[Mesa-dev] [PATCH 3/3] i965: Stop splitting fma() prior to optimization

[Connor Abbott]

On Thu, May 5, 2016 at 8:51 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> According to the GLSL spec, if the user uses fma directly and you have it
> in your hardware, you shouldn't split it.  For a while now, we've been
> splitting all fma's up-front and then planned to fuse them later.  The only
> reason why this possibly helped before was for ARB programs which is
> handled by the previous commit. This fixes rendering corruptions in Tomb
> Raider.

Where in the GLSL spec does it say this? In the definition of fma(), it says:

"Otherwise, in the absence of precise consumption, there are no
special constraints on the number of operations or difference in
precision between fma() and the expression
'a * b + c'."

So in other words, splitting them and then opportunistically combining
them is fine. I think the real problem is that the lower_ffma
transform isn't marked as inexact, when doing that transform does
violate the GLSL spec. That is, we shouldn't be splitting ffma
instructions marked as exact.

>
> Shader-db results on Haswell:
>
>    total instructions in shared programs: 7560300 -> 7561510 (0.02%)
>    instructions in affected programs: 56265 -> 57475 (2.15%)
>    helped: 86
>    HURT: 291
>
> The only shaders in the database that are affected are from Shadow of
> Mordor which is the first app in our database to use fma().
>
> Reported-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_compiler.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_compiler.c b/src/mesa/drivers/dri/i965/brw_compiler.c
> index 0ea5e8b..7969543 100644
> --- a/src/mesa/drivers/dri/i965/brw_compiler.c
> +++ b/src/mesa/drivers/dri/i965/brw_compiler.c
> @@ -72,7 +72,6 @@ shader_perf_log_mesa(void *data, const char *fmt, ...)
>      * split all ffma instructions during opt_algebraic and we then re-combine \
>      * them as a later step.                                                   \
>      */                                                                        \
> -   .lower_ffma = true,                                                        \
>     .lower_sub = true,                                                         \
>     .lower_fdiv = true,                                                        \
>     .lower_scmp = true,                                                        \
> --
> 2.5.0.400.gff86faf
>
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