i965: Disable Z16 on contexts that don't require it.

It appears that Z16 on Intel hardware is in fact slower than Z24, so
people are getting surprisingly hurt when trying to use Z16 as a
performance-versus-precision tradeoff, or when they're targeting GLES2 and
that's all you get.

GL 3.0+ have Z16 on the list of required exact format sizes, but GLES
doesn't, so choose the better-performing layout in that case.  Improves
GLB 2.7 trex performance at 1920x1080 by 10.7% +/- 1.1% (n=3) on my IVB
system.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

diff --git a/src/mesa/drivers/dri/i965/brw_wm_surface_state.c b/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
index a74b2c7..f197639 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
+++ b/src/mesa/drivers/dri/i965/brw_wm_surface_state.c
@@ -566,7 +566,20 @@ brw_init_surface_formats(struct brw_context *brw)
    ctx->TextureFormatSupported[MESA_FORMAT_X8_Z24] = true;
    ctx->TextureFormatSupported[MESA_FORMAT_Z32_FLOAT] = true;
    ctx->TextureFormatSupported[MESA_FORMAT_Z32_FLOAT_X24S8] = true;
-   ctx->TextureFormatSupported[MESA_FORMAT_Z16] = 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.
+    *
+    * However, desktop GL 3.0+ require that you get exactly 16 bits when
+    * asking for DEPTH_COMPONENT16, so we have to respect that.
+    */
+   if (_mesa_is_desktop_gl(ctx))
+      ctx->TextureFormatSupported[MESA_FORMAT_Z16] = true;
 
    /* On hardware that lacks support for ETC1, we map ETC1 to RGBX
     * during glCompressedTexImage2D(). See intel_mipmap_tree::wraps_etc1.