From: Kenneth Graunke Date: Sun, 13 Apr 2014 21:15:49 +0000 (-0700) Subject: i965: Update comments about Z16 being slow. X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=be000b4d1911d2d520ec7b2366403d2ae3cf8bdc;p=mesa.git i965: Update comments about Z16 being slow. We've learned a few things since we originally disabled Z16; this attempts to summarize the issue. I am no expert on this subject, though, so the comment may not be totally accurate. I did some benchmarking on GM45 and Ironlake, and discovered that for GLBenchmark 2.7 EgyptHD, using Z16 was 3% slower on GM45 (n=15), and 4.5% slower on Ironlake (n=95). So, we can drop the "on Ivybridge" aspect of the comment - it's always slower. Signed-off-by: Kenneth Graunke Reviewed-by: Matt Turner Reviewed-by: Chia-I Wu --- diff --git a/src/mesa/drivers/dri/i965/brw_surface_formats.c b/src/mesa/drivers/dri/i965/brw_surface_formats.c index cef4020d9ab..196f13930d0 100644 --- a/src/mesa/drivers/dri/i965/brw_surface_formats.c +++ b/src/mesa/drivers/dri/i965/brw_surface_formats.c @@ -620,13 +620,16 @@ brw_init_surface_formats(struct brw_context *brw) ctx->TextureFormatSupported[MESA_FORMAT_Z_FLOAT32] = true; ctx->TextureFormatSupported[MESA_FORMAT_Z32_FLOAT_S8X24_UINT] = true; - /* It appears that Z16 is slower than Z24 (on Intel Ivybridge and newer - * hardware at least), so there's no real reason to prefer it unless you're - * under memory (not memory bandwidth) pressure. Our speculation is that - * this is due to either increased fragment shader execution from - * GL_LEQUAL/GL_EQUAL depth tests at the reduced precision, or due to - * increased depth stalls from a cacheline-based heuristic for detecting - * depth stalls. + /* Benchmarking shows that Z16 is slower than Z24, so there's no reason to + * use it unless you're under memory (not memory bandwidth) pressure. + * + * Apparently, the GPU's depth scoreboarding works on a 32-bit granularity, + * which corresponds to one pixel in the depth buffer for Z24 or Z32 formats. + * However, it corresponds to two pixels with Z16, which means both need to + * hit the early depth case in order for it to happen. + * + * Other speculation is that we may be hitting increased fragment shader + * execution from GL_LEQUAL/GL_EQUAL depth tests at reduced precision. * * However, desktop GL 3.0+ require that you get exactly 16 bits when * asking for DEPTH_COMPONENT16, so we have to respect that.