nir: optimise min/max fadd combos

shader-db results BDW:

total instructions in shared programs: 13060410 -> 13060313 (-0.00%)
instructions in affected programs: 24533 -> 24436 (-0.40%)
helped: 88
HURT: 0

total cycles in shared programs: 256585692 -> 256586698 (0.00%)
cycles in affected programs: 647290 -> 648296 (0.16%)
helped: 35
HURT: 30

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir_opt_algebraic.py

@@ -522,6 +522,10 @@ late_optimizations = [
 
    (('b2f(is_used_more_than_once)', ('inot', a)), ('bcsel', a, 0.0, 1.0)),
    (('fneg(is_used_more_than_once)', ('b2f', ('inot', a))), ('bcsel', a, -0.0, -1.0)),
+
+   # we do these late so that we don't get in the way of creating ffmas
+   (('fmin', ('fadd(is_used_once)', '#c', a), ('fadd(is_used_once)', '#c', b)), ('fadd', c, ('fmin', a, b))),
+   (('fmax', ('fadd(is_used_once)', '#c', a), ('fadd(is_used_once)', '#c', b)), ('fadd', c, ('fmax', a, b))),
 ]
 
 print nir_algebraic.AlgebraicPass("nir_opt_algebraic", optimizations).render()

src/compiler/nir/nir_search_helpers.h

@@ -130,6 +130,28 @@ is_used_more_than_once(nir_alu_instr *instr)
    return true;
 }
 
+static inline bool
+is_used_once(nir_alu_instr *instr)
+{
+   bool zero_if_use = list_empty(&instr->dest.dest.ssa.if_uses);
+   bool zero_use = list_empty(&instr->dest.dest.ssa.uses);
+
+   if (zero_if_use && zero_use)
+      return false;
+
+   if (!zero_if_use && list_is_singular(&instr->dest.dest.ssa.uses))
+     return false;
+
+   if (!zero_use && list_is_singular(&instr->dest.dest.ssa.if_uses))
+     return false;
+
+   if (!list_is_singular(&instr->dest.dest.ssa.if_uses) &&
+       !list_is_singular(&instr->dest.dest.ssa.uses))
+      return false;
+
+   return true;
+}
+
 static inline bool
 is_not_used_by_if(nir_alu_instr *instr)
 {