[Mesa-dev] [PATCH] i965/fs: Disable opt_sampler_eot for more message types

Neil Roberts neil at linux.intel.com
Tue Oct 20 02:56:15 PDT 2015

In bfdae9149e0 I disabled the opt_sampler_eot optimisation for TG4
message types because I found by experimentation that it doesn't work.
I wrote in the comment that I couldn't find any documentation for this
problem. However I've now found the documentation and it has
additional restrictions on further message types so this patch updates
the comment and adds the others.

That paragraph in the spec also mentions further restrictions that we
should probably worry about like that the shader shouldn't combine
this optimisation with any other render target data port read/writes.

It also has a fairly pessimistic note saying the optimisation is only
really good for large polygons in a GUI-like workload. I wonder
whether we should be doing some more benchmarking to decide whether
it's really a good idea to enable this as a general optimisation even
for games.

 src/mesa/drivers/dri/i965/brw_fs.cpp | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 49323eb..bf9ff84 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -2238,13 +2238,14 @@ fs_visitor::opt_sampler_eot()
    if (unlikely(tex_inst->is_head_sentinel()) || !tex_inst->is_tex())
       return false;
-   /* This optimisation doesn't seem to work for textureGather for some
-    * reason. I can't find any documentation or known workarounds to indicate
-    * that this is expected, but considering that it is probably pretty
-    * unlikely that a shader would directly write out the results from
-    * textureGather we might as well just disable it.
+   /* 3D Sampler » Messages » Message Format
+    *
+    * “Response Length of zero is allowed on all SIMD8* and SIMD16* sampler
+    *  messages except sample+killpix, resinfo, sampleinfo, LOD, and gather4*”
-   if (tex_inst->opcode == SHADER_OPCODE_TG4 ||
+   if (tex_inst->opcode == SHADER_OPCODE_TXS ||
+       tex_inst->opcode == SHADER_OPCODE_LOD ||
+       tex_inst->opcode == SHADER_OPCODE_TG4 ||
        tex_inst->opcode == SHADER_OPCODE_TG4_OFFSET)
       return false;

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