[Mesa-dev] [PATCH] i965/fs: Don't try 16-wide if 8-wide uses more than half the registers.

[Kenneth Graunke]

16-wide programs use roughly twice as many registers as 8-wide, and we
don't support spilling in 16-wide.  So if an 8-wide program uses more
than half the available GRFs, the 16-wide one almost certainly will fail
to compile during register allocation.

Not only that, but attempting to compiling such shaders is expensive:
programs that use a lot of registers tend to be quite complex, meaning
that we spend more time than usual generating and optimizing code.  If
we fail at register allocation, we've failed at the last step, after
needlessly burning through a lot of CPU time.

To make things worse, such shader compilation typically happens at the
first draw call using the shader, so it can cause the GPU to stall.

With all that in mind, it makes sense to short-circuit the 16-wide
attempt if the 8-wide program uses too many registers.  I've chosen 75
to be conservative---if we /can/ compile a SIMD16 program, we want to.

Reduces the number of GPU stalls due to fragment shader recompiles
in Left 4 Dead 2 by about 20%, and reduces the duration of many of the
remaining stalls by about half.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_fs.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index e2dafdc..a113105 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -2100,7 +2100,10 @@ brw_wm_fs_emit(struct brw_context *brw, struct brw_wm_compile *c,
       return false;
    }
 
-   if (intel->gen >= 5 && c->prog_data.nr_pull_params == 0) {
+   if (v.grf_used >= 75) {
+      perf_debug("Too many registers to attempt compiling a 16-wide shader; "
+                 "falling back to 8-wide at a 10-20%% performance cost.\n");
+   } else if (intel->gen >= 5 && c->prog_data.nr_pull_params == 0) {
       c->dispatch_width = 16;
       fs_visitor v2(c, prog, shader);
       v2.import_uniforms(&v);
-- 
1.7.11.4