[Mesa-dev] [PATCH] i965: Don't add barrier deps for FB write messages.

[Connor Abbott]

Reviewed-by: Connor Abbott <cwabbott0 at gmail.com>

FWIW, in this area, another place where we unnecessarily introduce
dependencies between instructions is when multiple instructions write
to different parts of a virtual register, for example when setting up
message headers. Instead of tracking dependencies per-vgrf, we should
be tracking them per-register (similar to what liveness analysis does)
to avoid that. The glassy mesa repo has some changes to that effect,
but their implementation is broken.

On Mon, Feb 8, 2016 at 2:31 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> There are never render target reads, so there are no scheduling hazards.
>
> Giving the extra flexibility to the scheduler makes it possible to do
> FB writes as soon as their sources are available, reducing register
> pressure.  It also makes it possible to do the payload setup for more
> than one FB write message at a time, which could better hide latency.
>
> shader-db results on Skylake:
>
> total instructions in shared programs: 9110254 -> 9110211 (-0.00%)
> instructions in affected programs: 2898 -> 2855 (-1.48%)
> helped: 3
> HURT:   0
> LOST:   0
> GAINED: 1
>
> A reduction in instruction counts is surprising, but legitimate:
> the three shaders helped were spilling, and reducing register
> pressure allowed us to issue fewer spills/fills.
>
> total cycles in shared programs: 69035108 -> 68928820 (-0.15%)
> cycles in affected programs: 4412402 -> 4306114 (-2.41%)
> helped: 4457
> HURT: 213
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp b/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
> index 60f7fd9..4f97577 100644
> --- a/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
> @@ -939,8 +939,9 @@ fs_instruction_scheduler::calculate_deps()
>     foreach_in_list(schedule_node, n, &instructions) {
>        fs_inst *inst = (fs_inst *)n->inst;
>
> -      if (inst->opcode == FS_OPCODE_PLACEHOLDER_HALT ||
> -         inst->has_side_effects())
> +      if ((inst->opcode == FS_OPCODE_PLACEHOLDER_HALT ||
> +           inst->has_side_effects()) &&
> +          inst->opcode != FS_OPCODE_FB_WRITE)
>           add_barrier_deps(n);
>
>        /* read-after-write deps. */
> @@ -1195,7 +1196,7 @@ vec4_instruction_scheduler::calculate_deps()
>     foreach_in_list(schedule_node, n, &instructions) {
>        vec4_instruction *inst = (vec4_instruction *)n->inst;
>
> -      if (inst->has_side_effects())
> +      if (inst->has_side_effects() && inst->opcode != FS_OPCODE_FB_WRITE)
>           add_barrier_deps(n);
>
>        /* read-after-write deps. */
> --
> 2.7.0
>
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