When emitting a sampler message, we allocate a temporary destination
large enough to hold 4 values (or 5 for sparse). This is the maximum
size needed to hold any result. However, we shrink the size written by
the sampler message to skip writing any trailing components that NIR
tells us are never read. So we may not write the entire temporary.
The NIR texture instruction has a destination VGRF which is sized
assuming that all components are present. We issue a LOAD_PAYLOAD
instruction to copy our sampler result temporary to the NIR destination.
When we reduce the response length of the sampler messages, then some of
these temporary components have undefined values. The correct way to
indicate that is by using a BAD_FILE source. Unfortunately, we were
naively reading offsets of the temporary that were never written, but
are still part of a larger VGRF. This complicates things.
For example, sampling and only using RGB (not RGBA) was producing this:
txl_logical(8) (written: 3) vgrf3+0.0:F, ...
undef(8) (written: 4) vgrf4:UD
load_payload(8) (written: 4) vgrf4:F, vgrf3+0.0:F, vgrf3+1.0:F, vgrf3+2.0:F, vgrf3+3.0:F
The last source, vgrf3+3.0:F, is undefined, and should be BAD_FILE.
Doing so allows VGRF splitting and other optimizations to work better.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28971>
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