[Mesa-dev] [PATCH] i965: Fix brw_swap_cmod() for LE/GE comparisons.

[Kenneth Graunke]

The idea here is to rewrite comparisons like 2 >= x with x <= 2; we want
to simply exchange arguments, not negate the condition.  If equality was
part of the original comparison, it should remain part of the swapped
version.

This is the true cause of bug #50298.  It didn't manifest itself on
Sandybridge because we embed the conditional modifier in the IF
instruction rather than emitting a CMP.  All other platforms use CMP.

It also didn't manifest itself on the master branch because commit
be5f27a84d ("glsl: Refine the loop instruction counting.") papered over
the problem.

NOTE: This is a candidate for stable release branches.

Cc: Eric Anholt <eric at anholt.net>
Cc: Chad Versace <chad.versace at linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50298
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_eu.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Since this fixes an Ivybridge regression that occurred on the 8.0 branch (!),
I'd really like to get it merged and cherry-picked before Jakob does the
release on Monday.

diff --git a/src/mesa/drivers/dri/i965/brw_eu.c b/src/mesa/drivers/dri/i965/brw_eu.c
index 00b9671..2c432a9 100644
--- a/src/mesa/drivers/dri/i965/brw_eu.c
+++ b/src/mesa/drivers/dri/i965/brw_eu.c
@@ -47,13 +47,13 @@ brw_swap_cmod(uint32_t cmod)
    case BRW_CONDITIONAL_NZ:
       return cmod;
    case BRW_CONDITIONAL_G:
-      return BRW_CONDITIONAL_LE;
-   case BRW_CONDITIONAL_GE:
       return BRW_CONDITIONAL_L;
+   case BRW_CONDITIONAL_GE:
+      return BRW_CONDITIONAL_LE;
    case BRW_CONDITIONAL_L:
-      return BRW_CONDITIONAL_GE;
-   case BRW_CONDITIONAL_LE:
       return BRW_CONDITIONAL_G;
+   case BRW_CONDITIONAL_LE:
+      return BRW_CONDITIONAL_GE;
    default:
       return ~0;
    }
-- 
1.7.10.4