[Mesa-dev] [PATCH] i965/clear: Quantize the depth clear value based on the format

[Jason Ekstrand]

In f9fd976e8adba733b08d we changed the clear value to be stored as an
isl_color_value.  This had the side-effect same clear value check is now
happening directly between the f32[0] field of the isl_color_value and
ctx->Depth.Clear.  This isn't what we want for two reasons.  One is that
the comparison happens in floating point even for Z16 and Z24 formats.
Worse than that, ctx->Depth.Clear is a double so, even for 32-bit float
formats, we were comparing as doubles and not floats.  This means that
the test basically always fails for anything other than 0.0f and 1.0f.
This caused a slight performance regression in Lightsmark 2008 because
it was using a depth clear value of 0.999 which can't be stored in a
32-bit float so we were doing unneeded resolves.

Cc: Kenneth Graunke <kenneth at whitecape.org>
Bugzilla: https://bugs.freedesktop.org/101678
---
 src/mesa/drivers/dri/i965/brw_clear.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_clear.c b/src/mesa/drivers/dri/i965/brw_clear.c
index 5eb2423..b7bf03d 100644
--- a/src/mesa/drivers/dri/i965/brw_clear.c
+++ b/src/mesa/drivers/dri/i965/brw_clear.c
@@ -159,12 +159,22 @@ brw_fast_clear_depth(struct gl_context *ctx)
       break;
    }
 
+   /* Quantize the clear value to what can be stored in the actual depth
+    * buffer.  This makes the following check more accurate because it now
+    * checks if the actual depth bits will match.  It also prevents us from
+    * getting a too-accurate depth value during depth testing or when sampling
+    * with HiZ enabled.
+    */
+   float clear_value =
+      mt->format == MESA_FORMAT_Z_FLOAT32 ? ctx->Depth.Clear :
+      (unsigned)(ctx->Depth.Clear * fb->_DepthMax) / (float)fb->_DepthMax;
+
    const uint32_t num_layers = depth_att->Layered ? depth_irb->layer_count : 1;
 
    /* If we're clearing to a new clear value, then we need to resolve any clear
     * flags out of the HiZ buffer into the real depth buffer.
     */
-   if (mt->fast_clear_color.f32[0] != ctx->Depth.Clear) {
+   if (mt->fast_clear_color.f32[0] != clear_value) {
       for (uint32_t level = mt->first_level; level <= mt->last_level; level++) {
          if (!intel_miptree_level_has_hiz(mt, level))
             continue;
@@ -201,7 +211,7 @@ brw_fast_clear_depth(struct gl_context *ctx)
          }
       }
 
-      mt->fast_clear_color.f32[0] = ctx->Depth.Clear;
+      mt->fast_clear_color.f32[0] = clear_value;
    }
 
    bool need_clear = false;
-- 
2.5.0.400.gff86faf