[Mesa-dev] [PATCH 1/5] i965/vec4: nir_emit_if doesn't need to predicate based on all the channels

[Alejandro Piñeiro]

---

I already talked about this with Jason Ekstrand and Matt Turner
privately, but just in case somebody else jump to the review:

When using BRW_PREDICATE_NORMAL, the if will use all the channels of
the register flag.  But nir_if only reads from one channel, so that
is not needed. Another hint showing that this is safe: the MOV that
put the condition on f0 is calling get_nir_src with just one
component. That will return always a source with swizzle
BRW_SWIZZLE_XXXX, so that component is the only to be used.

This commit is not needed/solving anything per-se, but it is needed in
order to be able to implement vec4_cmod_propagation with a good
overall outcome.

 src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
index 41bd80d..e05745f 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
@@ -193,7 +193,9 @@ vec4_visitor::nir_emit_if(nir_if *if_stmt)
    vec4_instruction *inst = emit(MOV(dst_null_d(), condition));
    inst->conditional_mod = BRW_CONDITIONAL_NZ;
 
-   emit(IF(BRW_PREDICATE_NORMAL));
+   /* We can just predicate based on the X channel, as the condition only
+    * reads from one channel */
+   emit(IF(BRW_PREDICATE_ALIGN16_REPLICATE_X));
 
    nir_emit_cf_list(&if_stmt->then_list);
 
-- 
2.1.4