Test truncation of subnormals (#4623)

Add a test that specifically captures the result of trunc() on a
subnormal value which has been operated on to make it even smaller and
no longer representable as subnormal. Whether the result of that
operation is flushed to zero or not, the result of trunc() should
still be zero.

Before https://dawn-review.googlesource.com/c/dawn/+/302655 was landed
in Tint, this CTS test would fail on a Windows 11 device with an Intel
driver (the result would be 1.0). Now it should pass on all known
devices.

Author: sudonatalie

src/webgpu/listing_meta.json

@@ -1780,6 +1780,7 @@
   "webgpu:shader,execution,expression,call,builtin,trunc:abstract_float:*": { "subcaseMS": 12197.517 },
   "webgpu:shader,execution,expression,call,builtin,trunc:f16:*": { "subcaseMS": 120.204 },
   "webgpu:shader,execution,expression,call,builtin,trunc:f32:*": { "subcaseMS": 48.544 },
+  "webgpu:shader,execution,expression,call,builtin,trunc:subnormal_division:*": { "subcaseMS": 103.258 },
   "webgpu:shader,execution,expression,call,builtin,unpack2x16float:unpack:*": { "subcaseMS": 11.651 },
   "webgpu:shader,execution,expression,call,builtin,unpack2x16snorm:unpack:*": { "subcaseMS": 9.275 },
   "webgpu:shader,execution,expression,call,builtin,unpack2x16unorm:unpack:*": { "subcaseMS": 8.701 },

src/webgpu/shader/execution/expression/call/builtin/trunc.spec.ts

@@ -10,8 +10,15 @@ Component-wise when T is a vector.
 
 import { makeTestGroup } from '../../../../../../common/framework/test_group.js';
 import { AllFeaturesMaxLimitsGPUTest } from '../../../../../gpu_test.js';
-import { Type } from '../../../../../util/conversion.js';
-import { allInputSources, onlyConstInputSource, run } from '../../expression.js';
+import { kValue } from '../../../../../util/constants.js';
+import { Type, f32, f16 } from '../../../../../util/conversion.js';
+import { FP } from '../../../../../util/floating_point.js';
+import {
+  allInputSources,
+  onlyConstInputSource,
+  run,
+  basicExpressionBuilder,
+} from '../../expression.js';
 
 import { abstractFloatBuiltin, builtin } from './builtin.js';
 import { d } from './trunc.cache.js';
@@ -60,3 +67,39 @@ g.test('f16')
     const cases = await d.get('f16');
     await run(t, builtin('trunc'), [Type.f16], Type.f16, t.params, cases);
   });
+
+g.test('subnormal_division')
+  .specURL('https://www.w3.org/TR/WGSL/#float-builtin-functions')
+  .desc(`truncate subnormals made even smaller to zero`)
+  .params(u => u.combine('inputSource', allInputSources).combine('type', ['f32', 'f16'] as const))
+  .fn(async t => {
+    const type = t.params.type;
+    if (type === 'f16') {
+      t.skipIfDeviceDoesNotHaveFeature('shader-f16');
+    }
+
+    const trait = FP[type as 'f32' | 'f16'];
+    const scalarBuilder = type === 'f32' ? f32 : f16;
+    const constant = kValue[type as 'f32' | 'f16'];
+    const cases = [
+      // Smallest subnormals (closest to zero)
+      {
+        input: [scalarBuilder(constant.negative.subnormal.max)],
+        expected: trait.toInterval(constant.negative.zero),
+      },
+      {
+        input: [scalarBuilder(constant.positive.subnormal.min)],
+        expected: trait.toInterval(constant.positive.zero),
+      },
+    ];
+
+    await run(
+      t,
+      // Divided by 10.0 to make subnormal even smaller and unrepresentable as subnormal, should truncate to zero
+      basicExpressionBuilder(values => `trunc(${values[0]} / 10.0)`),
+      [Type[type as 'f32' | 'f16']],
+      Type[type as 'f32' | 'f16'],
+      t.params,
+      cases
+    );
+  });