Jason Ekstrand
68092df8d8
Intel hardware supports 8-bit arithmetic but it's tricky and annoying:
- Byte operations don't actually execute with a byte type. The
execution type for byte operations is actually word. (I don't know
if this has implications for the HW implementation. Probably?)
- Destinations are required to be strided out to at least the
execution type size. This means that B-type operations always have
a stride of at least 2. This means wreaks havoc on the back-end in
multiple ways.
- Thanks to the strided destination, we don't actually save register
space by storing things in bytes. We could, in theory, interleave
two byte values into a single 2B-strided register but that's both a
pain for RA and would lead to piles of false dependencies pre-Gen12
and on Gen12+, we'd need some significant improvements to the SWSB
pass.
- Also thanks to the strided destination, all byte writes are treated
as partial writes by the back-end and we don't know how to copy-prop
them.
- On Gen11, they added a new hardware restriction that byte types
aren't allowed in the 2nd and 3rd sources of instructions. This
means that we have to emit B->W conversions all over to resolve
things. If we emit said conversions in NIR, instead, there's a
chance NIR can get rid of some of them for us.
We can get rid of a lot of this pain by just asking NIR to get rid of
8-bit arithmetic for us. It may lead to a few more conversions in some
cases but having back-end copy-prop actually work is probably a bigger
bonus. There is still a bit we have to handle in the back-end. In
particular, basic MOVs and conversions because 8-bit load/store ops
still require 8-bit types.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7482>
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