nir/range-analysis: Handle constants in nir_op_mov just like nir_op_bcsel

I discovered this while looking at a shader that was hurt by some other
work I'm doing.  When I examined the changes, I was confused that one
instance of a comparison that was used in a discard_if was (incorrectly)
eliminated, while another instance used by a bcsel was (correctly) not
eliminated.  I had to use NIR_PRINT=true to see exactly where things
when wrong.

A bunch of shaders in Goat Simulator, Dungeon Defenders, Sanctum 2, and
Strike Suit Zero were impacted.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Fixes: 405de7ccb6c ("nir/range-analysis: Rudimentary value range analysis pass")

All Intel platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 16280659 -> 16281075 (<.01%)
instructions in affected programs: 21042 -> 21458 (1.98%)
helped: 0
HURT: 136
HURT stats (abs)   min: 1 max: 9 x̄: 3.06 x̃: 3
HURT stats (rel)   min: 1.16% max: 6.12% x̄: 2.23% x̃: 2.03%
95% mean confidence interval for instructions value: 2.93 3.19
95% mean confidence interval for instructions %-change: 2.08% 2.37%
Instructions are HURT.

total cycles in shared programs: 367168270 -> 367170313 (<.01%)
cycles in affected programs: 172020 -> 174063 (1.19%)
helped: 14
HURT: 111
helped stats (abs) min: 2 max: 80 x̄: 21.21 x̃: 9
helped stats (rel) min: 0.10% max: 4.47% x̄: 1.35% x̃: 0.79%
HURT stats (abs)   min: 2 max: 584 x̄: 21.08 x̃: 5
HURT stats (rel)   min: 0.12% max: 17.28% x̄: 1.55% x̃: 0.40%
95% mean confidence interval for cycles value: 5.41 27.28
95% mean confidence interval for cycles %-change: 0.64% 1.81%
Cycles are HURT.

(cherry picked from commit 7dba7df5e577b94e009848a2ca3e0b0a41629fe9)

1 files changed, 9 insertions, 2 deletions

diff --git a/src/compiler/nir/nir_range_analysis.c b/src/compiler/nir/nir_range_analysis.c
index b7a2f20d473..6dfaea167ac 100644
--- a/src/compiler/nir/nir_range_analysis.c
+++ b/src/compiler/nir/nir_range_analysis.c
@@ -610,9 +610,16 @@ analyze_expression(const nir_alu_instr *instr, unsigned src,
       r = (struct ssa_result_range){analyze_expression(alu, 0, ht).range, false};
       break;
 
-   case nir_op_mov:
-      r = analyze_expression(alu, 0, ht);
+   case nir_op_mov: {
+      const struct ssa_result_range left = analyze_expression(alu, 0, ht);
+
+      /* See commentary in nir_op_bcsel for the reasons this is necessary. */
+      if (nir_src_is_const(alu->src[0].src) && left.range != eq_zero)
+         return (struct ssa_result_range){unknown, false};
+
+      r = left;
       break;
+   }
 
    case nir_op_fneg:
       r = analyze_expression(alu, 0, ht);