Mesa (main): nir/algebraic: Add algebraic opt for float comparisons with identical operands.

Thu May 20 02:09:22 UTC 2021

Module: Mesa
Branch: main
Commit: d246c31ec1adbedf46d0b82a946f1e5645dbf460
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=d246c31ec1adbedf46d0b82a946f1e5645dbf460

Author: Ian Romanick <ian.d.romanick at intel.com>
Date:   Mon Aug 10 15:34:42 2020 -0700

nir/algebraic: Add algebraic opt for float comparisons with identical operands.

The flt version could have been added in 56e21647e20d, but our
collective understanding of NaN and comparisons was poor in 2015.  The
new "is_a_number" predicate makes the others possible.

All of the helped shaders in shader-db are either from Mad Max or Skia.
Some of the Skia shaders just get decimated by this change:

instructions helped:   shaders/skia/580-4.shader_test FS SIMD8:          81 -> 29 (-64.20%) (scheduled: top-down)

I looked at a couple of those shaders, and they had sequences like:

        vec1 32 ssa_44 = flt32 ssa_32, ssa_32
        vec1 32 ssa_45 = b32csel ssa_44, ssa_43, ssa_0
        vec1 32 ssa_46 = fge32 ssa_32, ssa_32
        vec1 32 ssa_47 = b32csel ssa_46, ssa_0, ssa_45
        vec1 32 ssa_48 = iand ssa_46, ssa_44
        vec1 32 ssa_49 = b32csel ssa_48, ssa_43, ssa_0

ssa_44 is replaced with False.  Then ssa_47 selects between ssa_0 and
ssa_0, so ssa_47 and ssa_46 are eliminated.  ssa_48 is (False && don't
care), so ssa_48 and ssa_49 are eliminated.  After that, many
calculations now involve constants of zero, so they are optimized down
too.  So it continues until there's not much left!

Reviewed-by: Rhys Perry <pendingchaos02 at gmail.com>

All Intel platforms had similar results. (Tiger Lake shown)
total instructions in shared programs: 21072238 -> 21071386 (<.01%)
instructions in affected programs: 33722 -> 32870 (-2.53%)
helped: 146
HURT: 1
helped stats (abs) min: 1 max: 62 x̄: 5.84 x̃: 2
helped stats (rel) min: 0.19% max: 62.35% x̄: 4.09% x̃: 1.07%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.20% max: 0.20% x̄: 0.20% x̃: 0.20%
95% mean confidence interval for instructions value: -7.94 -3.65
95% mean confidence interval for instructions %-change: -5.87% -2.25%
Instructions are helped.

total cycles in shared programs: 856203326 -> 856192238 (<.01%)
cycles in affected programs: 749966 -> 738878 (-1.48%)
helped: 148
HURT: 0
helped stats (abs) min: 1 max: 1226 x̄: 74.92 x̃: 18
helped stats (rel) min: 0.07% max: 49.70% x̄: 2.69% x̃: 0.46%
95% mean confidence interval for cycles value: -104.82 -45.02
95% mean confidence interval for cycles %-change: -4.01% -1.37%
Cycles are helped.

LOST:   4
GAINED: 0

Fossil-db results:

Tiger Lake
Instructions in all programs: 160915223 -> 160898354 (-0.0%)
SENDs in all programs: 6812780 -> 6812780 (+0.0%)
Loops in all programs: 38340 -> 38340 (+0.0%)
Cycles in all programs: 7434144207 -> 7433978462 (-0.0%)
Spills in all programs: 192582 -> 192582 (+0.0%)
Fills in all programs: 304537 -> 304537 (+0.0%)

Ice Lake
Instructions in all programs: 145296298 -> 145279531 (-0.0%)
SENDs in all programs: 6863692 -> 6863692 (+0.0%)
Loops in all programs: 38334 -> 38334 (+0.0%)
Cycles in all programs: 8800257014 -> 8800088384 (-0.0%)
Spills in all programs: 216880 -> 216880 (+0.0%)
Fills in all programs: 334248 -> 334248 (+0.0%)

Skylake
Instructions in all programs: 135891664 -> 135874910 (-0.0%)
SENDs in all programs: 6802946 -> 6802946 (+0.0%)
Loops in all programs: 38331 -> 38331 (+0.0%)
Cycles in all programs: 8444273433 -> 8444130932 (-0.0%)
Spills in all programs: 194839 -> 194839 (+0.0%)
Fills in all programs: 301114 -> 301114 (+0.0%)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10012>

---

 src/compiler/nir/nir_opt_algebraic.py | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 0c441b8fb8c..fa9ecf7c531 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -972,15 +972,20 @@ optimizations.extend([
    (('iand', 'a at bool16', 1.0), ('b2f', a)),
    (('iand', 'a at bool32', 1.0), ('b2f', a)),
    (('flt', ('fneg', ('b2f', 'a at 1')), 0), a), # Generated by TGSI KILL_IF.
-   # Comparison with the same args.  Note that these are not done for
-   # the float versions because NaN always returns false on float
-   # inequalities.
+   # Comparison with the same args.  Note that these are only done for the
+   # float versions when the source must be a number.  Generally, NaN cmp NaN
+   # produces the opposite result of X cmp X.  flt is the outlier.  NaN < NaN
+   # is false, and, for any number X, X < X is also false.
    (('ilt', a, a), False),
    (('ige', a, a), True),
    (('ieq', a, a), True),
    (('ine', a, a), False),
    (('ult', a, a), False),
    (('uge', a, a), True),
+   (('flt', a, a), False),
+   (('fge', 'a(is_a_number)', a), True),
+   (('feq', 'a(is_a_number)', a), True),
+   (('fneu', 'a(is_a_number)', a), False),
    # Logical and bit operations
    (('iand', a, a), a),
    (('iand', a, ~0), a),

