Mesa (master): nir: make fsat return 0.0 with NaN instead of passing it through

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Module: Mesa
Branch: master
Commit: abc4a8285776dcded21d0b7f3035c9858d061611
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=abc4a8285776dcded21d0b7f3035c9858d061611

Author: Rhys Perry <pendingchaos02 at gmail.com>
Date:   Mon Feb  3 21:19:52 2020 +0000

nir: make fsat return 0.0 with NaN instead of passing it through

This is how lower_fsat and ACO implements fsat and is a more useful
definition since it can be exactly created from fmin(fmax(a, 0.0), 1.0).

Signed-off-by: Rhys Perry <pendingchaos02 at gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3716>

---

 src/compiler/nir/nir_opcodes.py       |  4 +---
 src/compiler/nir/nir_opt_algebraic.py | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/compiler/nir/nir_opcodes.py b/src/compiler/nir/nir_opcodes.py
index 830fb346a4d..e1c9788b4f0 100644
--- a/src/compiler/nir/nir_opcodes.py
+++ b/src/compiler/nir/nir_opcodes.py
@@ -199,9 +199,7 @@ unop("fsign", tfloat, ("bit_size == 64 ? " +
 unop("isign", tint, "(src0 == 0) ? 0 : ((src0 > 0) ? 1 : -1)")
 unop("iabs", tint, "(src0 < 0) ? -src0 : src0")
 unop("fabs", tfloat, "fabs(src0)")
-unop("fsat", tfloat, ("bit_size == 64 ? " +
-                      "((src0 > 1.0) ? 1.0 : ((src0 <= 0.0) ? 0.0 : src0)) : " +
-                      "((src0 > 1.0f) ? 1.0f : ((src0 <= 0.0f) ? 0.0f : src0))"))
+unop("fsat", tfloat, ("fmin(fmax(src0, 0.0), 1.0)"))
 unop("frcp", tfloat, "bit_size == 64 ? 1.0 / src0 : 1.0f / src0")
 unop("frsq", tfloat, "bit_size == 64 ? 1.0 / sqrt(src0) : 1.0f / sqrtf(src0)")
 unop("fsqrt", tfloat, "bit_size == 64 ? sqrt(src0) : sqrtf(src0)")
diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 58ff35af0b0..4112ccb0aa4 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -543,9 +543,17 @@ optimizations.extend([
    (('fmax', a, ('fneg', a)), ('fabs', a)),
    (('imax', a, ('ineg', a)), ('iabs', a)),
    (('~fmax', ('fabs', a), 0.0), ('fabs', a)),
-   (('~fmin', ('fmax', a, 0.0), 1.0), ('fsat', a), '!options->lower_fsat'),
+   (('fmin', ('fmax', a, 0.0), 1.0), ('fsat', a), '!options->lower_fsat'),
+   # fmax(fmin(a, 1.0), 0.0) is inexact because it returns 1.0 on NaN, while
+   # fsat(a) returns 0.0.
    (('~fmax', ('fmin', a, 1.0), 0.0), ('fsat', a), '!options->lower_fsat'),
+   # fmin(fmax(a, -1.0), 0.0) is inexact because it returns -1.0 on NaN, while
+   # fneg(fsat(fneg(a))) returns -0.0 on NaN.
    (('~fmin', ('fmax', a, -1.0),  0.0), ('fneg', ('fsat', ('fneg', a))), '!options->lower_fsat'),
+   # fmax(fmin(a, 0.0), -1.0) is inexact because it returns 0.0 on NaN, while
+   # fneg(fsat(fneg(a))) returns -0.0 on NaN. This only matters if
+   # SignedZeroInfNanPreserve is set, but we don't currently have any way of
+   # representing this in the optimizations other than the usual ~.
    (('~fmax', ('fmin', a,  0.0), -1.0), ('fneg', ('fsat', ('fneg', a))), '!options->lower_fsat'),
    (('fsat', ('fsign', a)), ('b2f', ('flt', 0.0, a))),
    (('fsat', ('b2f', a)), ('b2f', a)),
@@ -557,8 +565,11 @@ optimizations.extend([
    (('fmin', ('fmax', ('fmin', ('fmax', a, b), c), b), c), ('fmin', ('fmax', a, b), c)),
    (('imin', ('imax', ('imin', ('imax', a, b), c), b), c), ('imin', ('imax', a, b), c)),
    (('umin', ('umax', ('umin', ('umax', a, b), c), b), c), ('umin', ('umax', a, b), c)),
+   # Both the left and right patterns are "b" when isnan(a), so this is exact.
    (('fmax', ('fsat', a), '#b at 32(is_zero_to_one)'), ('fsat', ('fmax', a, b))),
-   (('fmin', ('fsat', a), '#b at 32(is_zero_to_one)'), ('fsat', ('fmin', a, b))),
+   # The left pattern is 0.0 when isnan(a) (because fmin(fsat(NaN), b) ->
+   # fmin(0.0, b)) while the right one is "b", so this optimization is inexact.
+   (('~fmin', ('fsat', a), '#b at 32(is_zero_to_one)'), ('fsat', ('fmin', a, b))),
 
    # If a in [0,b] then b-a is also in [0,b].  Since b in [0,1], max(b-a, 0) =
    # fsat(b-a).