[Mesa-dev] [PATCH 1/5] i965/fs: Don't CSE negated multiplies with saturation.

[Matt Turner]

It's not correct to CSE these multiplies

   mul.sat dst1, -a, b
   mul.sat dst2,  a, b

by emitting a negated MOV from dst1 to dst2:

   mul.sat dst1, -a, b
   mov     dst2, -dst1

Take 2.0*2.0 for example. The first multiply would produce 0.0 and the
second would produce 1.0.

Fixes bad generated code in 18 to 22 shaders:

instructions in affected programs: 432 -> 464 (7.41%)
helped: 4
HURT: 18

Cc: mesa-stable at lists.freedesktop.org
---
 src/mesa/drivers/dri/i965/brw_fs_cse.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
index cde6566..0e743de 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
@@ -139,6 +139,8 @@ operands_match(const fs_inst *a, const fs_inst *b, bool *negate)
       ys[1].f = ys1_imm;
 
       *negate = (xs0_negate != xs1_negate) != (ys0_negate != ys1_negate);
+      if (*negate && (a->saturate || b->saturate))
+         return false;
       return ret;
    } else if (!a->is_commutative()) {
       bool match = true;
-- 
2.4.10