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.
---
I don't like that we aren't totally sure of the mechanism behind the
performance improvement, but in the absence of any data against this,
I think we should drop Z16 at this point.
src/mesa/drivers/dri/i965/brw_wm_surface_state.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
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
--- 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.
--
1.7.10.4
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