[lld][WebAssembly] Don't GC library objects under `--whole-archive`

[sbc100]

Summary

Followup on https://reviews.llvm.org/D85062 which ignores
entire library objects when no symbols are used within them.
This is shouldn't apply with --whole-archive since this
is specified to treat them like direct object inputs.

[sbc100]

Looks like we raced to write this patch :)

[sunfish]

LGTM; thanks for fixing this!

[sbc100]

I think mind is slightly less intrusive.. and I like having the single test. So I'll land this then.

lld/test/wasm/ctor-gc.test

 ; Verify that constructors from a .o file which it initially depends on but
 ; doesn't ultimately contribute to the final link are not included.
 ;
 ; RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/ctor-ctor.s -o %t.ctor.o
 ; RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/ctor-lib.s -o %t.lib.o
 ; RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/ctor-start.s -o %t.start.o
 ; RUN: rm -f %t.lib.a
 ; RUN: llvm-ar rcs %t.lib.a %t.lib.o %t.ctor.o
 ; RUN: wasm-ld %t.start.o %t.lib.a -o %t.wasm
+; RUN: wasm-ld %t.start.o --whole-archive %t.lib.a -o %t2.wasm
 ; RUN: obj2yaml %t.wasm | FileCheck %s
+; RUN: obj2yaml %t2.wasm | FileCheck %s -check-prefix=WHOLEARCHIVE
 
 ; CHECK-NOT: __wasm_call_ctors
+
+; WHOLEARCHIVE: __wasm_call_ctors

lld/wasm/Driver.cpp

void LinkerDriver::addFile(StringRef path) {
   case file_magic::archive: {
     SmallString<128> importFile = path;
     path::replace_extension(importFile, ".imports");
     if (fs::exists(importFile))
       readImportFile(importFile.str());
 
     // Handle -whole-archive.
     if (inWholeArchive) {
-      for (MemoryBufferRef &m : getArchiveMembers(mbref))
-        files.push_back(createObjectFile(m, path));
+      for (MemoryBufferRef &m : getArchiveMembers(mbref)) {
+        auto object = createObjectFile(m, path);
+        // Mark object as live; object members are normally not
+        // live by default but -whole-archive is designed to treat
+        // them as such.
+        object->markLive();
+        files.push_back(object);
+      }
+
       return;
     }
 
     std::unique_ptr<Archive> file =
         CHECK(Archive::create(mbref), path + ": failed to parse archive");
 
     if (!file->isEmpty() && !file->hasSymbolTable()) {
       error(mbref.getBufferIdentifier() +

lld/wasm/InputFiles.cpp

Optional<MemoryBufferRef> readFile(StringRef path) {
   MemoryBufferRef mbref = mb->getMemBufferRef();
   make<std::unique_ptr<MemoryBuffer>>(std::move(mb)); // take MB ownership
 
   if (tar)
     tar->append(relativeToRoot(path), mbref.getBuffer());
   return mbref;
 }
 
-InputFile *createObjectFile(MemoryBufferRef mb,
-                                       StringRef archiveName) {
+InputFile *createObjectFile(MemoryBufferRef mb, StringRef archiveName) {
   file_magic magic = identify_magic(mb.getBuffer());
   if (magic == file_magic::wasm_object) {
     std::unique_ptr<Binary> bin =
         CHECK(createBinary(mb), mb.getBufferIdentifier());
     auto *obj = cast<WasmObjectFile>(bin.get());
     if (obj->isSharedObject())
       return make<SharedFile>(mb);
     return make<ObjFile>(mb, archiveName);