nir/range-analysis: Adjust result range of exp2 to account for flush-to-zero

Fixes piglit tests (new in piglit!110):

    - fs-underflow-exp2-compare-zero.shader_test

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111308
Fixes: 405de7ccb6c ("nir/range-analysis: Rudimentary value range analysis pass")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

Most of the shaders affected are, unsurprisingly, in Unigine Heaven.

All Gen6+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 16278207 -> 16278465 (<.01%)
instructions in affected programs: 11374 -> 11632 (2.27%)
helped: 0
HURT: 58
HURT stats (abs)   min: 2 max: 13 x̄: 4.45 x̃: 4
HURT stats (rel)   min: 0.54% max: 4.11% x̄: 2.42% x̃: 2.82%
95% mean confidence interval for instructions value: 3.77 5.13
95% mean confidence interval for instructions %-change: 2.19% 2.64%
Instructions are HURT.

total cycles in shared programs: 367134284 -> 367135159 (<.01%)
cycles in affected programs: 81207 -> 82082 (1.08%)
helped: 17
HURT: 36
helped stats (abs) min: 6 max: 356 x̄: 90.35 x̃: 6
helped stats (rel) min: 0.69% max: 21.45% x̄: 5.71% x̃: 0.78%
HURT stats (abs)   min: 4 max: 235 x̄: 66.97 x̃: 16
HURT stats (rel)   min: 0.35% max: 27.58% x̄: 5.34% x̃: 1.09%
95% mean confidence interval for cycles value: -20.36 53.38
95% mean confidence interval for cycles %-change: -1.08% 4.67%
Inconclusive result (value mean confidence interval includes 0).

No changes on any earlier platforms.

(cherry picked from commit 33ad2bab4bcb52c0f6be56e2f9cce5f52601a4ea)

1 files changed, 14 insertions, 2 deletions

diff --git a/src/compiler/nir/nir_range_analysis.c b/src/compiler/nir/nir_range_analysis.c
index f9756ef080c..3acaa4774ae 100644
--- a/src/compiler/nir/nir_range_analysis.c
+++ b/src/compiler/nir/nir_range_analysis.c
@@ -453,9 +453,21 @@ analyze_expression(const nir_alu_instr *instr, unsigned src,
       break;
    }
 
-   case nir_op_fexp2:
-      r = (struct ssa_result_range){gt_zero, analyze_expression(alu, 0, ht).is_integral};
+   case nir_op_fexp2: {
+      /* If the parameter might be less than zero, the mathematically result
+       * will be on (0, 1).  For sufficiently large magnitude negative
+       * parameters, the result will flush to zero.
+       */
+      static const enum ssa_ranges table[last_range + 1] = {
+      /* unknown  lt_zero  le_zero  gt_zero  ge_zero  ne_zero  eq_zero */
+         ge_zero, ge_zero, ge_zero, gt_zero, gt_zero, ge_zero, gt_zero
+      };
+
+      r = analyze_expression(alu, 0, ht);
+
+      r.range = table[r.range];
       break;
+   }
 
    case nir_op_fmax: {
       const struct ssa_result_range left = analyze_expression(alu, 0, ht);