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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 "InputEvent.h"
#include "InputGlobal.h"
#include "SymbolTable.h"
#include "Symbols.h"
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
void lld::wasm::markLive() {
if (!Config->GcSections)
return;
LLVM_DEBUG(dbgs() << "markLive\n");
SmallVector<InputChunk *, 256> Q;
auto Enqueue = [&](Symbol *Sym) {
if (!Sym || Sym->isLive())
return;
LLVM_DEBUG(dbgs() << "markLive: " << Sym->getName() << "\n");
Sym->markLive();
if (InputChunk *Chunk = Sym->getChunk())
Q.push_back(Chunk);
};
// Add GC root symbols.
if (!Config->Entry.empty())
Enqueue(Symtab->find(Config->Entry));
Enqueue(WasmSym::CallCtors);
// We need to preserve any exported symbol
for (Symbol *Sym : Symtab->getSymbols())
if (Sym->isExported())
Enqueue(Sym);
// The ctor functions are all used in the synthetic __wasm_call_ctors
// function, but since this function is created in-place it doesn't contain
// relocations 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.Symbol));
}
// Follow relocations to mark all reachable chunks.
while (!Q.empty()) {
InputChunk *C = Q.pop_back_val();
for (const WasmRelocation Reloc : C->getRelocations()) {
if (Reloc.Type == R_WEBASSEMBLY_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_WEBASSEMBLY_TABLE_INDEX_SLEB ||
Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_I32) {
auto *FuncSym = cast<FunctionSymbol>(Sym);
if (FuncSym->hasTableIndex() && FuncSym->getTableIndex() == 0)
continue;
}
Enqueue(Sym);
}
}
// 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));
}
}
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