[Mesa-dev] [PATCH 0/5] Gallium support for TXF and TXL with level = 0

[Roland Scheidegger]

I was thinking the same thing. It should be possible for backends to do
this on their own.
That said, for the d3d10 opcodes with shadow comparison we actually have
SAMPLE_C_LZ already since d3d10 exposes that (unlike glsl), though we
don't really make any use of it (gallivm just will put in a zero as the
lod parameter in the end...).
The TEX opcodes have a bit quirky encoding due to the "packed"
arguments, so I guess you'd have to pass the target along.
It sounds to me like this just introduces gallium opcodes to paper over
a backend deficiency. Though since IIRC most (all recent?) hw actually
supports such instructions directly, why not...
(Another question would then be, does this really warrant a cap bit (the
ever-increasing cap hell...), or should it just be mandatory.)

Roland



Am 07.03.2017 um 13:38 schrieb Ilia Mirkin:
> Why can't the compiler perform this opt? At least we do that in nouveau...
> 
> On Mar 7, 2017 7:33 AM, "Marek Olšák" <maraeo at gmail.com
> <mailto:maraeo at gmail.com>> wrote:
> 
>     Hi,
> 
>     These new TGSI opcodes are for better code generation in drivers. If you
>     know the mipmap level or LOD is 0, you don't have to pass 0 to texture
>     instructions on hardware that has dedicated instructions for level-zero
>     fetches. This saves a temporary register that is usually required for
>     the level or LOD parameter.
> 
>     This change surprisingly reduces VGPR register spilling for DiRT
>     Showdown
>     on radeonsi, but that doesn't seem to have a visible effect on
>     performance.
>     We can't make this improvement at the LLVM level though.
> 
>     Note that TXF with an MSAA target and TG4 already imply level = 0.
> 
>     Please review.
> 
>     Marek
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