[WebAssembly] Support wasm exports with zero-length names.

[sunfish]

Summary

Zero-length strings are valid export names in WebAssembly, so allow
users to specify them.

[sbc100]

lld/wasm/InputChunks.h

134

Is there a shorter way to write that? Does just {} work?

llvm/include/llvm/BinaryFormat/Wasm.h

181–182

Would it make sense to do the same for there two too? Or is that a followup?

[sbc100]

Shall we land this?

[sunfish]

lld/wasm/InputChunks.h

134

Just {} didn't work, so I don't know of a shorter way offhand.

llvm/include/llvm/BinaryFormat/Wasm.h

181–182

Let's do that in a followup.

lld/test/wasm/export-name.ll

 ; RUN: llc -filetype=obj %s -o %t.o
 ; RUN: wasm-ld -o %t.wasm %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target triple = "wasm32-unknown-unknown"
 
 define void @foo() #0 {
     ret void
 }
 
+define void @qux() #1 {
+    ret void
+}
+
 define void @_start() {
     call void @foo()
     ret void
 }
 
 attributes #0 = { "wasm-export-name"="bar" }
 
+attributes #1 = { "wasm-export-name"="" }
+
 ; CHECK:       - Type:            EXPORT
 ; CHECK-NEXT:    Exports:
 ; CHECK-NEXT:      - Name:            memory
 ; CHECK-NEXT:        Kind:            MEMORY
 ; CHECK-NEXT:        Index:           0
 ; CHECK-NEXT:      - Name:            bar
 ; CHECK-NEXT:        Kind:            FUNCTION
 ; CHECK-NEXT:        Index:           0
-; CHECK-NEXT:      - Name:            _start
+; CHECK-NEXT:      - Name:            ''
 ; CHECK-NEXT:        Kind:            FUNCTION
 ; CHECK-NEXT:        Index:           1
+; CHECK-NEXT:      - Name:            _start
+; CHECK-NEXT:        Kind:            FUNCTION
+; CHECK-NEXT:        Index:           2

lld/wasm/InputChunks.h

public:
   static bool classof(const InputChunk *c) {
     return c->kind() == InputChunk::Function ||
            c->kind() == InputChunk::SyntheticFunction;
   }
 
   void writeTo(uint8_t *sectionStart) const override;
   StringRef getName() const override { return function->SymbolName; }
   StringRef getDebugName() const override { return function->DebugName; }
-  StringRef getExportName() const {
-    return function ? function->ExportName : "";
+  llvm::Optional<StringRef> getExportName() const {
+    return function ? function->ExportName : llvm::Optional<StringRef>();
   }
   uint32_t getComdat() const override { return function->Comdat; }
   uint32_t getFunctionInputOffset() const { return getInputSectionOffset(); }
   uint32_t getFunctionCodeOffset() const { return function->CodeOffset; }
   uint32_t getSize() const override {
     if (config->compressRelocations && file) {
       assert(compressedSize);
       return compressedSize;

lld/wasm/Writer.cpp

for (Symbol *sym : symtab->getSymbols()) {
     if (!sym->isExported())
       continue;
     if (!sym->isLive())
       continue;
 
     StringRef name = sym->getName();
     WasmExport export_;
     if (auto *f = dyn_cast<DefinedFunction>(sym)) {
-      StringRef exportName = f->function->getExportName();
-      if (!exportName.empty()) {
-        name = exportName;
+      if (Optional<StringRef> exportName = f->function->getExportName()) {
+        name = *exportName;
       }
       export_ = {name, WASM_EXTERNAL_FUNCTION, f->getFunctionIndex()};
     } else if (auto *g = dyn_cast<DefinedGlobal>(sym)) {
       // TODO(sbc): Remove this check once to mutable global proposal is
       // implement in all major browsers.
       // See: https://github.com/WebAssembly/mutable-global
       if (g->getGlobalType()->Mutable) {
         // Only __stack_pointer and __tls_base should ever be create as mutable.

llvm/include/llvm/BinaryFormat/Wasm.h

 
 struct WasmFunction {
   uint32_t Index;
   std::vector<WasmLocalDecl> Locals;
   ArrayRef<uint8_t> Body;
   uint32_t CodeSectionOffset;
   uint32_t Size;
   uint32_t CodeOffset;  // start of Locals and Body
-  StringRef ExportName; // from the "export" section
+  Optional<StringRef> ExportName; // from the "export" section
   StringRef SymbolName; // from the "linking" section
   StringRef DebugName;  // from the "name" section
   uint32_t Comdat;      // from the "comdat info" section
 };
 
 struct WasmDataSegment {
   uint32_t InitFlags;
   uint32_t MemoryIndex; // present if InitFlags & WASM_SEGMENT_HAS_MEMINDEX

 struct WasmInitFunc {
   uint32_t Priority;
   uint32_t Symbol;
 };
 
 struct WasmSymbolInfo {
   StringRef Name;
   uint8_t Kind;
   uint32_t Flags;
   // For undefined symbols the module of the import
   Optional<StringRef> ImportModule;
   // For undefined symbols the name of the import
   Optional<StringRef> ImportName;
   // For symbols to be exported from the final module
   Optional<StringRef> ExportName;
   union {
     // For function or global symbols, the index in function or global index
     // space.