[wasm] Differentiate unnamed and empty names

src/wasm/wasm-function-name-table.cc

Index: src/wasm/wasm-function-name-table.cc
diff --git a/src/wasm/wasm-function-name-table.cc b/src/wasm/wasm-function-name-table.cc
index f08270410da6c6b1fa1c2a71535c92529c9de9d2..1380b3048b671440f10b7797d33ab3759f34594c 100644
--- a/src/wasm/wasm-function-name-table.cc
+++ b/src/wasm/wasm-function-name-table.cc
@@ -14,10 +14,12 @@ namespace wasm {
 // module, then the first (kIntSize * (N+1)) bytes are integer entries.
 // The first integer entry encodes the number of functions in the module.
 // The entries 1 to N contain offsets into the second part of this array.
+// If a function is unnamed (not to be confused with an empty name), then the
+// integer entry is the negative offset of the next function name.
 // After these N+1 integer entries, the second part begins, which holds a
 // concatenation of all function names.
 //
-// Returns undefined if the array length would not fit in an integer value
+// Returns undefined if the array length would not fit in an integer value.
 Handle<Object> BuildFunctionNamesTable(Isolate* isolate, WasmModule* module) {
   uint64_t func_names_length = 0;
   for (auto& func : module->functions) func_names_length += func.name_length;
@@ -36,37 +38,48 @@ Handle<Object> BuildFunctionNamesTable(Isolate* isolate, WasmModule* module) {
   int func_index = 0;
   for (WasmFunction& fun : module->functions) {
     WasmName name = module->GetNameOrNull(&fun);
-    func_names_array->copy_in(current_offset,
-                              reinterpret_cast<const byte*>(name.start()),
-                              name.length());
-    func_names_array->set_int(func_index + 1, current_offset);
-    current_offset += name.length();
+    if (name.start() == nullptr) {
+      func_names_array->set_int(func_index + 1, -current_offset);
+    } else {
+      func_names_array->copy_in(current_offset,
+                                reinterpret_cast<const byte*>(name.start()),
+                                name.length());
+      func_names_array->set_int(func_index + 1, current_offset);
+      current_offset += name.length();
+    }
     ++func_index;
   }
   return func_names_array;
 }
 
+// Extract the function name for the given function index from the function name
+// table.
+//
+// Returns undefined if the function index is invalid, the respective function
+// is unnamed (not to be confused with empty names), or the function name is not
+// a valid UTF-8 string.
 Handle<Object> GetWasmFunctionNameFromTable(Handle<ByteArray> func_names_array,
                                             uint32_t func_index) {
   uint32_t num_funcs = static_cast<uint32_t>(func_names_array->get_int(0));
   DCHECK(static_cast<int>(num_funcs) >= 0);
-  auto undefined = [&func_names_array]() -> Handle<Object> {
-    return func_names_array->GetIsolate()->factory()->undefined_value();
-  };
-  if (func_index >= num_funcs) return undefined();
+  Factory* factory = func_names_array->GetIsolate()->factory();
+  if (func_index >= num_funcs) return factory->undefined_value();
   int offset = func_names_array->get_int(func_index + 1);
+  if (offset < 0) return factory->undefined_value();
   int next_offset = func_index == num_funcs - 1
                         ? func_names_array->length()
-                        : func_names_array->get_int(func_index + 2);
+                        : abs(func_names_array->get_int(func_index + 2));
   ScopedVector<byte> buffer(next_offset - offset);
   func_names_array->copy_out(offset, buffer.start(), next_offset - offset);
   if (!unibrow::Utf8::Validate(buffer.start(), buffer.length())) {
-    return undefined();
+    return factory->undefined_value();
+  }
+  Handle<Object> maybe_name;
+  if (!factory->NewStringFromUtf8(Vector<const char>::cast(buffer))
+           .ToHandle(&maybe_name)) {
+    maybe_name = factory->undefined_value();
   }
-  MaybeHandle<Object> maybe_name =
-      func_names_array->GetIsolate()->factory()->NewStringFromUtf8(
-          Vector<const char>::cast(buffer));
-  return maybe_name.is_null() ? undefined() : maybe_name.ToHandleChecked();
+  return maybe_name;
 }
 
 }  // namespace wasm