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From: Jonathan Wakely <>
To: Joseph Myers <>
Cc: Jonathan Wakely <>,
	Jakub Jelinek <>,,
Subject: Re: [RFC PATCH] libstdc++, v2: Partial library support for std::float{16, 32, 64, 128}_t
Date: Mon, 17 Oct 2022 22:45:13 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, 17 Oct 2022 at 22:33, Joseph Myers <> wrote:
> On Mon, 17 Oct 2022, Jonathan Wakely via Gcc-patches wrote:
> > > And I/O etc. support is missing, not sure I'm able to handle that and if it
> > > is e.g. possible to keep that support out of, because what
> > > extended floating point types one has on a particular arch could change over
> > > time (I mean e.g. bfloat16_t support or float16_t support can be added
> > > etc.).
> >
> > Yes, I think we can add the I/O functions as always_inline because all
> > they're going to do is convert the argument to float, double, or long
> > double and then call the existing overloads. There will be no new
> > virtual functions.
> As with fma, note that doing conversions from strings to floating-point is
> a case where doing the operation on float and then narrowing is
> technically incorrect because double rounding can occur (the rounded float
> result can be half way between two values of the narrower type, without
> that being the exact mathematical result) but the operation should be
> correctly rounding.  It's fine to use float or double operations in the
> other direction (floating-point to strings), of course.

Yes, this is called out in the C++23 draft:

[ Note: When the extended floating-point type has a floating-point
conversion rank that is not equal to the rank of any standard
floating-point type, then double rounding during the conversion can
result in inaccurate results. from_chars can be used in situations
where maximum accuracy is important. - end note ]

The alternative is an ABI break, which we didn't want to force on
implementors. For libstdc++ we're not going to break the ABI, so we're
going to live with the double rounding.

  reply	other threads:[~2022-10-17 21:45 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-27  8:45 [RFC PATCH] libstdc++: Partial library support for std::float{16,32,64,128}_t Jakub Jelinek
2022-10-16 10:20 ` [RFC PATCH] libstdc++, v2: " Jakub Jelinek
2022-10-17 13:07   ` Jonathan Wakely
2022-10-17 16:25     ` [PATCH] libstdc++, v3: Partial library support for std::float{16,32,64,128}_t and std::bfloat16_t Jakub Jelinek
2022-10-18  9:18       ` Jonathan Wakely
2022-10-17 21:33     ` [RFC PATCH] libstdc++, v2: Partial library support for std::float{16, 32, 64, 128}_t Joseph Myers
2022-10-17 21:45       ` Jonathan Wakely [this message]
2022-10-17 21:50       ` Jakub Jelinek

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