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//===- MarkLive.cpp -------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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 "SymbolTable.h"
#include "Symbols.h"
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
using namespace lld;
using namespace lld::wasm;
void lld::wasm::markLive() {
if (!Config->GcSections)
return;
DEBUG(dbgs() << "markLive\n");
SmallVector<InputChunk *, 256> Q;
auto Enqueue = [&](Symbol *Sym) {
if (!Sym)
return;
InputChunk *Chunk = Sym->getChunk();
if (!Chunk || Chunk->Live)
return;
Chunk->Live = true;
Q.push_back(Chunk);
};
// Add GC root symbols.
if (!Config->Entry.empty())
Enqueue(Symtab->find(Config->Entry));
Enqueue(WasmSym::CallCtors);
// By default we export all non-hidden, so they are gc roots too
for (Symbol *Sym : Symtab->getSymbols())
if (!Sym->isHidden())
Enqueue(Sym);
// The ctor fuctions are all used the synthetic __wasm_call_ctors function,
// but since this function is created in-place it doesn't contain reloctations
// which mean we have to manually mark the ctors.
for (const ObjFile *Obj : Symtab->ObjectFiles) {
const WasmLinkingData &L = Obj->getWasmObj()->linkingData();
for (const WasmInitFunc &F : L.InitFunctions)
Enqueue(Obj->getFunctionSymbol(F.FunctionIndex));
}
auto EnqueueSuccessors = [Enqueue](InputChunk &Chunk) {
for (const WasmRelocation Reloc : Chunk.getRelocations()) {
switch (Reloc.Type) {
case R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
case R_WEBASSEMBLY_TABLE_INDEX_I32:
case R_WEBASSEMBLY_TABLE_INDEX_SLEB:
Enqueue(Chunk.File->getFunctionSymbol(Reloc.Index));
break;
case R_WEBASSEMBLY_GLOBAL_INDEX_LEB:
case R_WEBASSEMBLY_MEMORY_ADDR_LEB:
case R_WEBASSEMBLY_MEMORY_ADDR_SLEB:
case R_WEBASSEMBLY_MEMORY_ADDR_I32:
Enqueue(Chunk.File->getGlobalSymbol(Reloc.Index));
break;
}
}
};
while (!Q.empty())
EnqueueSuccessors(*Q.pop_back_val());
// Report garbage-collected sections.
if (Config->PrintGcSections) {
auto CheckChunk = [](const InputChunk *C) {
if (!C->Live)
message("removing unused section '" + C->getName() + "' in file '" +
C->getFileName() + "'");
};
for (const ObjFile *Obj : Symtab->ObjectFiles) {
for (InputChunk *C : Obj->Functions)
CheckChunk(C);
for (InputChunk *C : Obj->Segments)
CheckChunk(C);
}
}
}
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