On 15 August 2012 16:37, Kenneth Graunke <<a href="mailto:kenneth@whitecape.org" target="_blank">kenneth@whitecape.org</a>> wrote: <div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> 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 <<a href="mailto:kenneth@whitecape.org">kenneth@whitecape.org</a>> --- 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) { </blockquote><div> It looks like this code will give the perf warning even for situations where we couldn't do 16-wide anyhow (intel->gen == 4 || c->prog_data.nr_pull_params != 0). To avoid confusing people, we should probably only give the perf warning if register count is the *only* reason we can't do 16-wide. </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> c->dispatch_width = 16; fs_visitor v2(c, prog, shader); v2.import_uniforms(&v); -- 1.7.11.4 _______________________________________________ mesa-dev mailing list <a href="mailto:mesa-dev@lists.freedesktop.org">mesa-dev@lists.freedesktop.org</a> <a href="http://lists.freedesktop.org/mailman/listinfo/mesa-dev" target="_blank">http://lists.freedesktop.org/mailman/listinfo/mesa-dev</a> </blockquote></div>