[Mesa-dev] [PATCH] nir: add flt comparision simplification

[Timothy Arceri]

Didn't turn out as useful as I'd hoped, but it will help alot more on
i965 by reducing regressions when we drop brw_do_channel_expressions()
and brw_do_vector_splitting().

I'm not sure how much sense 'is_not_used_by_conditional' makes on
platforms other than i965 but since this is a new opt it at least
won't do any harm.

shader-db BDW:

total instructions in shared programs: 13060410 -> 13060242 (-0.00%)
instructions in affected programs: 44880 -> 44712 (-0.37%)
helped: 86
HURT: 0

total cycles in shared programs: 256585692 -> 256584248 (-0.00%)
cycles in affected programs: 8060766 -> 8059322 (-0.02%)
helped: 177
HURT: 33
---
 src/compiler/nir/nir_opt_algebraic.py |  4 ++++
 src/compiler/nir/nir_search_helpers.h | 15 +++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 3c8318c..59f9e43 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -159,6 +159,10 @@ optimizations = [
    # a != 0.0
    (('flt', 0.0, ('fabs', a)), ('fne', a, 0.0)),
 
+   # ignore this opt when the result is used by a bcsel or if so we can make
+   # use of conditional modifiers on supported hardware.
+   (('flt(is_not_used_by_conditional)', ('fadd(is_used_once)', a, ('fneg', b)), 0.0), ('flt', a, b)),
+
    (('fge', ('fneg', ('fabs', a)), 0.0), ('feq', a, 0.0)),
    (('bcsel', ('flt', b, a), b, a), ('fmin', a, b)),
    (('bcsel', ('flt', a, b), b, a), ('fmax', a, b)),
diff --git a/src/compiler/nir/nir_search_helpers.h b/src/compiler/nir/nir_search_helpers.h
index ddaff52..05bd317 100644
--- a/src/compiler/nir/nir_search_helpers.h
+++ b/src/compiler/nir/nir_search_helpers.h
@@ -158,4 +158,19 @@ is_not_used_by_if(nir_alu_instr *instr)
    return list_empty(&instr->dest.dest.ssa.if_uses);
 }
 
+static inline bool
+is_not_used_by_conditional(nir_alu_instr *instr)
+{
+   if (!is_not_used_by_if(instr))
+      return false;
+
+   nir_foreach_use(use, &instr->dest.dest.ssa) {
+      if (use->parent_instr->type == nir_instr_type_alu &&
+          nir_instr_as_alu(use->parent_instr)->op == nir_op_bcsel)
+         return false;
+   }
+
+   return true;
+}
+
 #endif /* _NIR_SEARCH_ */
-- 
2.9.3