[Mesa-dev] [PATCH 3/6] i965/fs: Improve register coalescing interference check.

[Matt Turner]

I always thought that the is_control_flow() -> return false check was a
bad hack, and some previous attempts to remove it have failed and have
been reverted.

The previous two patches fix some problems that caused register
coalescing to not notice some interference between registers, which the
is_control_flow() check apparently works around.

With that fixed, we can calculate interference more accurately.

total instructions in shared programs: 6261319 -> 6257917 (-0.05%)
instructions in affected programs:     346282 -> 342880 (-0.98%)
helped:                                1552
---
 .../drivers/dri/i965/brw_fs_register_coalesce.cpp     | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
index 218cc61..452aee5 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
@@ -110,27 +110,30 @@ can_coalesce_vars(brw::fs_live_variables *live_intervals,
        (end_from > end_to && start_to < start_from))
       return false;
 
-   int start_ip = MIN2(start_to, start_from);
+   /* Check for a write to either register in the intersection of their live
+    * ranges.
+    */
+   int start_ip = MAX2(start_to, start_from);
+   int end_ip = MIN2(end_to, end_from);
    int scan_ip = -1;
 
    foreach_block_and_inst(block, fs_inst, scan_inst, cfg) {
       scan_ip++;
 
+      /* Ignore anything before the intersection of the live ranges */
       if (scan_ip < start_ip)
          continue;
 
-      if (scan_inst->is_control_flow())
-         return false;
-
-      if (scan_ip <= live_intervals->start[var_to])
+      /* Ignore the copying instruction itself */
+      if (scan_inst == inst)
          continue;
 
-      if (scan_ip > live_intervals->end[var_to])
-         return true;
+      if (scan_ip > end_ip)
+         return true; /* registers do not interfere */
 
       if (scan_inst->overwrites_reg(inst->dst) ||
           scan_inst->overwrites_reg(inst->src[0]))
-         return false;
+         return false; /* registers interfere */
    }
 
    return true;
-- 
2.4.6