[dart2wasm] Remove the --string-data-segments option

In WasmGC MVP, the `array.new_data` instruction will not be a constant
instruction. This means we can't implement the option that places
eager string constants in data segments and reads them using
`array.new_data`. We can still do this for lazy strings.

Also clean out the string function generator (which is dead since we
started using `array.new_data` for lazy strings) and the special case
for the empty string (which is obsolete since we started using
`array.new_fixed` for eager strings).

Change-Id: If28bdf6913ae0cd0f4faf7d87d64cb9412e56bb5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/271120
Reviewed-by: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/bin/dart2wasm.dart

@@ -35,9 +35,6 @@ final List<Option> options = [
       defaultsTo: _d.translatorOptions.printKernel),
   Flag("print-wasm", (o, value) => o.translatorOptions.printWasm = value,
       defaultsTo: _d.translatorOptions.printWasm),
-  Flag("string-data-segments",
-      (o, value) => o.translatorOptions.stringDataSegments = value,
-      defaultsTo: _d.translatorOptions.stringDataSegments),
   IntOption(
       "inlining-limit", (o, value) => o.translatorOptions.inliningLimit = value,
       defaultsTo: "${_d.translatorOptions.inliningLimit}"),

pkg/dart2wasm/dart2wasm.md

@@ -20,7 +20,6 @@ where *options* include:
 | `--`[`no-`]`print-kernel`               | no      | Print IR for each function before compiling it.
 | `--`[`no-`]`print-wasm`                 | no      | Print Wasm instructions of each compiled function.
 | `--shared-memory-max-pages` *pagecount* |         | Max size of the imported memory buffer. If `--shared-import-memory` is specified, this must also be specified.
-| `--`[`no-`]`string-data-segments`       | no      | Use experimental array init from data segment for string constants.
 | `--watch` *offset*                      |         | Print stack trace leading to the byte at offset *offset* in the `.wasm` output file. Can be specified multiple times.
 
 The resulting `.wasm` file can be run with:

pkg/dart2wasm/lib/constants.dart

@@ -49,39 +49,16 @@ class Constants {
   final Map<Constant, ConstantInfo> constantInfo = {};
   w.DataSegment? oneByteStringSegment;
   w.DataSegment? twoByteStringSegment;
-  late final w.DefinedGlobal emptyString;
   late final w.DefinedGlobal emptyTypeList;
   late final ClassInfo typeInfo = translator.classInfo[translator.typeClass]!;
 
   bool currentlyCreating = false;
 
   Constants(this.translator) {
-    _initEmptyString();
     _initEmptyTypeList();
   }
 
   w.Module get m => translator.m;
-  bool get stringDataSegments => translator.options.stringDataSegments;
-
-  void _initEmptyString() {
-    ClassInfo info = translator.classInfo[translator.oneByteStringClass]!;
-    translator.functions.allocateClass(info.classId);
-    w.ArrayType arrayType =
-        (info.struct.fields.last.type as w.RefType).heapType as w.ArrayType;
-
-    w.RefType emptyStringType = info.nonNullableType;
-    emptyString = m.addGlobal(w.GlobalType(emptyStringType, mutable: false));
-    w.Instructions ib = emptyString.initializer;
-    ib.i32_const(info.classId);
-    ib.i32_const(initialIdentityHash);
-    ib.array_new_fixed(arrayType, 0);
-    ib.struct_new(info.struct);
-    ib.end(); // end of global initializer expression
-
-    Constant emptyStringConstant = StringConstant("");
-    constantInfo[emptyStringConstant] =
-        ConstantInfo(emptyStringConstant, emptyString, null);
-  }
 
   void _initEmptyTypeList() {
     ClassInfo info = translator.classInfo[translator.immutableListClass]!;
@@ -120,63 +97,6 @@ class Constants {
         translator.typeParameterIndex[info.cls!.typeParameters.single]!);
   }
 
-  /// Create one of the two Wasm functions (one for each string type) called
-  /// from every lazily initialized string constant (of that type) to create and
-  /// initialize the string.
-  ///
-  /// The function signature is (i32 offset, i32 length) -> (ref stringClass)
-  /// where offset and length are measured in characters and indicate the place
-  /// in the corresponding string data segment from which to copy this string.
-  w.DefinedFunction makeStringFunction(Class cls) {
-    ClassInfo info = translator.classInfo[cls]!;
-    w.FunctionType ftype = m.addFunctionType(
-        const [w.NumType.i32, w.NumType.i32], [info.nonNullableType]);
-    return m.addFunction(ftype, "makeString ${cls.name}");
-  }
-
-  void makeStringFunctionBody(Class cls, w.DefinedFunction function,
-      void Function(w.Instructions) emitLoad) {
-    ClassInfo info = translator.classInfo[cls]!;
-    w.ArrayType arrayType =
-        (info.struct.fields.last.type as w.RefType).heapType as w.ArrayType;
-
-    w.Local offset = function.locals[0];
-    w.Local length = function.locals[1];
-    w.Local array =
-        function.addLocal(w.RefType.def(arrayType, nullable: false));
-    w.Local index = function.addLocal(w.NumType.i32);
-
-    w.Instructions b = function.body;
-    b.local_get(length);
-    b.array_new_default(arrayType);
-    b.local_set(array);
-
-    b.i32_const(0);
-    b.local_set(index);
-    w.Label loop = b.loop();
-    b.local_get(array);
-    b.local_get(index);
-    b.local_get(offset);
-    b.local_get(index);
-    b.i32_add();
-    emitLoad(b);
-    b.array_set(arrayType);
-    b.local_get(index);
-    b.i32_const(1);
-    b.i32_add();
-    b.local_tee(index);
-    b.local_get(length);
-    b.i32_lt_u();
-    b.br_if(loop);
-    b.end();
-
-    b.i32_const(info.classId);
-    b.i32_const(initialIdentityHash);
-    b.local_get(array);
-    b.struct_new(info.struct);
-    b.end();
-  }
-
   /// Makes a type list [ListConstant].
   ListConstant makeTypeList(List<DartType> types) => ListConstant(
       InterfaceType(translator.typeClass, Nullability.nonNullable),
@@ -417,7 +337,7 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
 
       b.i32_const(info.classId);
       b.i32_const(initialIdentityHash);
-      if (lazy || constants.stringDataSegments) {
+      if (lazy) {
         // Initialize string contents from passive data segment.
         w.DataSegment segment;
         Uint8List bytes;

pkg/dart2wasm/lib/translator.dart

@@ -39,7 +39,6 @@ class TranslatorOptions {
   bool printKernel = false;
   bool printWasm = false;
   int? sharedMemoryMaxPages;
-  bool stringDataSegments = false;
   List<int>? watchPoints = null;
 }