From: Joseph Myers <joseph@codesourcery.com>
To: FX <fxcoudert@gmail.com>
Cc: GCC Mailing List <gcc@gcc.gnu.org>, <fortran@gcc.gnu.org>
Subject: Re: Need for __builtin_issignaling()
Date: Thu, 30 Dec 2021 18:42:15 +0000 [thread overview]
Message-ID: <alpine.DEB.2.22.394.2112301832320.2246536@digraph.polyomino.org.uk> (raw)
In-Reply-To: <8ABA7142-4AF1-4857-89AA-40471703F95A@gmail.com>
On Wed, 29 Dec 2021, FX via Gcc wrote:
> Is it particularly hard to do? I came across a post in the list archives
> from Joseph, who said it would be good to have. I’d be willing to try
> and put something together, unless you think it’s a big project. Any
> pointers as to how to start would be appreciated.
Start by looking at Tamar Christina's patch for bugs 77925, 77926, 66462,
which got reverted because of problems it caused. In particular, see my
comment 18 in bug 66462 pointing to some of the reports of issues, and
look at the relevant discussion in June 2017. I think the following
comment 19 is incorrect (when it refers to June and November, those are
June 2017 and November 2016, so the November version is an *older* one,
but that comments is under the apprehension that it was a newer one).
There's no need to use that patch as a starting point, but it may well be
helpful to do so, or at least to get ideas from it. It didn't add
__builtin_issignaling, but did add implementations of other related
built-in functions based on bit-manipulation, and __builtin_issignaling
would need to be implemented based on such bit-manipulation.
Apart from avoiding the bugs in that patch, for __builtin_issignaling
there isn't any other implementation approach to fall back on, and there
isn't any defined external-linkage function to fall back on either. So
it's strongly desirable to have a built-in function that works (is
expanded inline) for *all* floating-point formats supported by GCC, not
just some. (For formats not supporting signaling NaNs, it can trivially
return 0 after evaluating the argument for its side effects.) Note that
supporting all formats includes working for formats where integer
arithmetic on a same-size integer isn't supported (TFmode on 32-bit
architectures where there isn't TImode integer arithmetic, in particular),
so you need to be careful about working correctly in that case.
--
Joseph S. Myers
joseph@codesourcery.com
prev parent reply other threads:[~2021-12-30 18:42 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-29 14:25 FX
2021-12-30 18:42 ` Joseph Myers [this message]
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