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//===- MarkLive.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
//
//===----------------------------------------------------------------------===//
//
// This file implements --gc-sections, which is a feature to remove unused
// chunks from the output. Unused chunks are those that are not reachable from
// known root symbols or chunks. This feature is implemented as a mark-sweep
// garbage collector.
//
// Here's how it works. Each InputChunk has a "Live" bit. The bit is off by
// default. Starting with the GC-roots, visit all reachable chunks and set their
// Live bits. The Writer will then ignore chunks whose Live bits are off, so
// that such chunk are not appear in the output.
//
//===----------------------------------------------------------------------===//
#include "MarkLive.h"
#include "Config.h"
#include "InputChunks.h"
#include "InputEvent.h"
#include "InputGlobal.h"
#include "SymbolTable.h"
#include "Symbols.h"
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
namespace lld {
namespace wasm {
namespace {
class MarkLive {
public:
void run();
private:
void enqueue(Symbol *sym);
void markSymbol(Symbol *sym);
void mark();
// A list of chunks to visit.
SmallVector<InputChunk *, 256> queue;
};
} // namespace
void MarkLive::enqueue(Symbol *sym) {
if (!sym || sym->isLive())
return;
LLVM_DEBUG(dbgs() << "markLive: " << sym->getName() << "\n");
sym->markLive();
if (InputChunk *chunk = sym->getChunk())
queue.push_back(chunk);
// The ctor functions are all referenced by the synthetic callCtors
// function. However, this function does not contain relocations so we
// have to manually mark the ctors as live if callCtors itself is live.
if (sym == WasmSym::callCtors) {
if (config->isPic)
enqueue(WasmSym::applyRelocs);
for (const ObjFile *obj : symtab->objectFiles) {
const WasmLinkingData &l = obj->getWasmObj()->linkingData();
for (const WasmInitFunc &f : l.InitFunctions) {
auto* initSym = obj->getFunctionSymbol(f.Symbol);
if (!initSym->isDiscarded())
enqueue(initSym);
}
}
}
}
void MarkLive::run() {
// Add GC root symbols.
if (!config->entry.empty())
enqueue(symtab->find(config->entry));
// We need to preserve any no-strip or exported symbol
for (Symbol *sym : symtab->getSymbols())
if (sym->isNoStrip() || sym->isExported())
enqueue(sym);
// For relocatable output, we need to preserve all the ctor functions
if (config->relocatable) {
for (const ObjFile *obj : symtab->objectFiles) {
const WasmLinkingData &l = obj->getWasmObj()->linkingData();
for (const WasmInitFunc &f : l.InitFunctions)
enqueue(obj->getFunctionSymbol(f.Symbol));
}
}
if (config->isPic)
enqueue(WasmSym::callCtors);
if (config->sharedMemory && !config->shared)
enqueue(WasmSym::initMemory);
mark();
}
void MarkLive::mark() {
// Follow relocations to mark all reachable chunks.
while (!queue.empty()) {
InputChunk *c = queue.pop_back_val();
for (const WasmRelocation reloc : c->getRelocations()) {
if (reloc.Type == R_WASM_TYPE_INDEX_LEB)
continue;
Symbol *sym = c->file->getSymbol(reloc.Index);
// If the function has been assigned the special index zero in the table,
// the relocation doesn't pull in the function body, since the function
// won't actually go in the table (the runtime will trap attempts to call
// that index, since we don't use it). A function with a table index of
// zero is only reachable via "call", not via "call_indirect". The stub
// functions used for weak-undefined symbols have this behaviour (compare
// equal to null pointer, only reachable via direct call).
if (reloc.Type == R_WASM_TABLE_INDEX_SLEB ||
reloc.Type == R_WASM_TABLE_INDEX_I32) {
auto *funcSym = cast<FunctionSymbol>(sym);
if (funcSym->hasTableIndex() && funcSym->getTableIndex() == 0)
continue;
}
enqueue(sym);
}
}
}
void markLive() {
if (!config->gcSections)
return;
LLVM_DEBUG(dbgs() << "markLive\n");
MarkLive marker;
marker.run();
// Report garbage-collected sections.
if (config->printGcSections) {
for (const ObjFile *obj : symtab->objectFiles) {
for (InputChunk *c : obj->functions)
if (!c->live)
message("removing unused section " + toString(c));
for (InputChunk *c : obj->segments)
if (!c->live)
message("removing unused section " + toString(c));
for (InputGlobal *g : obj->globals)
if (!g->live)
message("removing unused section " + toString(g));
for (InputEvent *e : obj->events)
if (!e->live)
message("removing unused section " + toString(e));
}
for (InputChunk *c : symtab->syntheticFunctions)
if (!c->live)
message("removing unused section " + toString(c));
for (InputGlobal *g : symtab->syntheticGlobals)
if (!g->live)
message("removing unused section " + toString(g));
}
}
} // namespace wasm
} // namespace lld
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