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util: Keep quiet NaNs quiet when converting to half float.

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We don't want to be throwing exceptions and changing float values later by
emitting a signaling binary16 nan.

If we don't do this, then when we convert back to f32 in NIR constant
expression evaluation, the signaling NaN can end up giving NaN for
fmax(NaN, 0.0), instead of 0.0.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5933
Cc: mesa-stable
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16233>
(cherry picked from commit 27e33d5c96)
This commit is contained in:
Chia-I Wu 2022-04-28 14:25:36 -07:00 • committed by Dylan Baker
parent d29fe64c7e
commit 9febc11af9
3 changed files with 13 additions and 4 deletions
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2
.pick_status.json
View file

@ -274,7 +274,7 @@
"description": "util: Keep quiet NaNs quiet when converting to half float.",
"nominated": true,
"nomination_type": 0,
"resolution": 0,
"resolution": 1,
"main_sha": null,
"because_sha": null
},

8
src/util/half_float.c
View file

@ -83,8 +83,12 @@ _mesa_float_to_half_slow(float val)
e = 31;
}
else if ((flt_e == 0xff) && (flt_m != 0)) {
/* NaN */
m = 1;
/* Retain the top bits of a NaN to make sure that the quiet/signaling
* status stays the same.
*/
m = flt_m >> 13;
if (!m)
m = 1;
e = 31;
}
else {

7
src/util/softfloat.c
View file

@ -1452,7 +1452,12 @@ _mesa_float_to_half_rtz_slow(float val)
if (flt_m != 0) {
/* 'val' is a NaN, return NaN */
e = 0x1f;
m = 0x1;
/* Retain the top bits of a NaN to make sure that the quiet/signaling
* status stays the same.
*/
m = flt_m >> 13;
if (!m)
m = 1;
return (s << 15) + (e << 10) + m;
}