[WebAssembly] Add support for --gc-sections
In this initial version we only GC symbols with `hidden` visibility since
other symbols we export to the embedder.
We could potentially modify this the future and only use symbols
explicitly passed via `--export` as GC roots.
This version of the code only does GC of data and code. GC for the
types section is coming soon.
Differential Revision: https://reviews.llvm.org/D42511
llvm-svn: 323842
diff --git a/lld/wasm/MarkLive.cpp b/lld/wasm/MarkLive.cpp
new file mode 100644
index 0000000..9bd2a83
--- /dev/null
+++ b/lld/wasm/MarkLive.cpp
@@ -0,0 +1,107 @@
+//===- 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(Config->CtorSymbol);
+
+ // 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);
+ }
+ }
+}