Mesa (master): nir: Fix i64tof32 lowering
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Mon Aug 17 21:27:56 UTC 2020
Module: Mesa
Branch: master
Commit: 199bea0fd80e65178a9d12c705a9f0aaf0a36ceb
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=199bea0fd80e65178a9d12c705a9f0aaf0a36ceb
Author: Boris Brezillon <boris.brezillon at collabora.com>
Date: Wed Aug 12 14:28:48 2020 +0200
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: 936c58c8fcce ("nir: Extend nir_lower_int64() to support i2f/f2i lowering")
Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
Tested-by: Jesse Natalie <jenatali at microsoft.com>
Acked-by: Matt Turner <mattst88 at gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6290>
---
src/compiler/nir/nir_lower_int64.c | 47 +++++++++++++++++---------------------
1 file changed, 21 insertions(+), 26 deletions(-)
diff --git a/src/compiler/nir/nir_lower_int64.c b/src/compiler/nir/nir_lower_int64.c
index 03d7c156807..9d268a84363 100644
--- a/src/compiler/nir/nir_lower_int64.c
+++ b/src/compiler/nir/nir_lower_int64.c
@@ -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));
-
- /* 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.
- */
- 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.
+ nir_ssa_def *significand =
+ COND_LOWER_CAST(b, u2u32, COND_LOWER_OP(b, ushr, x, discard));
+
+ /* 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
*/
- 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;
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