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Fix fragment input deductions in max_variables_count,input #4546

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ErichDonGubler wants to merge 3 commits into from
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Fix fragment input deductions in max_variables_count,input #4546

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656d74b
refactor: lower some vars. in `max_variables_count,input`
ErichDonGubler Dec 27, 2025
d0205a1
fix: correctly count fragment input deductions for built-ins in `max_…
ErichDonGubler Dec 27, 2025
e0157af
refactor: `max_variables_count,input`: hoist `success` to hidden case…
ErichDonGubler Jan 9, 2026
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15 changes: 7 additions & 8 deletions src/webgpu/api/validation/render_pipeline/inter_stage.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -315,23 +315,22 @@ g.test('max_variables_count,input')
u.combine('isAsync', [false, true]).combineWithParams([
// Number of user-defined output variables in test shader =
// device.limits.maxInterStageShaderVariables + numVariablesDelta
{ numVariablesDelta: 0, useExtraBuiltinInputs: false },
{ numVariablesDelta: 1, useExtraBuiltinInputs: false },
{ numVariablesDelta: 0, useExtraBuiltinInputs: true },
{ numVariablesDelta: -1, useExtraBuiltinInputs: true },
{ numVariablesDelta: 0, useExtraBuiltinInputs: false, _success: true },
{ numVariablesDelta: 1, useExtraBuiltinInputs: false, _success: false },
{ numVariablesDelta: -1, useExtraBuiltinInputs: true, _success: true },
{ numVariablesDelta: -2, useExtraBuiltinInputs: true, _success: false },
{ numVariablesDelta: -3, useExtraBuiltinInputs: true, _success: true },
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@greggman greggman Jan 9, 2026

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AFAICT numVariablesDelta is how many to add to maxInterStageShaderVariables. I don't think you can really decide where how many to subtract and statically determine success without changing the rest of the test. If the test is in compatibility mode then only 1 extra builtin will be added. If it's in core, 3 will be added.

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@kainino0x kainino0x Jan 9, 2026

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There is a thread about this above #4546 (comment)
and that's how we ended up with #4555. Actually I should close this PR (at least for now) in favor of that one.

] as const)
)
.fn(t => {
const { isAsync, numVariablesDelta, useExtraBuiltinInputs } = t.params;
const { isAsync, numVariablesDelta, useExtraBuiltinInputs, _success: success } = t.params;

const numVec4 = t.device.limits.maxInterStageShaderVariables + numVariablesDelta;
const numExtraVariables = useExtraBuiltinInputs ? 1 : 0;
const numUsedVariables = numVec4 + numExtraVariables;
const success = numUsedVariables <= t.device.limits.maxInterStageShaderVariables;

const outputs = range(numVec4, i => `@location(${i}) vout${i}: vec4<f32>`);
const inputs = range(numVec4, i => `@location(${i}) fin${i}: vec4<f32>`);

// NOTE: `numVec4` matches `inputs.length` here. We use this property later.
if (useExtraBuiltinInputs) {
inputs.push('@builtin(front_facing) front_facing_in: bool');
if (!t.isCompatibility) {
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