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nir: Fix i64tof32 lowering 

The round-to-nearest-even implementation found in lower_2f() is incorrect
for any value having a significand that is not directly representable
and whose non-representable part lies between 1 and half the minimum
representable value. In this case, the significand is rounded up instead
of being rounded down.

Fixes: 936c58c8fc ("nir: Extend nir_lower_int64() to support i2f/f2i lowering")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6290>
This commit is contained in:
Boris Brezillon 2020-08-12 14:28:48 +02:00 committed by Marge Bot
parent f9e773b920
commit 199bea0fd8
1 changed files with 20 additions and 25 deletions
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45
src/compiler/nir/nir_lower_int64.c
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@ -714,35 +714,30 @@ lower_2f(nir_builder *b, nir_ssa_def *x, unsigned dest_bit_size,
unreachable("Invalid dest_bit_size");
}
/* We keep one more bit than can fit in the significand field to let the
* u2f32 conversion do the rounding for us.
*/
nir_ssa_def *discard =
nir_imax(b, nir_isub(b, exp, nir_imm_int(b, significand_bits + 1)),
nir_imax(b, nir_isub(b, exp, nir_imm_int(b, significand_bits)),
nir_imm_int(b, 0));
nir_ssa_def *significand =
COND_LOWER_CAST(b, u2u32, COND_LOWER_OP(b, ushr, x, discard));
/* Part of the "round to nearest" has to be taken care of before we discard
* the LSB, and that's what this extra iadd is for.
* "Round to nearest even" is handled by u2f. That works because the
* shifted value either fits in the significand field (which means no
* rounding is required) or contains one extra bit that forces the
* conversion op to round things properly.
/* Round-to-nearest-even implementation:
* - if the non-representable part of the significand is higher than half
* the minimum representable significand, we round-up
* - if the non-representable part of the significand is equal to half the
* minimum representable significand and the representable part of the
* significand is odd, we round-up
* - in any other case, we round-down
*/
nir_ssa_def *add = COND_LOWER_OP(b, ishl, nir_imm_int64(b, 1), discard);
add = COND_LOWER_OP(b, isub, add, nir_imm_int64(b, 1));
nir_ssa_def *rounded_x = COND_LOWER_OP(b, iadd, x, add);
/* Signed Values can't overflow because we've saved the sign and promoted
* them to unsigned values.
*/
if (!src_is_signed) {
nir_ssa_def *overflow = COND_LOWER_CMP(b, ult, rounded_x, x);
rounded_x = COND_LOWER_OP(b, bcsel, overflow,
nir_imm_int64(b, UINT64_MAX), rounded_x);
}
nir_ssa_def *significand = COND_LOWER_OP(b, ushr, rounded_x, discard);
significand = COND_LOWER_CAST(b, u2u32, significand);
nir_ssa_def *lsb_mask = COND_LOWER_OP(b, ishl, nir_imm_int64(b, 1), discard);
nir_ssa_def *rem_mask = COND_LOWER_OP(b, isub, lsb_mask, nir_imm_int64(b, 1));
nir_ssa_def *half = COND_LOWER_OP(b, ishr, lsb_mask, nir_imm_int(b, 1));
nir_ssa_def *rem = COND_LOWER_OP(b, iand, x, rem_mask);
nir_ssa_def *halfway = nir_iand(b, COND_LOWER_CMP(b, ieq, rem, half),
nir_ine(b, discard, nir_imm_int(b, 0)));
nir_ssa_def *is_odd = nir_i2b(b, nir_iand(b, significand, nir_imm_int(b, 1)));
nir_ssa_def *round_up = nir_ior(b, COND_LOWER_CMP(b, ilt, half, rem),
nir_iand(b, halfway, is_odd));
significand = nir_iadd(b, significand, nir_b2i32(b, round_up));
nir_ssa_def *res;