[lld][WebAssembly] Fix for weak references to data symbols in archives

[sbc100]

Summary

Fix a bug where were not handling relocations against weakly undefined
data symbol. Add a test for this case. Also ensure that the weak
references to data symbols are not pulled in from archive files by
default (but are if -u <name> is added to the command line).

Fixes: PR43696

[sbc100]

• comment

[MaskRay]

lld/test/wasm/archive-weak-undefined.ll

15–16

Add a comment saying an undefined weak will not fetch archive members

[ruiu]

LGTM

[sbc100]

• comment

lld/test/wasm/archive-weak-undefined.ll

+; Test that weak undefined symbols do not fetch members from archive files.
 ; RUN: llc -filetype=obj %s -o %t.o
-; RUN: llc -filetype=obj %S/Inputs/ret32.ll -o %t.a1.o
+; RUN: llc -filetype=obj %S/Inputs/ret32.ll -o %t.ret32.o
+; RUN: llc -filetype=obj %S/Inputs/hello.ll -o %t.hello.o
 ; RUN: rm -f %t.a
-; RUN: llvm-ar rcs %t.a %t.a1.o
+; RUN: llvm-ar rcs %t.a %t.ret32.o %t.hello.o
+
 ; RUN: wasm-ld %t.o %t.a -o %t.wasm
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
+; RUN: wasm-ld -u hello_str %t.o %t.a -o %t2.wasm
+; RUN: obj2yaml %t2.wasm | FileCheck %s -check-prefix=CHECK-DATA
+
 target triple = "wasm32-unknown-unknown"
 
+; Weak external function symbol
 declare extern_weak i32 @ret32()
 
+; Weak external data symbol
+@hello_str = extern_weak global i8*, align 4
+
 define void @_start() {
-entry:
+  br i1 icmp ne (i8** @hello_str, i8** null), label %if.then, label %if.end
+
+if.then:
   %call1 = call i32 @ret32()
+  br label %if.end
+
+if.end:
   ret void
 }
 
+; Ensure we have no data section.  If we do, would mean that hello_str was
+; pulled out of the library.
+; CHECK-NOT:  Type:            DATA
+; CHECK-DATA: Type:            DATA
+
 ; CHECK: Name: 'undefined:ret32'
 ; CHECK-NOT: Name: ret32

lld/wasm/InputFiles.cpp

if (reloc.Type == R_WASM_TABLE_INDEX_REL_SLEB)
       index -= config->tableBase;
     return index;
 
   }
   case R_WASM_MEMORY_ADDR_SLEB:
   case R_WASM_MEMORY_ADDR_I32:
   case R_WASM_MEMORY_ADDR_LEB:
   case R_WASM_MEMORY_ADDR_REL_SLEB:
-    if (isa<UndefinedData>(sym))
+    if (isa<UndefinedData>(sym) || sym->isUndefWeak())
       return 0;
     return cast<DefinedData>(sym)->getVirtualAddress() + reloc.Addend;
   case R_WASM_TYPE_INDEX_LEB:
     return typeMap[reloc.Index];
   case R_WASM_FUNCTION_INDEX_LEB:
     return getFunctionSymbol(reloc.Index)->getFunctionIndex();
   case R_WASM_GLOBAL_INDEX_LEB:
     if (auto gs = dyn_cast<GlobalSymbol>(sym))