[benchmarks] Add SimdFields, covering SIMD values in instance fields

Catches an existing regression where Int32x4 values are boxed in instance
fields.

Issue: #64170
Change-Id: I3339f02922b9b5c7adbc38674a5ea0f59cd2bc23
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/543020
Auto-Submit: Modestas Valauskas <valauskasmodestas@gmail.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: modulovalue

benchmarks/SimdFields/dart/SimdFields.dart

@@ -0,0 +1,135 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Measures SIMD values held in instance fields.
+//
+// Each type has a field variant and a local variant over identical work.
+
+import 'dart:typed_data';
+
+import 'package:benchmark_harness/benchmark_harness.dart';
+
+const int iterations = 100000;
+
+class F32x4Holder {
+  Float32x4 v = Float32x4.zero();
+}
+
+class I32x4Holder {
+  Int32x4 v = Int32x4(0, 0, 0, 0);
+}
+
+class F64x2Holder {
+  Float64x2 v = Float64x2.zero();
+}
+
+abstract class FieldBench extends BenchmarkBase {
+  FieldBench(String name) : super('SimdFields.$name');
+}
+
+class F32x4Field extends FieldBench {
+  F32x4Field() : super('f32x4Field');
+  final h = F32x4Holder();
+  final d = Float32x4(1.0, 1.0, 1.0, 1.0);
+  @override
+  void run() {
+    final holder = h;
+    final delta = d;
+    for (int i = 0; i < iterations; i++) {
+      holder.v = holder.v + delta;
+    }
+  }
+}
+
+class F32x4Local extends FieldBench {
+  F32x4Local() : super('f32x4Local');
+  final d = Float32x4(1.0, 1.0, 1.0, 1.0);
+  @override
+  void run() {
+    final delta = d;
+    var v = Float32x4.zero();
+    for (int i = 0; i < iterations; i++) {
+      v = v + delta;
+    }
+    if (v.x < 0.0) throw StateError('unreachable');
+  }
+}
+
+class I32x4Field extends FieldBench {
+  I32x4Field() : super('i32x4Field');
+  final h = I32x4Holder();
+  final d = Int32x4(1, 1, 1, 1);
+  @override
+  void run() {
+    final holder = h;
+    final delta = d;
+    for (int i = 0; i < iterations; i++) {
+      holder.v = holder.v + delta;
+    }
+  }
+}
+
+class I32x4Local extends FieldBench {
+  I32x4Local() : super('i32x4Local');
+  final d = Int32x4(1, 1, 1, 1);
+  @override
+  void run() {
+    final delta = d;
+    var v = Int32x4(0, 0, 0, 0);
+    for (int i = 0; i < iterations; i++) {
+      v = v + delta;
+    }
+    if (v.x == 0x7fffffff) throw StateError('unreachable');
+  }
+}
+
+class F64x2Field extends FieldBench {
+  F64x2Field() : super('f64x2Field');
+  final h = F64x2Holder();
+  final d = Float64x2(1.0, 1.0);
+  @override
+  void run() {
+    final holder = h;
+    final delta = d;
+    for (int i = 0; i < iterations; i++) {
+      holder.v = holder.v + delta;
+    }
+  }
+}
+
+class F64x2Local extends FieldBench {
+  F64x2Local() : super('f64x2Local');
+  final d = Float64x2(1.0, 1.0);
+  @override
+  void run() {
+    final delta = d;
+    var v = Float64x2.zero();
+    for (int i = 0; i < iterations; i++) {
+      v = v + delta;
+    }
+    if (v.x < 0.0) throw StateError('unreachable');
+  }
+}
+
+void main() {
+  const benchmarks = [
+    F32x4Field.new,
+    F32x4Local.new,
+    I32x4Field.new,
+    I32x4Local.new,
+    F64x2Field.new,
+    F64x2Local.new,
+  ];
+
+  for (final bm in benchmarks) {
+    bm()
+      ..setup()
+      ..run()
+      ..run();
+  }
+
+  for (final bm in benchmarks) {
+    bm().report();
+  }
+}