nir/flrp: Reassociate add in flrp(±1, b, c) lowering path

With this reassociation, this lowering path is still beneficial.

Ice Lake
total instructions in shared programs: 17220191 -> 17207181 (-0.08%)
instructions in affected programs: 999871 -> 986861 (-1.30%)
helped: 3703
HURT: 17
helped stats (abs) min: 1 max: 686 x̄: 3.52 x̃: 3
helped stats (rel) min: 0.09% max: 51.97% x̄: 2.21% x̃: 1.35%
HURT stats (abs)   min: 1 max: 9 x̄: 1.47 x̃: 1
HURT stats (rel)   min: 0.08% max: 4.55% x̄: 0.78% x̃: 0.55%
95% mean confidence interval for instructions value: -4.01 -2.99
95% mean confidence interval for instructions %-change: -2.29% -2.11%
Instructions are helped.

total cycles in shared programs: 360871298 -> 360755040 (-0.03%)
cycles in affected programs: 9931334 -> 9815076 (-1.17%)
helped: 2388
HURT: 1569
helped stats (abs) min: 1 max: 10228 x̄: 93.54 x̃: 18
helped stats (rel) min: <.01% max: 74.11% x̄: 3.36% x̃: 1.07%
HURT stats (abs)   min: 1 max: 1917 x̄: 68.27 x̃: 22
HURT stats (rel)   min: <.01% max: 44.90% x̄: 3.44% x̃: 1.72%
95% mean confidence interval for cycles value: -39.48 -19.28
95% mean confidence interval for cycles %-change: -0.86% -0.46%
Cycles are helped.

total spills in shared programs: 12355 -> 12159 (-1.59%)
spills in affected programs: 295 -> 99 (-66.44%)
helped: 2
HURT: 1

total fills in shared programs: 25398 -> 25207 (-0.75%)
fills in affected programs: 288 -> 97 (-66.32%)
helped: 2
HURT: 1

LOST:   3
GAINED: 44

Iron Lake
total instructions in shared programs: 8169225 -> 8159729 (-0.12%)
instructions in affected programs: 1025712 -> 1016216 (-0.93%)
helped: 3352
HURT: 0
helped stats (abs) min: 1 max: 6 x̄: 2.83 x̃: 3
helped stats (rel) min: 0.15% max: 12.00% x̄: 1.51% x̃: 1.05%
95% mean confidence interval for instructions value: -2.86 -2.80
95% mean confidence interval for instructions %-change: -1.56% -1.46%
Instructions are helped.

total cycles in shared programs: 188656796 -> 188612280 (-0.02%)
cycles in affected programs: 18633584 -> 18589068 (-0.24%)
helped: 3085
HURT: 14
helped stats (abs) min: 2 max: 72 x̄: 14.45 x̃: 12
helped stats (rel) min: 0.02% max: 5.73% x̄: 0.73% x̃: 0.31%
HURT stats (abs)   min: 2 max: 4 x̄: 3.71 x̃: 4
HURT stats (rel)   min: <.01% max: <.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: -14.55 -14.18
95% mean confidence interval for cycles %-change: -0.76% -0.69%
Cycles are helped.

GM45
total instructions in shared programs: 5026905 -> 5021856 (-0.10%)
instructions in affected programs: 584169 -> 579120 (-0.86%)
helped: 1776
HURT: 0
helped stats (abs) min: 1 max: 6 x̄: 2.84 x̃: 3
helped stats (rel) min: 0.15% max: 11.11% x̄: 1.43% x̃: 0.98%
95% mean confidence interval for instructions value: -2.88 -2.80
95% mean confidence interval for instructions %-change: -1.50% -1.37%
Instructions are helped.

total cycles in shared programs: 129047376 -> 129018918 (-0.02%)
cycles in affected programs: 12941924 -> 12913466 (-0.22%)
helped: 1722
HURT: 14
helped stats (abs) min: 4 max: 72 x̄: 16.56 x̃: 18
helped stats (rel) min: 0.02% max: 5.73% x̄: 0.72% x̃: 0.30%
HURT stats (abs)   min: 2 max: 4 x̄: 3.71 x̃: 4
HURT stats (rel)   min: <.01% max: <.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: -16.65 -16.13
95% mean confidence interval for cycles %-change: -0.76% -0.66%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/compiler/nir/nir_lower_flrp.c

@@ -171,7 +171,7 @@ replace_with_fast(struct nir_builder *bld, struct u_vector *dead_flrp,
 }
 
 /**
- * Replace flrp(a, b, c) with (b*c ± c) + a
+ * Replace flrp(a, b, c) with (b*c ± c) + a => b*c + (a ± c)
  *
  * \note: This only works if a = ±1.
  */
@@ -193,14 +193,14 @@ replace_with_expanded_ffma_and_add(struct nir_builder *bld,
       nir_ssa_def *const neg_c = nir_fneg(bld, c);
       nir_instr_as_alu(neg_c->parent_instr)->exact = alu->exact;
 
-      inner_sum = nir_fadd(bld, b_times_c, neg_c);
+      inner_sum = nir_fadd(bld, a, neg_c);
    } else {
-      inner_sum = nir_fadd(bld, b_times_c, c);
+      inner_sum = nir_fadd(bld, a, c);
    }
 
    nir_instr_as_alu(inner_sum->parent_instr)->exact = alu->exact;
 
-   nir_ssa_def *const outer_sum = nir_fadd(bld, inner_sum, a);
+   nir_ssa_def *const outer_sum = nir_fadd(bld, inner_sum, b_times_c);
    nir_instr_as_alu(outer_sum->parent_instr)->exact = alu->exact;
 
    nir_ssa_def_rewrite_uses(&alu->dest.dest.ssa, nir_src_for_ssa(outer_sum));