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nir/algebraic: Add lowering for 64-bit iadd_sat and isub_sat 

v2: Rearranged and expand the comment about the optimizations applied to
the lowering.  Suggested by Caio.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/767>
This commit is contained in:
Ian Romanick 2019-11-27 13:22:38 -08:00 committed by Marge Bot
parent dcadbd2dd2
commit d3d970166c
1 changed files with 50 additions and 0 deletions
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50
src/compiler/nir/nir_opt_algebraic.py
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@ -1013,6 +1013,56 @@ optimizations.extend([
(('usub_sat', a, b), ('bcsel', ('ult', a, b), 0, ('isub', a, b)), 'options->lower_add_sat'),
(('usub_sat@64', a, b), ('bcsel', ('ult', a, b), 0, ('isub', a, b)), 'options->lower_usub_sat64 || (options->lower_int64_options & nir_lower_iadd64) != 0'),
# int64_t sum = a + b;
#
# if (a < 0 && b < 0 && a < sum)
# sum = INT64_MIN;
# } else if (a >= 0 && b >= 0 && sum < a)
# sum = INT64_MAX;
# }
#
# A couple optimizations are applied.
#
# 1. a < sum => sum >= 0. This replacement works because it is known that
# a < 0 and b < 0, so sum should also be < 0 unless there was
# underflow.
#
# 2. sum < a => sum < 0. This replacement works because it is known that
# a >= 0 and b >= 0, so sum should also be >= 0 unless there was
# overflow.
#
# 3. Invert the second if-condition and swap the order of parameters for
# the bcsel. !(a >= 0 && b >= 0 && sum < 0) becomes !(a >= 0) || !(b >=
# 0) || !(sum < 0), and that becomes (a < 0) || (b < 0) || (sum >= 0)
#
# On Intel Gen11, this saves ~11 instructions.
(('iadd_sat@64', a, b), ('bcsel',
('iand', ('iand', ('ilt', a, 0), ('ilt', b, 0)), ('ige', ('iadd', a, b), 0)),
0x8000000000000000,
('bcsel',
('ior', ('ior', ('ilt', a, 0), ('ilt', b, 0)), ('ige', ('iadd', a, b), 0)),
('iadd', a, b),
0x7fffffffffffffff)),
'(options->lower_int64_options & nir_lower_iadd64) != 0'),
# int64_t sum = a - b;
#
# if (a < 0 && b >= 0 && a < sum)
# sum = INT64_MIN;
# } else if (a >= 0 && b < 0 && a >= sum)
# sum = INT64_MAX;
# }
#
# Optimizations similar to the iadd_sat case are applied here.
(('isub_sat@64', a, b), ('bcsel',
('iand', ('iand', ('ilt', a, 0), ('ige', b, 0)), ('ige', ('isub', a, b), 0)),
0x8000000000000000,
('bcsel',
('ior', ('ior', ('ilt', a, 0), ('ige', b, 0)), ('ige', ('isub', a, b), 0)),
('isub', a, b),
0x7fffffffffffffff)),
'(options->lower_int64_options & nir_lower_iadd64) != 0'),
# Alternative lowering that doesn't rely on bfi.
(('bitfield_insert', 'base', 'insert', 'offset', 'bits'),
('bcsel', ('ult', 31, 'bits'),