[Mesa-dev] [PATCH 5/5] i965: Switch to scalar TCS by default.

Kenneth Graunke kenneth at whitecape.org
Tue May 3 23:28:53 UTC 2016

Normally, we expect SIMD8 shaders to be more instructions than SIMD4x2
shaders, as it takes four instructions to operate on a vec4, rather than
a single instruction.  However, the benefit is that it can process 8
objects per shader thread instead of 2.

Surprisingly, the shader-db statistics show an improvement in both
instruction and cycle counts:

Synmark: -31.25% instructions, -29.27% cycles, 0 hurt.
Tessmark: -36.92% instructions, -37.81% cycles, 0 hurt.
Unigine Heaven: -3.42% instructions, -17.95% cycles, 0 hurt.
Shadow of Mordor:
   +13.24% instructions (26 with fewer instructions, 45 with more),
   -5.23% cycles (44 with fewer cycles, 27 with more cycles).

Presumably, this is because the SIMD8 URB messages are a much more
natural fit than the SIMD4x2 URB messages - there's a ton less header

I benchmarked Shadow of Mordor and Unigine Heaven on my Skylake GT3e,
and the performance seems to be the same or increase ever so slightly
(< 1 FPS difference).  So I believe it's strictly superior.

There's also a lot more optimization potential we can do in scalar mode.

This will also help us finish fp64 support, as scalar support is going
to land much sooner than vec4-mode support.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
 src/mesa/drivers/dri/i965/brw_compiler.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_compiler.c b/src/mesa/drivers/dri/i965/brw_compiler.c
index 61bb5ad..f83ec8f 100644
--- a/src/mesa/drivers/dri/i965/brw_compiler.c
+++ b/src/mesa/drivers/dri/i965/brw_compiler.c
@@ -153,7 +153,7 @@ brw_compiler_create(void *mem_ctx, const struct brw_device_info *devinfo)
    compiler->scalar_stage[MESA_SHADER_VERTEX] =
       devinfo->gen >= 8 && !(INTEL_DEBUG & DEBUG_VEC4VS);
    compiler->scalar_stage[MESA_SHADER_TESS_CTRL] =
-      devinfo->gen >= 8 && env_var_as_boolean("INTEL_SCALAR_TCS", false);
+      devinfo->gen >= 8 && env_var_as_boolean("INTEL_SCALAR_TCS", true);
    compiler->scalar_stage[MESA_SHADER_TESS_EVAL] =
       devinfo->gen >= 8 && env_var_as_boolean("INTEL_SCALAR_TES", true);
    compiler->scalar_stage[MESA_SHADER_GEOMETRY] =

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