[Mesa-dev] [PATCH v2 7/7] nir/opt_algebraic: Add some constant bcsel reductions

[Jason Ekstrand]

total instructions in shared programs: 5998190 -> 5997603 (-0.01%)
instructions in affected programs:     54276 -> 53689 (-1.08%)
helped:                                293
---
 src/glsl/nir/nir_opt_algebraic.py | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/glsl/nir/nir_opt_algebraic.py b/src/glsl/nir/nir_opt_algebraic.py
index e3b309c..3223119 100644
--- a/src/glsl/nir/nir_opt_algebraic.py
+++ b/src/glsl/nir/nir_opt_algebraic.py
@@ -120,11 +120,37 @@ optimizations = [
    # Boolean simplifications
    (('ine', 'a at bool', 0), 'a'),
    (('ieq', 'a at bool', 0), ('inot', 'a')),
-   (('bcsel', 'a at bool', True, False), 'a'),
-   (('bcsel', 'a at bool', False, True), ('inot', 'a')),
+   (('bcsel', a, True, False), ('ine', a, 0)),
+   (('bcsel', a, False, True), ('ieq', a, 0)),
+   (('bcsel', True, b, c), b),
+   (('bcsel', False, b, c), c),
+   # The result of this should be hit by constant propagation and, in the
+   # next round of opt_algebraic, get picked up by one of the above two.
+   (('bcsel', '#a', b, c), ('bcsel', ('ine', 'a', 0), b, c)),
 
 # This one may not be exact
    (('feq', ('fadd', a, b), 0.0), ('feq', a, ('fneg', b))),
 ]
 
+# Add optimizations to handle the case where the result of a ternary is
+# compared to a constant.  This way we can take things like
+#
+# (a ? 0 : 1) > 0
+#
+# and turn it into
+#
+# a ? (0 > 0) : (1 > 0)
+#
+# which constant folding will eat for lunch.  The resulting ternary will
+# further get cleaned up by the boolean reductions above ane we will be
+# left with just the original variable "a".
+for op in ['flt', 'fge', 'feq', 'fne',
+           'ilt', 'ige', 'ieq', 'ine', 'ult', 'uge']:
+   optimizations += [
+      ((op, ('bcsel', 'a', '#b', '#c'), '#d'),
+       ('bcsel', 'a', (op, 'b', 'd'), (op, 'c', 'd'))),
+      ((op, '#d', ('bcsel', a, '#b', '#c')),
+       ('bcsel', 'a', (op, 'd', 'b'), (op, 'd', 'c'))),
+   ]
+
 print nir_algebraic.AlgebraicPass("nir_opt_algebraic", optimizations).render()
-- 
2.2.2