llvmpipe: turn depth clears into full depth/stencil clears for d24x8 formats

If we have a d24x8 format, there is no stencil. Therefore, we can always
clear these bits too, which means this will be some kind of memset rather
than read-modify-write.
This is good for some 7% increase or so in gears with huge window size -
seems to have a bigger effect if things aren't in caches. Of course, any
real app won't spend nearly as much time comparatively in clearing
depth buffer in the first place, so the speedup will be much lower.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/drivers/llvmpipe/lp_setup.c

@@ -476,27 +476,30 @@ lp_setup_try_clear_zs(struct lp_setup_context *setup,
    uint64_t zsvalue = 0;
    uint32_t zmask32;
    uint8_t smask8;
+   enum pipe_format format = setup->fb.zsbuf->format;
 
    LP_DBG(DEBUG_SETUP, "%s state %d\n", __FUNCTION__, setup->state);
 
    zmask32 = (flags & PIPE_CLEAR_DEPTH) ? ~0 : 0;
    smask8 = (flags & PIPE_CLEAR_STENCIL) ? ~0 : 0;
 
-   zsvalue = util_pack64_z_stencil(setup->fb.zsbuf->format,
-                                   depth,
-                                   stencil);
+   zsvalue = util_pack64_z_stencil(format, depth, stencil);
 
-   /*
-    * XXX: should make a full mask here for things like D24X8,
-    * otherwise we'll do a read-modify-write clear later which
-    * should be unnecessary.
-    */
-   zsmask = util_pack64_mask_z_stencil(setup->fb.zsbuf->format,
-                                       zmask32,
-                                       smask8);
+   zsmask = util_pack64_mask_z_stencil(format, zmask32, smask8);
 
    zsvalue &= zsmask;
 
+   if (format == PIPE_FORMAT_Z24X8_UNORM ||
+       format == PIPE_FORMAT_X8Z24_UNORM) {
+      /*
+       * Make full mask if there's "X" bits so we can do full
+       * clear (without rmw).
+       */
+      uint32_t zsmask_full = 0;
+      zsmask_full = util_pack_mask_z_stencil(format, ~0, ~0);
+      zsmask |= ~zsmask_full;
+   }
+
    if (setup->state == SETUP_ACTIVE) {
       struct lp_scene *scene = setup->scene;