forked from OSchip/llvm-project
121 lines
4.2 KiB
C++
121 lines
4.2 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 "InputEvent.h"
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#include "InputGlobal.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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void lld::wasm::markLive() {
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if (!Config->GcSections)
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return;
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LLVM_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 || Sym->isLive())
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return;
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LLVM_DEBUG(dbgs() << "markLive: " << Sym->getName() << "\n");
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Sym->markLive();
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if (InputChunk *Chunk = Sym->getChunk())
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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(WasmSym::CallCtors);
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// We need to preserve any exported symbol
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for (Symbol *Sym : Symtab->getSymbols())
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if (Sym->isExported())
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Enqueue(Sym);
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// The ctor functions are all used in the synthetic __wasm_call_ctors
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// function, but since this function is created in-place it doesn't contain
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// relocations 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.Symbol));
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}
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// Follow relocations to mark all reachable chunks.
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while (!Q.empty()) {
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InputChunk *C = Q.pop_back_val();
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for (const WasmRelocation Reloc : C->getRelocations()) {
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if (Reloc.Type == R_WEBASSEMBLY_TYPE_INDEX_LEB)
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continue;
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Symbol *Sym = C->File->getSymbol(Reloc.Index);
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// If the function has been assigned the special index zero in the table,
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// the relocation doesn't pull in the function body, since the function
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// won't actually go in the table (the runtime will trap attempts to call
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// that index, since we don't use it). A function with a table index of
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// zero is only reachable via "call", not via "call_indirect". The stub
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// functions used for weak-undefined symbols have this behaviour (compare
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// equal to null pointer, only reachable via direct call).
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if (Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_SLEB ||
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Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_I32) {
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FunctionSymbol *FuncSym = cast<FunctionSymbol>(Sym);
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if (FuncSym->hasTableIndex() && FuncSym->getTableIndex() == 0)
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continue;
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}
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Enqueue(Sym);
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}
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}
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// Report garbage-collected sections.
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if (Config->PrintGcSections) {
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for (const ObjFile *Obj : Symtab->ObjectFiles) {
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for (InputChunk *C : Obj->Functions)
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if (!C->Live)
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message("removing unused section " + toString(C));
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for (InputChunk *C : Obj->Segments)
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if (!C->Live)
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message("removing unused section " + toString(C));
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for (InputGlobal *G : Obj->Globals)
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if (!G->Live)
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message("removing unused section " + toString(G));
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for (InputEvent *E : Obj->Events)
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if (!E->Live)
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message("removing unused section " + toString(E));
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}
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for (InputChunk *C : Symtab->SyntheticFunctions)
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if (!C->Live)
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message("removing unused section " + toString(C));
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for (InputGlobal *G : Symtab->SyntheticGlobals)
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if (!G->Live)
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message("removing unused section " + toString(G));
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}
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}
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