From: Joseph Myers <joseph@codesourcery.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Jason Merrill <jason@redhat.com>,
Jonathan Wakely <jwakely@redhat.com>,
Richard Earnshaw <richard.earnshaw@arm.com>,
<richard.sandiford@arm.com>, <gcc-patches@gcc.gnu.org>
Subject: Re: [RFC PATCH] c++, i386, arm, aarch64, libgcc: std::bfloat16_t and __bf16 arithmetic support
Date: Fri, 30 Sep 2022 18:21:04 +0000 [thread overview]
Message-ID: <alpine.DEB.2.22.394.2209301804390.609132@digraph.polyomino.org.uk> (raw)
In-Reply-To: <Yzb4SikTcfSimsIn@tucnak>
On Fri, 30 Sep 2022, Jakub Jelinek via Gcc-patches wrote:
> What isn't in the patch but I think we'll need to also change are some
> minimal set of __builtin_*bf16 builtins. Seems for _Float16, GCC provides
> all the __builtin_*f16 (and for C/ObjC even with *f16 names), but there is
> no glibc support for any of that, so builtins that are expanded by the
> compiler are fine, but what should be fall back to libm won't work.
> Maybe at least for now it is acceptable to implement most <cmath> and
> <complex> std::float16_t and std::bfloat16_t overloads with using
> __builtin_*f and explicitly narrow down, but I think at least nextafter
> (and apparently nexttoward as an alias for it for extended floating
> point types) needs to be specific for the particular floating point format.
C doesn't have nexttoward (the function whose second argument is always
long double, independent of the type of the first argument) for any of the
_FloatN or _FloatNx types; why does C++ have in (but with second argument
the same type as the first?) for those types?
(C2x also doesn't have fmax or fmin for the _FloatN or _FloatNx types, but
that's because the operations those are bound to are replaced in IEEE
754-2019 by different max/min operations with different corresponding
functions; glibc still provides the fmax and fmin operations for _FloatN
and _FloatNx, as in TS 18661-3, as extensions.)
Providing a full set of _Float16 operations in glibc would be reasonable
(modulo the issues with implications for the minimum GCC version for
building glibc for a given architecture). It's rather less clear that
__bf16 operations would be appropriate.
There may also be double rounding considerations for implementing some
functions (that are required to be correctly rounded) via operations on
float; at least for fma (if the result of fmaf is half way between two
representable values of the narrower type, but not exactly equal to the
value of a*b+c).
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2022-09-30 18:21 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-29 15:55 Jakub Jelinek
2022-09-30 13:49 ` Jason Merrill
2022-09-30 14:08 ` Jakub Jelinek
2022-09-30 18:21 ` Joseph Myers [this message]
2022-09-30 18:38 ` Jakub Jelinek
2022-09-30 19:27 ` Jonathan Wakely
2022-10-04 9:06 ` [PATCH] middle-end, c++, i386, " Jakub Jelinek
2022-10-04 15:54 ` Joseph Myers
2022-10-04 21:50 ` Jason Merrill
2022-10-05 13:47 ` Jakub Jelinek
2022-10-05 20:02 ` Jason Merrill
2022-10-12 8:23 ` [PATCH] machmode: Introduce GET_MODE_NEXT_MODE with previous GET_MODE_WIDER_MODE meaning, add new GET_MODE_WIDER_MODE Jakub Jelinek
2022-10-12 10:15 ` Richard Sandiford
2022-10-12 11:07 ` [PATCH] machmode, v2: " Jakub Jelinek
2022-10-12 11:49 ` Richard Sandiford
2022-10-12 10:37 ` [PATCH] machmode: " Eric Botcazou
2022-10-12 10:57 ` Jakub Jelinek
2022-10-13 16:50 ` [PATCH] middle-end, c++, i386, libgcc, v2: std::bfloat16_t and __bf16 arithmetic support Jakub Jelinek
2022-10-13 19:37 ` Jason Merrill
2022-10-13 21:11 ` Uros Bizjak
2022-10-13 21:35 ` Jakub Jelinek
2022-10-13 21:46 ` Uros Bizjak
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