[WebAssembly] Honor WASM_SYMBOL_EXPORT symbol flag

[sbc100]

Summary

This flag means that symbol should be exported in the final binary.

The reason for this change is to allow source level annotations to
trigger a given symbol to be exported:
https://github.com/WebAssembly/tool-conventions/issues/64

[ruiu]

LGTM

I think I like the idea of adding a way to annotate a symbol at source level so that the symbol is exported. It is unfortunate that there's no way to express distinction between module-local (DSO-local) symbols and (truly) global symbols in C by default.

lld/trunk/test/wasm/export.ll

 ; RUN: llc -filetype=obj %s -o %t.o
 ; RUN: not wasm-ld --export=missing -o %t.wasm %t.o 2>&1 | FileCheck -check-prefix=CHECK-ERROR %s
 ; RUN: wasm-ld --export=hidden_function -o %t.wasm %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
[e-mail] = appending global [1 x i8*] [i8* bitcast (i32 ()* @used_function to i8*)], section "llvm.metadata"
+
 target triple = "wasm32-unknown-unknown"
 
+; Not exported by default, but forced via commandline
 define hidden i32 @hidden_function() local_unnamed_addr {
 entry:
   ret i32 0
 }
 
+; Not exported by default
+define i32 @default_function() local_unnamed_addr {
+entry:
+  ret i32 0
+}
+
+; Exported because its part of llvm.used
+define i32 @used_function() local_unnamed_addr {
+entry:
+  ret i32 0
+}
+
+; Exported by default
 define void @_start() local_unnamed_addr {
 entry:
   ret void
 }
 
 ; CHECK-ERROR: error: symbol exported via --export not found: missing
 
+; CHECK-NOT: - Name: default_function
+
 ; CHECK:        - Type:            EXPORT
 ; CHECK-NEXT:     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:            hidden_function
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           1
-; CHECK-NEXT:       - Name:            _start
+; CHECK-NEXT:       - Name:            used_function
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:       - Name:            _start
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           3
 ; CHECK-NEXT:   - Type:            CODE

lld/trunk/wasm/Symbols.cpp

if (!isDefined() || isLocal())
     return false;
 
   if (ForceExport || Config->ExportAll)
     return true;
 
   if (Config->ExportDynamic && !isHidden())
     return true;
 
-  return false;
+  return Flags & WASM_SYMBOL_EXPORTED;
 }
 
 uint32_t FunctionSymbol::getFunctionIndex() const {
   if (auto *F = dyn_cast<DefinedFunction>(this))
     return F->Function->getFunctionIndex();
   assert(FunctionIndex != INVALID_INDEX);
   return FunctionIndex;
 }