nir: Mark negative re-distribution on fadd as imprecise

Otherwise, it would mutate `fneg(fadd(-0, 0))` into `fadd(0, -0)` which
isn't correct since -0 + (+0) = +0 + (-0) = +0.

This fixes the OpenCL contraction tests on Iris.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16041>

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 3c78fc0..9f73c33 100644 (file)
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -2857,7 +2857,7 @@ distribute_src_mods = [
 
    (('fneg', ('ffma(is_used_once)', a, b, c)), ('ffma', ('fneg', a), b, ('fneg', c))),
    (('fneg', ('flrp(is_used_once)', a, b, c)), ('flrp', ('fneg', a), ('fneg', b), c)),
-   (('fneg', ('fadd(is_used_once)', a, b)), ('fadd', ('fneg', a), ('fneg', b))),
+   (('fneg', ('~fadd(is_used_once)', a, b)), ('fadd', ('fneg', a), ('fneg', b))),
 
    # Note that fmin <-> fmax.  I don't think there is a way to distribute
    # fabs() into fmin or fmax.