Clarification regarding Coherent decoration

[Karpov-Ruslan]

SPIR-V spec states:

This (Coherent decoration) indicates the memory backing the object is coherent.

It doesn't sound entirely clear, especially when you think about the cases where this decoration is actually needed. I understand that this can be found out from the GLSL specification for example, but it seems that the SPIR-V specification should be self-sufficient in this matter.

Can a clarifying description of this decoration be added? Specifically, the interaction of the coherent decoration with read-modify-write atomic operations? Something along the lines of this good explanation (7.14.4 Global vs Group/Local Coherency on Non-Atomic UAV Reads) in the D3D11 specification.

[Karpov-Ruslan]

I would like to add that one of the causes of the issue was insufficient description in the GLSL and OpenGL specifications. This is why there is still confusion about when to use the coherent qualifier.

[alegal-qc]

SPIR-V Teleconference on the June 17, 2026

We agree that the current state of the spec is not ideal.

The description of the Coherent decoration in the spec is circular — it effectively says "this indicates the object is coherent" without defining what that means. This wording was carried over from the GLSL specifications, which themselves are not very precise on this point.

The most relevant GLSL 4.6 language is in the Shader Memory Control Functions:

"When these functions return, the effects of any memory stores performed using coherent variables prior to the call will be visible to any future coherent access to the same memory performed by any other shader invocation."

This establishes Coherent as a necessary condition for memory barriers to have effect.

The decoration is effectively superseded by the Vulkan memory model and can be interpreted as:

Storing to a Coherent variable is analogous to performing a store with the availability bits set; loading from a Coherent variable is analogous to performing a load with the visibility bits set. The variable is implicitly treated as non-private.