summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorBoris Brezillon <boris.brezillon@collabora.com>2020-08-12 14:28:48 +0200
committerDylan Baker <dylan.c.baker@intel.com>2020-08-18 07:32:14 -0700
commit8cdfbd3a07004d96e6480adbd7e0bfb8da7a43f6 (patch)
tree9a5d51be0d395fb959b2c07813a355a18277290e
parent81b1dcf83a9a708787d4c62b10caf987240d75f1 (diff)
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@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> (cherry picked from commit 199bea0fd80e65178a9d12c705a9f0aaf0a36ceb)
-rw-r--r--.pick_status.json2
-rw-r--r--src/compiler/nir/nir_lower_int64.c47
2 files changed, 22 insertions, 27 deletions
diff --git a/.pick_status.json b/.pick_status.json
index 7bf5bc6d514..526d5be634d 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -247,7 +247,7 @@
"description": "nir: Fix i64tof32 lowering",
"nominated": true,
"nomination_type": 1,
- "resolution": 0,
+ "resolution": 1,
"master_sha": null,
"because_sha": "936c58c8fcceee086d3c492712595555afe82266"
},
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;