i965/vec4: check writemask when bailing out at register coalesce

opt_register_coalesce stopped to check previous instructions to
coalesce with if somebody else was writing on the same
destination. This can be optimized to check if somebody else was
writing to the same channels of the same destination using the
writemask.

Shader DB results (taking into account only vec4):

total instructions in shared programs: 1781593 -> 1734957 (-2.62%)
instructions in affected programs:     1238390 -> 1191754 (-3.77%)
helped:                                12782
HURT:                                  0
GAINED:                                0
LOST:                                  0

v2: removed some parenthesis, fixed indentation, as suggested by
    Matt Turner
v3: added brackets, for consistency, as suggested by Eduardo Lima

Reviewed-by: Matt Turner <mattst88@gmail.com>

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
index 893ff356afa3eb0a3b9381e789c45bb1a18b1571..c4da1a11be8e8f98105c29bf15a37b64a3217172 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
@@ -1090,11 +1090,13 @@ vec4_visitor::opt_register_coalesce()
         if (interfered)
            break;
 
-         /* If somebody else writes our destination here, we can't coalesce
-          * before that.
+         /* If somebody else writes the same channels of our destination here,
+          * we can't coalesce before that.
           */
-         if (inst->dst.in_range(scan_inst->dst, scan_inst->regs_written))
-           break;
+         if (inst->dst.in_range(scan_inst->dst, scan_inst->regs_written) &&
+             (inst->dst.writemask & scan_inst->dst.writemask) != 0) {
+            break;
+         }
 
          /* Check for reads of the register we're trying to coalesce into.  We
           * can't go rewriting instructions above that to put some other value