forked from OSchip/llvm-project
108 lines
3.3 KiB
C++
108 lines
3.3 KiB
C++
//===- MarkLive.cpp -------------------------------------------------------===//
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//
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// The LLVM Linker
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements --gc-sections, which is a feature to remove unused
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// chunks from the output. Unused chunks are those that are not reachable from
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// known root symbols or chunks. This feature is implemented as a mark-sweep
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// garbage collector.
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//
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// Here's how it works. Each InputChunk has a "Live" bit. The bit is off by
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// default. Starting with the GC-roots, visit all reachable chunks and set their
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// Live bits. The Writer will then ignore chunks whose Live bits are off, so
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// that such chunk are not appear in the output.
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//
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//===----------------------------------------------------------------------===//
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#include "MarkLive.h"
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#include "Config.h"
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#include "InputChunks.h"
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#include "SymbolTable.h"
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#include "Symbols.h"
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#define DEBUG_TYPE "lld"
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using namespace llvm;
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using namespace llvm::wasm;
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using namespace lld;
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using namespace lld::wasm;
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void lld::wasm::markLive() {
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if (!Config->GcSections)
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return;
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DEBUG(dbgs() << "markLive\n");
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SmallVector<InputChunk *, 256> Q;
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auto Enqueue = [&](Symbol *Sym) {
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if (!Sym)
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return;
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InputChunk *Chunk = Sym->getChunk();
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if (!Chunk || Chunk->Live)
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return;
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Chunk->Live = true;
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Q.push_back(Chunk);
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};
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// Add GC root symbols.
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if (!Config->Entry.empty())
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Enqueue(Symtab->find(Config->Entry));
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Enqueue(Config->CtorSymbol);
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// By default we export all non-hidden, so they are gc roots too
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for (Symbol *Sym : Symtab->getSymbols())
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if (!Sym->isHidden())
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Enqueue(Sym);
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// The ctor fuctions are all used the synthetic __wasm_call_ctors function,
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// but since this function is created in-place it doesn't contain reloctations
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// which mean we have to manually mark the ctors.
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for (const ObjFile *Obj : Symtab->ObjectFiles) {
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const WasmLinkingData &L = Obj->getWasmObj()->linkingData();
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for (const WasmInitFunc &F : L.InitFunctions)
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Enqueue(Obj->getFunctionSymbol(F.FunctionIndex));
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}
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auto EnqueueSuccessors = [Enqueue](InputChunk &Chunk) {
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for (const WasmRelocation Reloc : Chunk.getRelocations()) {
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switch (Reloc.Type) {
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case R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
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case R_WEBASSEMBLY_TABLE_INDEX_I32:
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case R_WEBASSEMBLY_TABLE_INDEX_SLEB:
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Enqueue(Chunk.File->getFunctionSymbol(Reloc.Index));
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break;
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case R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
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case R_WEBASSEMBLY_MEMORY_ADDR_LEB:
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case R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
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case R_WEBASSEMBLY_MEMORY_ADDR_I32:
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Enqueue(Chunk.File->getGlobalSymbol(Reloc.Index));
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break;
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}
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}
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};
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while (!Q.empty())
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EnqueueSuccessors(*Q.pop_back_val());
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// Report garbage-collected sections.
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if (Config->PrintGcSections) {
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auto CheckChunk = [](const InputChunk *C) {
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if (!C->Live)
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message("removing unused section '" + C->getName() + "' in file '" +
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C->getFileName() + "'");
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};
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for (const ObjFile *Obj : Symtab->ObjectFiles) {
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for (InputChunk *C : Obj->Functions)
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CheckChunk(C);
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for (InputChunk *C : Obj->Segments)
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CheckChunk(C);
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}
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}
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}
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