[WebAssembly] --export should fetch lazy symbols

[sbc100]

Summary

--export now implies --undefined.

This is really a requirement from emscripten but I think it
makes sense in general too.

[sbc100]

Second attempt at this change since I think it makes sense. Otherwise emscripten would need to pass --export=foo and --undefined=foo for each symbol which seems redundant.

[sbc100]

• update test

lld/trunk/test/wasm/archive-export.ll

+Test that --export will also fetch lazy symbols from archives
+
+RUN: llc -filetype=obj %S/Inputs/start.ll -o %t.o
+RUN: llc -filetype=obj %S/Inputs/archive1.ll -o %t.a1.o
+RUN: llc -filetype=obj %S/Inputs/archive2.ll -o %t.a2.o
+RUN: rm -f %t.a
+RUN: llvm-ar rcs %t.a %t.a1.o %t.a2.o
+RUN: wasm-ld --export=archive2_symbol -o %t.wasm %t.a %t.o
+RUN: obj2yaml %t.wasm | FileCheck %s
+RUN: wasm-ld -o %t.wasm %t.a %t.o
+RUN: obj2yaml %t.wasm | FileCheck %s -check-prefix=NOEXPORT
+
+CHECK:         Exports:
+CHECK-NEXT:       - Name:            memory
+CHECK-NEXT:         Kind:            MEMORY
+CHECK-NEXT:         Index:           0
+CHECK-NEXT:       - Name:            __heap_base
+CHECK-NEXT:         Kind:            GLOBAL
+CHECK-NEXT:         Index:           1
+CHECK-NEXT:       - Name:            __data_end
+CHECK-NEXT:         Kind:            GLOBAL
+CHECK-NEXT:         Index:           2
+CHECK-NEXT:       - Name:            foo
+CHECK-NEXT:         Kind:            FUNCTION
+CHECK-NEXT:         Index:           2
+CHECK-NEXT:       - Name:            bar
+CHECK-NEXT:         Kind:            FUNCTION
+CHECK-NEXT:         Index:           3
+CHECK-NEXT:       - Name:            archive2_symbol
+CHECK-NEXT:         Kind:            FUNCTION
+CHECK-NEXT:         Index:           4
+CHECK-NEXT:       - Name:            _start
+CHECK-NEXT:         Kind:            FUNCTION
+CHECK-NEXT:         Index:           1
+CHECK-NEXT:   - Type:            CODE
+
+NOEXPORT:         Exports:
+NOEXPORT-NEXT:       - Name:            memory
+NOEXPORT-NEXT:         Kind:            MEMORY
+NOEXPORT-NEXT:         Index:           0
+NOEXPORT-NEXT:       - Name:            __heap_base
+NOEXPORT-NEXT:         Kind:            GLOBAL
+NOEXPORT-NEXT:         Index:           1
+NOEXPORT-NEXT:       - Name:            __data_end
+NOEXPORT-NEXT:         Kind:            GLOBAL
+NOEXPORT-NEXT:         Index:           2
+NOEXPORT-NEXT:       - Name:            _start
+NOEXPORT-NEXT:         Kind:            FUNCTION
+NOEXPORT-NEXT:         Index:           1
+NOEXPORT-NEXT:   - Type:            CODE

lld/trunk/wasm/Driver.cpp

for (InputFile *F : Files)
     Symtab->addFile(F);
   if (errorCount())
     return;
 
   // Handle the `--undefined <sym>` options.
   for (auto *Arg : Args.filtered(OPT_undefined))
     handleUndefined(Arg->getValue());
 
+  // Handle the `--export <sym>` options
+  // This works like --undefined but also exports the symbol if its found
+  for (auto *Arg : Args.filtered(OPT_export)) {
+    Symbol *Sym = handleUndefined(Arg->getValue());
+    if (Sym && Sym->isDefined())
+      Sym->ForceExport = true;
+    else if (!Config->AllowUndefined)
+      error(Twine("symbol exported via --export not found: ") +
+            Arg->getValue());
+  }
+
   if (!Config->Relocatable) {
     // Add synthetic dummies for weak undefined functions.
     handleWeakUndefines();
 
     if (!Config->Entry.empty()) {
       EntrySym = handleUndefined(Config->Entry);
       if (!EntrySym)
         error("entry symbol not defined (pass --no-entry to supress): " +
               Config->Entry);
     }
 
     // Make sure we have resolved all symbols.
     if (!Config->AllowUndefined)
       Symtab->reportRemainingUndefines();
   }
 
   if (errorCount())
     return;
 
-  // Handle --export.
-  for (auto *Arg : Args.filtered(OPT_export)) {
-    StringRef Name = Arg->getValue();
-    Symbol *Sym = Symtab->find(Name);
-    if (Sym && Sym->isDefined())
-      Sym->ForceExport = true;
-    else if (!Config->AllowUndefined)
-      error("symbol exported via --export not found: " + Name);
-  }
-
   // Do link-time optimization if given files are LLVM bitcode files.
   // This compiles bitcode files into real object files.
   Symtab->addCombinedLTOObject();
   if (errorCount())
     return;
 
   if (EntrySym)
     EntrySym->setHidden(false);