Rule 8.5

Chapter 8, Functions and entry points Test

A collective operation (a derivative, an implicit-LOD texture sample, a barrier) must be in uniform control flow.

The rule text and the parts under it are the compiler's own English, as the design document writes them.

Rationale

a barrier outside uniform control flow is a shader-creation error. A derivative or an implicit-LOD sample outside it triggers the derivative_uniformity diagnostic, whose default severity is error and which a WGSL author can lower or turn off with a diagnostic filter. The compiler performs the analysis at the source line (Rule 12.6) and emits a filter only where the author writes @diagnostic on an entry (surface §54).

Derives from

Uniformity; Synchronization Built-in Functions (“all synchronization functions must only be invoked in uniform control flow”); Derivative Built-in Functions (“trigger a derivative_uniformity diagnostic if uniformity analysis cannot prove the call is in uniform control flow”); Filterable Triggering Rules (default severity error).

How it is verified

Checked by a test. A test, a gate script or a CI workflow names this rule, and the traceability check fails when a file listed below stops naming it.

Where the rule says the compiler enforces it:

TS8052 UNIFORMITY for a derivative, an implicit-LOD sample, a barrier or a workgroupUniformLoad reached under a condition on a non-uniform value (an if, a switch, a loop’s condition, the left side of && or ||, the condition of a ?: that WGSL writes as an if), or after a return, break or continue taken under one, naming the value as the author wrote it and the statement (#161, proposal 0008); @diagnostic("off", "derivative_uniformity") on the entry emits diagnostic(off, derivative_uniformity); and lifts the derivative half, never the barrier half; pinned by src/core/passes/uniformity.test.ts and src/compiler/ts/barriers.test.ts.

The files that verify it at commit 26de7be8, each at the first line that names the rule:

Explained in

The sections of the surface document that explain this rule, at commit 26de7be8:

§54 Derivative uniformity: the control flow a sample may be reached under

Error codes that enforce it

The diagnostic codes the rule names under Enforced by, or whose registry text names the rule:

TS8052 UNIFORMITY: A call that needs uniform control flow — textureSample and the other implicit-LOD forms, the derivatives, a barrier, or workgroupUniformLoad — reached under a condition that is not uniform across the invocations that run together (an if, a switch, a loop's condition, the left side of && or ||, the condition of a ?: WGSL writes as an if), or after a return, break or continue taken under one (§54).

Source

The rule at commit 26de7be8:

docs/language-design.md:649 (the design document)
reqs/rules/RULE-0805.md (its traceability item)

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