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nir: strengthen fmin/fmax definitions with signed zero

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SPIR-V strengthened the semantics around signed zero, requiring fmin(-0, +0) =
-0. Since nir_op_fmin is commutative, we must also require fmin(+0, -0) = -0 to
match, although it's unclear if SPIR-V requires that. We must strengthen NIR's
definitions accordingly.

This strengthening is additionally motivated by the existing nir_opt_algebraic
rule like:

   (('fmin', a, ('fneg', a)), ('fneg', ('fabs', a))),

With the strengthened new definition, this transform is clearly exact. With the
weaker definition, the transform could change the sign of zero based on
implementation-defined behaviours which ... while, not exactly unsound, is
undesireable semantically.

...

This is probably technically a bug fix, but I'm not convinced it's worth it's
weight in backporting.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30075>
This commit is contained in:
Alyssa Rosenzweig 2024-07-08 11:43:54 -04:00 • committed by Marge Bot
parent 7fc5a2296b
commit 6f48fa4ebe
1 changed files with 21 additions and 2 deletions
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23
src/compiler/nir/nir_opcodes.py
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@ -927,10 +927,29 @@ opcode("fdph", 1, tfloat, [3, 4], [tfloat, tfloat], False, "",
opcode("fdph_replicated", 0, tfloat, [3, 4], [tfloat, tfloat], False, "",
"src0.x * src1.x + src0.y * src1.y + src0.z * src1.z + src1.w")
binop("fmin", tfloat, _2src_commutative + associative, "fmin(src0, src1)")
# The C fmin/fmax functions have implementation-defined behaviour for signed
# zeroes. However, SPIR-V requires:
#
# fmin(-0, +0) = -0
# fmax(+0, -0) = +0
#
# The NIR opcodes match SPIR-V. Furthermore, the NIR opcodes are commutative, so
# we must also ensure:
#
# fmin(+0, -0) = -0
# fmax(-0, +0) = +0
#
# To implement the constant folding, when the sources are equal, we use the
# min/max of the bit patterns which will order the signed zeroes while
# preserving all other values.
for op, macro in [("fmin", "MIN2"), ("fmax", "MAX2")]:
binop(op, tfloat, _2src_commutative + associative,
"bit_size == 64 ? " +
f"(src0 == src1 ? uid({macro}((int64_t)dui(src0), (int64_t)dui(src1))) : {op}(src0, src1)) :"
f"(src0 == src1 ? uif({macro}((int32_t)fui(src0), (int32_t)fui(src1))) : {op}f(src0, src1))")
binop("imin", tint, _2src_commutative + associative, "MIN2(src0, src1)")
binop("umin", tuint, _2src_commutative + associative, "MIN2(src0, src1)")
binop("fmax", tfloat, _2src_commutative + associative, "fmax(src0, src1)")
binop("imax", tint, _2src_commutative + associative, "MAX2(src0, src1)")
binop("umax", tuint, _2src_commutative + associative, "MAX2(src0, src1)")