This is required for correctness in presence of multiple 4-wide flag
writes (e.g. 4-wide instructions with a conditional mod set) which
update a different portion of the same 8-bit flag subregister.

Right now we keep track of flag dataflow with 8-bit granularity and
consider flag writes to have killed any previous definition of the
same subregister even if the write was less than 8 channels wide,
which can cause live flag register updates to be dead
code-eliminated incorrectly.

Signed-off-by: Samuel Iglesias Gonsálvez <sigles...@igalia.com>
---
 src/intel/compiler/brw_vec4_dead_code_eliminate.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/intel/compiler/brw_vec4_dead_code_eliminate.cpp 
b/src/intel/compiler/brw_vec4_dead_code_eliminate.cpp
index 5b22a096dd1..c09a3d7ebe9 100644
--- a/src/intel/compiler/brw_vec4_dead_code_eliminate.cpp
+++ b/src/intel/compiler/brw_vec4_dead_code_eliminate.cpp
@@ -121,7 +121,7 @@ vec4_visitor::dead_code_eliminate()
             }
          }
 
-         if (inst->writes_flag() && !inst->predicate) {
+         if (inst->writes_flag() && !inst->predicate && inst->exec_size == 8) {
             for (unsigned c = 0; c < 4; c++)
                BITSET_CLEAR(flag_live, c);
          }
-- 
2.11.0

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