D790 - A format store writes only its format's components

Status: decided Date: 2026-10-11

A buffer_store_format_* with more channels than its descriptor's whole-word format has components writes the format's components, taken from the first channels, and leaves the rest of the element as it was. The descriptor's selects must still be the identity over the components it writes. Fewer channels than components, or more through a packed format, stays refused by name.

Assumed (known_by = assumed), grounded in Mesa: its GL image-buffer store hands buffer_store_format four channels whatever the texel buffer's format, expanding three to four (ac_nir_to_llvm.c:2281-2293). An r32ui image buffer is stored with a vec4 through a one-component descriptor, and that is correct only if the store writes the format's one component and nothing past it.

Why: PPSA25872's fill program stores xyzw through a 32_UINT descriptor (word3 0x14004, selects X, 0, 0, 0), and the translation refused the mismatch, stopping its first submission at the dispatch at byte 0x258.

Rejected:

• Writing all four channels into the element: a 32_UINT element is one word, so the other three would land on the next elements, which Mesa's image stores would corrupt.