap_base to be aligned already.
memoryPtr = alignTo(memoryPtr, heapAlignment);
log("mem: heap base = " + Twine(memoryPtr));
WasmSym::heapBase->setVA(memoryPtr);
}
uint64_t maxMemorySetting = 1ULL << 32;
if (ctx.arg.is64.value_or(false)) {
// TODO: Update once we decide on a reasonable limit here:
// https://github.com/WebAssembly/memory64/issues/33
maxMemorySetting = 1ULL << 34;
}
if (ctx.arg.initialHeap != 0) {
checkPageAligned("initial heap", ctx.arg.initialHeap);
uint64_t maxInitialHeap = maxMemorySetting - memoryPtr;
if (ctx.arg.initialHeap > maxInitialHeap)
error("initial heap too large, cannot be greater than " +
Twine(maxInitialHeap));
memoryPtr += ctx.arg.initialHeap;
}
if (ctx.arg.initialMemory != 0) {
checkPageAligned("initial memory", ctx.arg.initialMemory);
if (memoryPtr > ctx.arg.initialMemory)
error("initial memory too small, " + Twine(memoryPtr) + " bytes needed");
if (ctx.arg.initialMemory > maxMemorySetting)
error("initial memory too large, cannot be greater than " +
Twine(maxMemorySetting));
memoryPtr = ctx.arg.initialMemory;
}
memoryPtr = alignTo(memoryPtr, ctx.arg.pageSize);
out.memorySec->numMemoryPages = memoryPtr / ctx.arg.pageSize;
log("mem: total pages = " + Twine(out.memorySec->numMemoryPages));
if (WasmSym::heapEnd) {
// Set `__heap_end` to follow the end of the statically allocated linear
// memory. The fact that this comes last means that a malloc/brk
// implementation can grow the heap at runtime.
log("mem: heap end = " + Twine(memoryPtr));
WasmSym::heapEnd->setVA(memoryPtr);
}
uint64_t maxMemory = 0;
if (ctx.arg.maxMemory != 0) {
checkPageAligned("maximum memory", ctx.arg.maxMemory);
if (memoryPtr > ctx.arg.maxMemory)
error("maximum memory too small, " + Twine(memoryPtr) + " bytes needed");
if (ctx.arg.maxMemory > maxMemorySetting)
error("maximum memory too large, cannot be greater than " +
Twine(maxMemorySetting));
maxMemory = ctx.arg.maxMemory;
} else if (ctx.arg.noGrowableMemory) {
maxMemory = memoryPtr;
}
// If no maxMemory config was supplied but we are building with
// shared memory, we need to pick a sensible upper limit.
if (ctx.arg.sharedMemory && maxMemory == 0) {
if (ctx.isPic)
maxMemory = maxMemorySetting;
else
maxMemory = memoryPtr;
}
if (maxMemory != 0) {
out.memorySec->maxMemoryPages = maxMemory / ctx.arg.pageSize;
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 (ctx.arg.emitRelocs || ctx.arg.relocatable) {
createRelocSections();
}
addSection(out.nameSec);
addSection(out.producersSec);
addSection(out.targetFeaturesSec);
addSection(out.buildIdSec);
}
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> disallowed;
SmallSet<std::string, 8> &allowed = out.targetFeaturesSec->features;
bool tlsUsed = false;
if (ctx.isPic) {
// This should not be necessary because all PIC objects should
// contain the mutable-globals feature.
// TODO (https://github.com/llvm/llvm-project/issues/51681)
allowed.insert("mutable-globals");
}
if (ctx.arg.extraFeatures.has_value()) {
auto &extraFeatures = *ctx.arg.extraFeatures;
allowed.insert(extraFeatures.begin(), extraFeatures.end());
}
// Only infer used features if user did not specify features
bool inferFeatures = !ctx.arg.features.has_value();
if (!inferFeatures) {
auto &explicitFeatures = *ctx.arg.features;
allowed.insert(explicitFeatures.begin(), explicitFeatures.end());
if (!ctx.arg.checkFeatures)
goto done;
}
// Find the sets of used and disallowed features
for (ObjFile *file : ctx.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_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 || llvm::any_of(file->segments, isTLS);
}
if (inferFeatures)
for (const auto &key : used.keys())
allowed.insert(std::string(key));
if (!ctx.arg.checkFeatures)
goto done;
if (ctx.arg.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 (const auto &feature : used.keys()) {
if (!allowed.count(std::string(feature)))
error(Twine("Target feature '") + feature + "' used by " +
used[feature] + " is not allowed.");
}
}
// Validate the disallowed constraints for each file
for (ObjFile *file : ctx.objectFiles) {
StringRef fileName(file->getName());
SmallSet<std::string, 8> objectFeatures;
for (const 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.");
}
}
done:
// Normally we don't include bss segments in the binary. In particular if
// memory is not being imported then we can assume its zero initialized.
// In the case the memory is imported, and we can use the memory.fill
// instruction, then we can also avoid including the segments.
// Finally, if we are emitting relocations, they may refer to locations within
// the bss segments, so these segments need to exist in the binary.
if (ctx.arg.emitRelocs ||
(ctx.arg.memoryImport.has_value() && !allowed.count("bulk-memory")))
ctx.emitBssSegments = true;
if (allowed.count("extended-const"))
ctx.arg.extendedConst = true;
for (auto &feature : allowed)
log("Allowed feature: " + feature);
}
void Writer::checkImportExportTargetFeatures() {
if (ctx.arg.relocatable || !ctx.arg.checkFeatures)
return;
if (out.targetFeaturesSec->features.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.");
}
}
}
}
static bool shouldImport(Symbol *sym) {
// We don't generate imports for data symbols. They however can be imported
// as GOT entries.
if (isa<DataSymbol>(sym))
return false;
if (!sym->isLive())
return false;
if (!sym->isUsedInRegularObj)
return false;
// When a symbol is weakly defined in a shared library we need to allow
// it to be overridden by another module so need to both import
// and export the symbol.
if (ctx.arg.shared && sym->isWeak() && !sym->isUndefined() &&
!sym->isHidden())
return true;
if (sym->isShared())
return true;
if (!sym->isUndefined())
return false;
if (sym->isWeak() && !ctx.arg.relocatable && !ctx.isPic)
return false;
// In PIC mode we only need to import functions when they are called directly.
// Indirect usage all goes via GOT imports.
if (ctx.isPic) {
if (auto *f = dyn_cast<UndefinedFunction>(sym))
if (!f->isCalledDirectly)
return false;
}
if (ctx.isPic || ctx.arg.relocatable || ctx.arg.importUndefined ||
ctx.arg.unresolvedSymbols == UnresolvedPolicy::ImportDynamic)
return true;
if (ctx.arg.allowUndefinedSymbols.count(sym->getName()) != 0)
return true;
return sym->isImported();
}
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->symbols()) {
if (!shouldImport(sym))
continue;
if (sym == WasmSym::indirectFunctionTable)
continue;
LLVM_DEBUG(dbgs() << "import: " << sym->getName() << "\n");
out.importSec->addImport(sym);
}
}
void Writer::calculateExports() {
if (ctx.arg.relocatable)
return;
if (!ctx.arg.relocatable && ctx.arg.memoryExport.has_value()) {
out.exportSec->exports.push_back(
WasmExport{*ctx.arg.memoryExport, WASM_EXTERNAL_MEMORY, 0});
}
unsigned globalIndex =
out.importSec->getNumImportedGlobals() + out.globalSec->numGlobals();
for (Symbol *sym : symtab->symbols()) {
if (!sym->isExported())
continue;
if (!sym->isLive())
continue;
if (isa<SharedFunctionSymbol>(sym) || sym->isShared())
continue;
StringRef name = sym->getName();
LLVM_DEBUG(dbgs() << "Export: " << name << "\n");
WasmExport export_;
if (auto *f = dyn_cast<DefinedFunction>(sym)) {
if (std::optional<StringRef> exportName = f->function->getExportName()) {
name = *exportName;
}
export_ = {name, WASM_EXTERNAL_FUNCTION, f->getExportedFunctionIndex()};
} 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)) {
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()};
}
out.exportSec->exports.push_back(export_);
out.exportSec->exportedSymbols.push_back(sym);
}
}
void Writer::populateSymtab() {
if (!ctx.arg.relocatable && !ctx.arg.emitRelocs)
return;
for (Symbol *sym : symtab->symbols())
if (sym->isUsedInRegularObj && sym->isLive() && !sym->isShared())
out.linkingSec->addToSymtab(sym);
for (ObjFile *file : ctx.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 : ctx.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(!ctx.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 == nullptr)
return;
std::vector<DefinedFunction *> toWrap;
for (Symbol *sym : symtab->symbols())
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())
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 = ctx.arg.tableBase + out.elemSec->numEntries();
WasmLimits limits = {0, tableSize, 0, 0};
if (WasmSym::indirectFunctionTable->isDefined() && !ctx.arg.growableTable) {
limits.Flags |= WASM_LIMITS_FLAG_HAS_MAX;
limits.Maximum = limits.Minimum;
}
if (ctx.arg.is64.value_or(false))
limits.Flags |= WASM_LIMITS_FLAG_IS_64;
WasmSym::indirectFunctionTable->setLimits(limits);
}
static void scanRelocations() {
for (ObjFile *file : ctx.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 : ctx.syntheticFunctions)
out.functionSec->addFunction(func);
for (ObjFile *file : ctx.objectFiles) {
LLVM_DEBUG(dbgs() << "Functions: " << file->getName() << "\n");
for (InputFunction *func : file->functions)
out.functionSec->addFunction(func);
}
for (InputGlobal *global : ctx.syntheticGlobals)
out.globalSec->addGlobal(global);
for (ObjFile *file : ctx.objectFiles) {
LLVM_DEBUG(dbgs() << "Globals: " << file->getName() << "\n");
for (InputGlobal *global : file->globals)
out.globalSec->addGlobal(global);
}
for (ObjFile *file : ctx.objectFiles) {
LLVM_DEBUG(dbgs() << "Tags: " << file->getName() << "\n");
for (InputTag *tag : file->tags)
out.tagSec->addTag(tag);
}
for (ObjFile *file : ctx.objectFiles) {
LLVM_DEBUG(dbgs() << "Tables: " << file->getName() << "\n");
for (InputTable *table : file->tables)
out.tableSec->addTable(table);
}
for (InputTable *table : ctx.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";
if (!ctx.arg.mergeDataSegments)
return seg.name;
if (seg.name.starts_with(".text."))
return ".text";
if (seg.name.starts_with(".data."))
return ".data";
if (seg.name.starts_with(".bss."))
return ".bss";
if (seg.name.starts_with(".rodata."))
return ".rodata";
return seg.name;
}
OutputSegment *Writer::createOutputSegment(StringRef name) {
LLVM_DEBUG(dbgs() << "new segment: " << name << "\n");
OutputSegment *s = make<OutputSegment>(name);
if (ctx.arg.sharedMemory)
s->initFlags = WASM_DATA_SEGMENT_IS_PASSIVE;
if (!ctx.arg.relocatable && name.starts_with(".bss"))
s->isBss = true;
segments.push_back(s);
return s;
}
void Writer::createOutputSegments() {
for (ObjFile *file : ctx.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 (ctx.arg.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 restriction does not apply when the extended const extension is
// available: https://github.com/WebAssembly/extended-const
assert(!ctx.arg.extendedConst);
assert(ctx.isPic && !ctx.arg.sharedMemory);
if (segments.size() <= 1)
return;
OutputSegment *combined = make<OutputSegment>(".data");
combined->startVA = segments[0]->startVA;
std::vector<OutputSegment *> newSegments = {combined};
for (OutputSegment *s : segments) {
if (!s->requiredInBinary()) {
newSegments.push_back(s);
continue;
}
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->name
<< " oldVA=" << oldVA << " newVA=" << newVA << "\n");
assert(oldVA == newVA);
#endif
}
}
segments = newSegments;
}
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) {
// If bulk memory features is supported then we can perform bss initialization
// (via memory.fill) during `__wasm_init_memory`.
if (ctx.arg.memoryImport.has_value() && !segment->requiredInBinary())
return true;
return segment->initFlags & WASM_DATA_SEGMENT_IS_PASSIVE;
}
bool Writer::hasPassiveInitializedSegments() {
return llvm::any_of(segments, [this](const OutputSegment *s) {
return this->needsPassiveInitialization(s);
});
}
void Writer::createSyntheticInitFunctions() {
if (ctx.arg.relocatable)
return;
static WasmSignature nullSignature = {{}, {}};
createApplyDataRelocationsFunction();
// 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
// We also initialize bss segments (using memory.fill) as part of this
// function.
if (hasPassiveInitializedSegments()) {
WasmSym::initMemory = symtab->addSyntheticFunction(
"__wasm_init_memory", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_init_memory"));
WasmSym::initMemory->markLive();
if (ctx.arg.sharedMemory) {
// This global is assigned during __wasm_init_memory in the shared memory
// case.
WasmSym::tlsBase->markLive();
}
}
if (ctx.arg.sharedMemory) {
if (out.globalSec->needsTLSRelocations()) {
WasmSym::applyGlobalTLSRelocs = symtab->addSyntheticFunction(
"__wasm_apply_global_tls_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature,
"__wasm_apply_global_tls_relocs"));
WasmSym::applyGlobalTLSRelocs->markLive();
// TLS relocations depend on the __tls_base symbols
WasmSym::tlsBase->markLive();
}
auto hasTLSRelocs = [](const OutputSegment *segment) {
if (segment->isTLS())
for (const auto* is: segment->inputSegments)
if (is->getRelocations().size())
return true;
return false;
};
if (llvm::any_of(segments, hasTLSRelocs)) {
WasmSym::applyTLSRelocs = symtab->addSyntheticFunction(
"__wasm_apply_tls_relocs", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature,
"__wasm_apply_tls_relocs"));
WasmSym::applyTLSRelocs->markLive();
}
}
if (ctx.isPic && 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 there is only one start function we can just use that function
// itself as the Wasm start function, otherwise we need to synthesize
// a new function to call them in sequence.
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(hasPassiveInitializedSegments());
uint64_t flagAddress;
if (ctx.arg.sharedMemory) {
assert(WasmSym::initMemoryFlag);
flagAddress = WasmSym::initMemoryFlag->getVA();
}
bool is64 = ctx.arg.is64.value_or(false);
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
// Initialize memory in a thread-safe manner. The thread that successfully
// increments the flag from 0 to 1 is responsible for performing the memory
// initialization. Other threads go sleep on the flag until the first thread
// finishing initializing memory, increments the flag to 2, and wakes all
// the other threads. Once the flag has been set to 2, subsequently started
// threads will skip the sleep. All threads unconditionally drop their
// passive data segments once memory has been initialized. The generated
// code is as follows:
//
// (func $__wasm_init_memory
// (block $drop
// (block $wait
// (block $init
// (br_table $init $wait $drop
// (i32.atomic.rmw.cmpxchg align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 0)
// (i32.const 1)
// )
// )
// ) ;; $init
// ( ... initialize data segments ... )
// (i32.atomic.store align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 2)
// )
// (drop
// (i32.atomic.notify align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const -1u)
// )
// )
// (br $drop)
// ) ;; $wait
// (drop
// (i32.atomic.wait align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 1)
// (i32.const -1)
// )
// )
// ) ;; $drop
// ( ... drop data segments ... )
// )
//
// When we are building with PIC, calculate the flag location using:
//
// (global.get $__memory_base)
// (i32.const $__init_memory_flag)
// (i32.const 1)
auto writeGetFlagAddress = [&]() {
if (ctx.isPic) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, 0, "local 0");
} else {
writePtrConst(os, flagAddress, is64, "flag address");
}
};
if (ctx.arg.sharedMemory) {
// With PIC code we cache the flag address in local 0
if (ctx.isPic) {
writeUleb128(os, 1, "num local decls");
writeUleb128(os, 2, "local count");
writeU8(os, is64 ? WASM_TYPE_I64 : WASM_TYPE_I32, "address type");
writeU8(os, WASM_OPCODE_GLOBAL_GET, "GLOBAL_GET");
writeUleb128(os, WasmSym::memoryBase->getGlobalIndex(), "memory_base");
writePtrConst(os, flagAddress, is64, "flag address");
writeU8(os, is64 ? WASM_OPCODE_I64_ADD : WASM_OPCODE_I32_ADD, "add");
writeU8(os, WASM_OPCODE_LOCAL_SET, "local.set");
writeUleb128(os, 0, "local 0");
} else {
writeUleb128(os, 0, "num locals");
}
// Set up destination blocks
writeU8(os, WASM_OPCODE_BLOCK, "block $drop");
writeU8(os, WASM_TYPE_NORESULT, "block type");
writeU8(os, WASM_OPCODE_BLOCK, "block $wait");
writeU8(os, WASM_TYPE_NORESULT, "block type");
writeU8(os, WASM_OPCODE_BLOCK, "block $init");
writeU8(os, WASM_TYPE_NORESULT, "block type");
// Atomically check whether we win the race.
writeGetFlagAddress();
writeI32Const(os, 0, "expected flag value");
writeI32Const(os, 1, "new flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_RMW_CMPXCHG, "i32.atomic.rmw.cmpxchg");
writeMemArg(os, 2, 0);
// Based on the value, decide what to do next.
writeU8(os, WASM_OPCODE_BR_TABLE, "br_table");
writeUleb128(os, 2, "label vector length");
writeUleb128(os, 0, "label $init");
writeUleb128(os, 1, "label $wait");
writeUleb128(os, 2, "default label $drop");
// Initialize passive data segments
writeU8(os, WASM_OPCODE_END, "end $init");
} else {
writeUleb128(os, 0, "num local decls");
}
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s)) {
// For passive BSS segments we can simple issue a memory.fill(0).
// For non-BSS segments we do a memory.init. Both these
// instructions take as their first argument the destination
// address.
writePtrConst(os, s->startVA, is64, "destination address");
if (ctx.isPic) {
writeU8(os, WASM_OPCODE_GLOBAL_GET, "GLOBAL_GET");
writeUleb128(os, WasmSym::memoryBase->getGlobalIndex(),
"__memory_base");
writeU8(os, is64 ? WASM_OPCODE_I64_ADD : WASM_OPCODE_I32_ADD,
"i32.add");
}
// When we initialize the TLS segment we also set the `__tls_base`
// global. This allows the runtime to use this static copy of the
// TLS data for the first/main thread.
if (ctx.arg.sharedMemory && s->isTLS()) {
if (ctx.isPic) {
// Cache the result of the addionion in local 0
writeU8(os, WASM_OPCODE_LOCAL_TEE, "local.tee");
writeUleb128(os, 1, "local 1");
} else {
writePtrConst(os, s->startVA, is64, "destination address");
}
writeU8(os, WASM_OPCODE_GLOBAL_SET, "GLOBAL_SET");
writeUleb128(os, WasmSym::tlsBase->getGlobalIndex(),
"__tls_base");
if (ctx.isPic) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.tee");
writeUleb128(os, 1, "local 1");
}
}
if (s->isBss) {
writeI32Const(os, 0, "fill value");
writePtrConst(os, s->size, is64, "memory region size");
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_MEMORY_FILL, "memory.fill");
writeU8(os, 0, "memory index immediate");
} else {
writeI32Const(os, 0, "source segment offset");
writeI32Const(os, s->size, "memory region size");
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_MEMORY_INIT, "memory.init");
writeUleb128(os, s->index, "segment index immediate");
writeU8(os, 0, "memory index immediate");
}
}
}
if (ctx.arg.sharedMemory) {
// Set flag to 2 to mark end of initialization
writeGetFlagAddress();
writeI32Const(os, 2, "flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_STORE, "i32.atomic.store");
writeMemArg(os, 2, 0);
// Notify any waiters that memory initialization is complete
writeGetFlagAddress();
writeI32Const(os, -1, "number of waiters");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_ATOMIC_NOTIFY, "atomic.notify");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
// Branch to drop the segments
writeU8(os, WASM_OPCODE_BR, "br");
writeUleb128(os, 1, "label $drop");
// Wait for the winning thread to initialize memory
writeU8(os, WASM_OPCODE_END, "end $wait");
writeGetFlagAddress();
writeI32Const(os, 1, "expected flag value");
writeI64Const(os, -1, "timeout");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_WAIT, "i32.atomic.wait");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
// Unconditionally drop passive data segments
writeU8(os, WASM_OPCODE_END, "end $drop");
}
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s) && !s->isBss) {
// The TLS region should not be dropped since its is needed
// during the initialization of each thread (__wasm_init_tls).
if (ctx.arg.sharedMemory && s->isTLS())
continue;
// data.drop instruction
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_DATA_DROP, "data.drop");
writeUleb128(os, s->index, "segment index immediate");
}
}
// End the function
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::initMemory, bodyContent);
}
void Writer::createStartFunction() {
// If the start function exists when we have more than one function to call.
if (WasmSym::initMemory && WasmSym::applyGlobalRelocs) {
assert(WasmSym::startFunction);
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyGlobalRelocs->getFunctionIndex(),
"function index");
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::initMemory->getFunctionIndex(),
"function index");
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::startFunction, bodyContent);
} else if (WasmSym::initMemory) {
WasmSym::startFunction = WasmSym::initMemory;
} else if (WasmSym::applyGlobalRelocs) {
WasmSym::startFunction = WasmSym::applyGlobalRelocs;
}
}
// For -shared (PIC) output, we create create a synthetic function which will
// apply any relocations to the data segments on startup. This function is
// called `__wasm_apply_data_relocs` and is expected to be called before
// any user code (i.e. before `__wasm_call_ctors`).
void Writer::createApplyDataRelocationsFunction() {
LLVM_DEBUG(dbgs() << "createApplyDataRelocationsFunction\n");
// First write the body's contents to a string.
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
bool generated = false;
for (const OutputSegment *seg : segments)
if (!ctx.arg.sharedMemory || !seg->isTLS())
for (const InputChunk *inSeg : seg->inputSegments)
generated |= inSeg->generateRelocationCode(os);
if (!generated) {
LLVM_DEBUG(dbgs() << "skipping empty __wasm_apply_data_relocs\n");
return;
}
writeU8(os, WASM_OPCODE_END, "END");
}
// __wasm_apply_data_relocs
// Function that applies relocations to data segment post-instantiation.
static WasmSignature nullSignature = {{}, {}};
auto def = symtab->addSyntheticFunction(
"__wasm_apply_data_relocs",
WASM_SYMBOL_VISIBILITY_DEFAULT | WASM_SYMBOL_EXPORTED,
make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
def->markLive();
createFunction(def, bodyContent);
}
void Writer::createApplyTLSRelocationsFunction() {
LLVM_DEBUG(dbgs() << "createApplyTLSRelocationsFunction\n");
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
for (const OutputSegment *seg : segments)
if (seg->isTLS())
for (const InputChunk *inSeg : seg->inputSegments)
inSeg->generateRelocationCode(os);
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::applyTLSRelocs, bodyContent);
}
// Similar to createApplyDataRelocationsFunction but generates relocation code
// for WebAssembly globals. Because these globals are not shared between threads
// these relocation need to run on every thread.
void Writer::createApplyGlobalRelocationsFunction() {
// First write the body's contents to a string.
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
out.globalSec->generateRelocationCode(os, false);
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::applyGlobalRelocs, bodyContent);
}
// Similar to createApplyGlobalRelocationsFunction but for
// TLS symbols. This cannot be run during the start function
// but must be delayed until __wasm_init_tls is called.
void Writer::createApplyGlobalTLSRelocationsFunction() {
// First write the body's contents to a string.
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
out.globalSec->generateRelocationCode(os, true);
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::applyGlobalTLSRelocs, bodyContent);
}
// Create synthetic "__wasm_call_ctors" function based on ctor functions
// in input object.
void Writer::createCallCtorsFunction() {
// If __wasm_call_ctors isn't referenced, there aren't any ctors, don't
// define the `__wasm_call_ctors` function.
if (!WasmSym::callCtors->isLive() && initFunctions.empty())
return;
// First write the body's contents to a string.
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
// Call constructors
for (const WasmInitEntry &f : initFunctions) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, f.sym->getFunctionIndex(), "function index");
for (size_t i = 0; i < f.sym->signature->Returns.size(); i++) {
writeU8(os, WASM_OPCODE_DROP, "DROP");
}
}
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::callCtors, bodyContent);
}
// Create a wrapper around a function export which calls the
// static constructors and destructors.
void Writer::createCommandExportWrapper(uint32_t functionIndex,
DefinedFunction *f) {
// First write the body's contents to a string.
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
// Call `__wasm_call_ctors` which call static constructors (and
// applies any runtime relocations in Emscripten-style PIC mode)
if (WasmSym::callCtors->isLive()) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::callCtors->getFunctionIndex(),
"function index");
}
// Call the user's code, leaving any return values on the operand stack.
for (size_t i = 0; i < f->signature->Params.size(); ++i) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, i, "local index");
}
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, functionIndex, "function index");
// Call the function that calls the destructors.
if (DefinedFunction *callDtors = WasmSym::callDtors) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, callDtors->getFunctionIndex(), "function index");
}
// End the function, returning the return values from the user's code.
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(f, bodyContent);
}
void Writer::createInitTLSFunction() {
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
OutputSegment *tlsSeg = nullptr;
for (auto *seg : segments) {
if (seg->name == ".tdata") {
tlsSeg = seg;
break;
}
}
writeUleb128(os, 0, "num locals");
if (tlsSeg) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, 0, "local index");
writeU8(os, WASM_OPCODE_GLOBAL_SET, "global.set");
writeUleb128(os, WasmSym::tlsBase->getGlobalIndex(), "global index");
// FIXME(wvo): this local needs to be I64 in wasm64, or we need an extend op.
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, 0, "local index");
writeI32Const(os, 0, "segment offset");
writeI32Const(os, tlsSeg->size, "memory region size");
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_MEMORY_INIT, "MEMORY.INIT");
writeUleb128(os, tlsSeg->index, "segment index immediate");
writeU8(os, 0, "memory index immediate");
}
if (WasmSym::applyTLSRelocs) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyTLSRelocs->getFunctionIndex(),
"function index");
}
if (WasmSym::applyGlobalTLSRelocs) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyGlobalTLSRelocs->getFunctionIndex(),
"function index");
}
writeU8(os, WASM_OPCODE_END, "end function");
}
createFunction(WasmSym::initTLS, bodyContent);
}
// Populate InitFunctions vector with init functions from all input objects.
// This is then used either when creating the output linking section or to
// synthesize the "__wasm_call_ctors" function.
void Writer::calculateInitFunctions() {
if (!ctx.arg.relocatable && !WasmSym::callCtors->isLive())
return;
for (ObjFile *file : ctx.objectFiles) {
const WasmLinkingData &l = file->getWasmObj()->linkingData();
for (const WasmInitFunc &f : l.InitFunctions) {
FunctionSymbol *sym = file->getFunctionSymbol(f.Symbol);
// comdat exclusions can cause init functions be discarded.
if (sym->isDiscarded() || !sym->isLive())
continue;
if (sym->signature->Params.size() != 0)
error("constructor functions cannot take arguments: " + toString(*sym));
LLVM_DEBUG(dbgs() << "initFunctions: " << toString(*sym) << "\n");
initFunctions.emplace_back(WasmInitEntry{sym, f.Priority});
}
}
// Sort in order of priority (lowest first) so that they are called
// in the correct order.
llvm::stable_sort(initFunctions,
[](const WasmInitEntry &l, const WasmInitEntry &r) {
return l.priority < r.priority;
});
}
void Writer::createSyntheticSections() {
out.dylinkSec = make<DylinkSection>();
out.typeSec = make<TypeSection>();
out.importSec = make<ImportSection>();
out.functionSec = make<FunctionSection>();
out.tableSec = make<TableSection>();
out.memorySec = make<MemorySection>();
out.tagSec = make<TagSection>();
out.globalSec = make<GlobalSection>();
out.exportSec = make<ExportSection>();
out.startSec = make<StartSection>();
out.elemSec = make<ElemSection>();
out.producersSec = make<ProducersSection>();
out.targetFeaturesSec = make<TargetFeaturesSection>();
out.buildIdSec = make<BuildIdSection>();
}
void Writer::createSyntheticSectionsPostLayout() {
out.dataCountSec = make<DataCountSection>(segments);
out.linkingSec = make<LinkingSection>(initFunctions, segments);
out.nameSec = make<NameSection>(segments);
}
void Writer::run() {
// For PIC code the table base is assigned dynamically by the loader.
// For non-PIC, we start at 1 so that accessing table index 0 always traps.
if (!ctx.isPic && WasmSym::definedTableBase)
WasmSym::definedTableBase->setVA(ctx.arg.tableBase);
log("-- createOutputSegments");
createOutputSegments();
log("-- createSyntheticSections");
createSyntheticSections();
log("-- layoutMemory");
layoutMemory();
if (!ctx.arg.relocatable) {
// Create linker synthesized __start_SECNAME/__stop_SECNAME symbols
// This has to be done after memory layout is performed.
for (const OutputSegment *seg : segments) {
addStartStopSymbols(seg);
}
}
for (auto &pair : ctx.arg.exportedSymbols) {
Symbol *sym = symtab->find(pair.first());
if (sym && sym->isDefined())
sym->forceExport = true;
}
// Delay reporting errors about explicit exports until after
// addStartStopSymbols which can create optional symbols.
for (auto &name : ctx.arg.requiredExports) {
Symbol *sym = symtab->find(name);
if (!sym || !sym->isDefined()) {
if (ctx.arg.unresolvedSymbols == UnresolvedPolicy::ReportError)
error(Twine("symbol exported via --export not found: ") + name);
if (ctx.arg.unresolvedSymbols == UnresolvedPolicy::Warn)
warn(Twine("symbol exported via --export not found: ") + name);
}
}
log("-- populateTargetFeatures");
populateTargetFeatures();
// When outputting PIC code each segment lives at at fixes offset from the
// `__memory_base` import. Unless we support the extended const expression we
// can't do addition inside the constant expression, so we much combine the
// segments into a single one that can live at `__memory_base`.
if (ctx.isPic && !ctx.arg.extendedConst && !ctx.arg.sharedMemory) {
// In shared memory mode all data segments are passive and initialized
// via __wasm_init_memory.
log("-- combineOutputSegments");
combineOutputSegments();
}
log("-- createSyntheticSectionsPostLayout");
createSyntheticSectionsPostLayout();
log("-- populateProducers");
populateProducers();
log("-- calculateImports");
calculateImports();
log("-- scanRelocations");
scanRelocations();
log("-- finalizeIndirectFunctionTable");
finalizeIndirectFunctionTable();
log("-- createSyntheticInitFunctions");
createSyntheticInitFunctions();
log("-- assignIndexes");
assignIndexes();
log("-- calculateInitFunctions");
calculateInitFunctions();
if (!ctx.arg.relocatable) {
// Create linker synthesized functions
if (WasmSym::applyGlobalRelocs)
createApplyGlobalRelocationsFunction();
if (WasmSym::applyTLSRelocs)
createApplyTLSRelocationsFunction();
if (WasmSym::applyGlobalTLSRelocs)
createApplyGlobalTLSRelocationsFunction();
if (WasmSym::initMemory)
createInitMemoryFunction();
createStartFunction();
createCallCtorsFunction();
// Create export wrappers for commands if needed.
//
// If the input contains a call to `__wasm_call_ctors`, either in one of
// the input objects or an explicit export from the command-line, we
// assume ctors and dtors are taken care of already.
if (!ctx.arg.relocatable && !ctx.isPic &&
!WasmSym::callCtors->isUsedInRegularObj &&
!WasmSym::callCtors->isExported()) {
log("-- createCommandExportWrappers");
createCommandExportWrappers();
}
}
if (WasmSym::initTLS && WasmSym::initTLS->isLive()) {
log("-- createInitTLSFunction");
createInitTLSFunction();
}
if (errorCount())
return;
log("-- calculateTypes");
calculateTypes();
log("-- calculateExports");
calculateExports();
log("-- calculateCustomSections");
calculateCustomSections();
log("-- populateSymtab");
populateSymtab();
log("-- checkImportExportTargetFeatures");
checkImportExportTargetFeatures();
log("-- addSections");
addSections();
if (errorHandler().verbose) {
log("Defined Functions: " + Twine(out.functionSec->inputFunctions.size()));
log("Defined Globals : " + Twine(out.globalSec->numGlobals()));
log("Defined Tags : " + Twine(out.tagSec->inputTags.size()));
log("Defined Tables : " + Twine(out.tableSec->inputTables.size()));
log("Function Imports : " +
Twine(out.importSec->getNumImportedFunctions()));
log("Global Imports : " + Twine(out.importSec->getNumImportedGlobals()));
log("Tag Imports : " + Twine(out.importSec->getNumImportedTags()));
log("Table Imports : " + Twine(out.importSec->getNumImportedTables()));
}
createHeader();
log("-- finalizeSections");
finalizeSections();
log("-- writeMapFile");
writeMapFile(outputSections);
log("-- openFile");
openFile();
if (errorCount())
return;
writeHeader();
log("-- writeSections");
writeSections();
writeBuildId();
if (errorCount())
return;
if (Error e = buffer->commit())
fatal("failed to write output '" + buffer->getPath() +
"': " + toString(std::move(e)));
}
// Open a result file.
void Writer::openFile() {
log("writing: " + ctx.arg.outputFile);
Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
FileOutputBuffer::create(ctx.arg.outputFile, fileSize,
FileOutputBuffer::F_executable);
if (!bufferOrErr)
error("failed to open " + ctx.arg.outputFile + ": " +
toString(bufferOrErr.takeError()));
else
buffer = std::move(*bufferOrErr);
}
void Writer::createHeader() {
raw_string_ostream os(header);
writeBytes(os, WasmMagic, sizeof(WasmMagic), "wasm magic");
writeU32(os, WasmVersion, "wasm version");
fileSize += header.size();
}
void writeResult() { Writer().run(); }
} // namespace wasm::lld
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