nir: Optimize fpow with small constant exponents

They would be turned into exp(log(a)*b) instead, which is slow.

Totals from 2146 (2.52% of 85071) affected shaders:
MaxWaves: 35769 -> 35779 (+0.03%); split: +0.03%, -0.01%
Instrs: 6476835 -> 6465494 (-0.18%); split: -0.18%, +0.00%
CodeSize: 35382288 -> 35347092 (-0.10%); split: -0.10%, +0.00%
SpillSGPRs: 1055 -> 1017 (-3.60%)
Latency: 75211743 -> 75063623 (-0.20%); split: -0.20%, +0.00%
InvThroughput: 17525115 -> 17501745 (-0.13%); split: -0.14%, +0.00%
VClause: 200089 -> 200077 (-0.01%); split: -0.01%, +0.01%
SClause: 293566 -> 293480 (-0.03%); split: -0.03%, +0.00%
Copies: 649631 -> 640516 (-1.40%); split: -1.44%, +0.03%
Branches: 268441 -> 268325 (-0.04%)
PreSGPRs: 146868 -> 146045 (-0.56%)
PreVGPRs: 134125 -> 134128 (+0.00%); split: -0.00%, +0.01%

Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26727>

src/amd/ci/traces-amd.yml

@@ -52,7 +52,7 @@ traces:
       checksum: 0e7441a7f2c86c75eeac72269786e7a1
   gputest/gimark-v2.trace:
     gl-radeonsi-stoney:
-      checksum: 32d373e5ae6cd241c5a7b286c296113f
+      checksum: f2aee98f2b8813ad60d98cf241e42114
   gputest/pixmark-julia-fp32-v2.trace:
     gl-radeonsi-stoney:
       checksum: b5e0ae5ad680d884432f89a923f3e70d

src/compiler/nir/nir_opt_algebraic.py

@@ -1443,9 +1443,14 @@ optimizations.extend([
     ('~fmul', ('fpow', a, b), ('fpow', c, d)), '!options->lower_fpow'), # 2^(lg2(a) * b + lg2(c) + d) = a^b * c^d
    (('~fexp2', ('fmul', ('flog2', a), 0.5)), ('fsqrt', a)),
    (('~fexp2', ('fmul', ('flog2', a), 2.0)), ('fmul', a, a)),
+   (('~fexp2', ('fmul', ('flog2', a), 3.0)), ('fmul', ('fmul', a, a), a)),
    (('~fexp2', ('fmul', ('flog2', a), 4.0)), ('fmul', ('fmul', a, a), ('fmul', a, a))),
+   (('~fexp2', ('fmul', ('flog2', a), 5.0)), ('fmul', ('fmul', ('fmul', a, a), ('fmul', a, a)), a)),
+   (('~fexp2', ('fmul', ('flog2', a), 6.0)), ('fmul', ('fmul', ('fmul', a, a), ('fmul', a, a)), ('fmul', a, a))),
+   (('~fexp2', ('fmul', ('flog2', a), 8.0)), ('fmul', ('fmul', ('fmul', a, a), ('fmul', a, a)), ('fmul', ('fmul', a, a), ('fmul', a, a)))),
    (('~fpow', a, 1.0), a),
    (('~fpow', a, 2.0), ('fmul', a, a)),
+   (('~fpow', a, 3.0), ('fmul', ('fmul', a, a), a)),
    (('~fpow', a, 4.0), ('fmul', ('fmul', a, a), ('fmul', a, a))),
    (('~fpow', 2.0, a), ('fexp2', a)),
    (('~fpow', ('fpow', a, 2.2), 0.454545), a),

src/freedreno/ci/traces-freedreno.yml

@@ -344,7 +344,7 @@ traces:
       text: missing 3/4 of rendered image
       checksum: e7f01e62180b60aef8c67fc4977c90d1
     freedreno-a530:
-      checksum: 777ecb806510d9a038a0d06d44deeb6f
+      checksum: c0db49f777798ec7da518bb9eff099c8
     freedreno-a618:
       label: [skip]
     freedreno-a630:
@@ -353,7 +353,7 @@ traces:
         One of the material textures appears brighter on freedreno than
         i965 in a way that is probably wrong.
     zink-a618:
-      checksum: ba79e6d4a64b4391d3e9e20d3d630f0a
+      checksum: d4ac6b6dd39f5a63a7b6e0b39f81daa3
     zink-a630:
       checksum: e64bcc59d61d1c75ce5eeb109343d9bd
 

src/gallium/drivers/llvmpipe/ci/traces-llvmpipe.yml

@@ -111,7 +111,7 @@ traces:
       checksum: 58a6a276abc0e28fcb2a8acea3342712
   gputest/pixmark-piano-v2.trace:
     gl-vmware-llvmpipe:
-      checksum: 11e2a97c14c74e771483ca0d90f9bde3
+      checksum: edc09da55fea262e76686d99548f2cfd
   gputest/triangle-v2.trace:
     gl-vmware-llvmpipe:
       checksum: 7812de00011a3a059892e36cea19c696

src/gallium/drivers/virgl/ci/traces-virgl.yml

@@ -12,7 +12,7 @@ traces:
       checksum: 57ddd36b117adc9216c65c10d914a37e
   gputest/pixmark-piano-v2.trace:
     gl-virgl:
-      checksum: 0d875bda7edc01698342b157c6f51500
+      checksum: cbe50265c2d1a114fd75bf12407fbad9
   gputest/triangle-v2.trace:
     gl-virgl:
       checksum: 7812de00011a3a059892e36cea19c696