[Mesa-dev] [PATCH 2/5] i965/fs: Switch to per-subspan discard jumps.

[Jason Ekstrand]

This makes a lot of sense

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>

On Tue, Aug 16, 2016 at 1:54 PM, Francisco Jerez <currojerez at riseup.net>
wrote:

> ANY4H is more efficient than ANY8H and ANY16H because it makes sure
> that whenever a whole subspan hits a discard statement it gets
> disabled by the EU until the end of the program, regardless of whether
> the discard condition is uniform across all channels of the SIMD8-16
> thread.  OTOH ANY8H/ANY16H would cause the rest of the program to be
> executed for *all* channels if only one of the channels hadn't taken
> the discard branch, potentially increasing the bandwidth and ALU usage
> of the program unnecessarily.
>
> This change increases the FPS by over 3x of a simple micro-benchmark
> that discards a bunch of fragments and then does a single costly
> texturing operation.  I've just re-verified the FPS change on HSW and
> SKL, but I expect all platforms from Gen6 up to get a similar benefit.
>
> Note that we could potentially be more aggressive and use the NORMAL
> predicate to discard individual channels, but that would need to
> happen post-scheduling because the scheduler currently doesn't care to
> reorder HALT instructions with respect to other instructions, and the
> NORMAL predicate would cause the results of subsequent derivative
> computations to become undefined -- If the scheduler didn't reorder
> HALT instructions it would actually be safe to switch to NORMAL
> because the behavior of derivative computations after a non-uniform
> discard statement is undefined by the GLSL spec, but that would make
> the optimization implemented by one of the following commits somewhat
> more difficult.
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
> b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index d1ac80a..c5067cd 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -1394,9 +1394,7 @@ fs_visitor::emit_discard_jump()
>     fs_inst *discard_jump = bld.emit(FS_OPCODE_DISCARD_JUMP);
>     discard_jump->flag_subreg = 1;
>
> -   discard_jump->predicate = (dispatch_width == 8)
> -                             ? BRW_PREDICATE_ALIGN1_ANY8H
> -                             : BRW_PREDICATE_ALIGN1_ANY16H;
> +   discard_jump->predicate = BRW_PREDICATE_ALIGN1_ANY4H;
>     discard_jump->predicate_inverse = true;
>  }
>
> --
> 2.9.0
>
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