From: Jonathan Wakely <jwakely@redhat.com>
To: Patrick Palka <ppalka@redhat.com>
Cc: gcc-patches@gcc.gnu.org, jakub@redhat.com, libstdc++@gcc.gnu.org
Subject: Re: [PATCH] libstdc++: Fix ERANGE behavior for fallback FP std::from_chars
Date: Wed, 2 Nov 2022 11:20:36 +0000 [thread overview]
Message-ID: <CACb0b4=OE5Pzzzr51YiY-tmxx11OfYqf0cAmMA2r_PaLvV8nTg@mail.gmail.com> (raw)
In-Reply-To: <20221101213029.940043-1-ppalka@redhat.com>
On Tue, 1 Nov 2022 at 21:30, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The fallback implementation of floating-point std::from_chars for e.g.
> float80 just calls the C library's strtod family of functions. In case
> of overflow of the parsed result, the behavior of these functions is
> rigidly specified:
>
> If the correct value overflows and default rounding is in effect, plus
> or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to
> the return type and sign of the value), and the value of the macro
> ERANGE is stored in errno.
>
> But in case of underflow, implementations are given more leeway:
>
> If the result underflows the functions return a value whose magnitude
> is no greater than the smallest normalized positive number in the
> return type; whether errno acquires the value ERANGE is
> implementation-defined.
>
> Thus we can (and do) portably detect overflow, but we can't portably
> detect underflow. However, glibc (and presumably other high-quality C
> library implementations) will reliably set errno to ERANGE in case of
> underflow too, and it will also return the nearest denormal number to
> the parsed result (including zero in case of true underflow).
>
> Since we can't be perfect here, this patch takes the best effort
> approach of assuming a high quality C library implementation that
> allows us to distinguish between a denormal parsed result and true
> underflow by inspecting the return value.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk? Dunno
OK for trunk.
> if we should backport this too. No test because we can't portably
> test this IIUC.
I think it's worth backporting to 11 and 12 because this is a C++17
feature and that's our default mode since GCC 11.
But give it some soak time on trunk first please.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++17/floating_from_chars.cc (from_chars_impl): In the
> ERANGE case, also check for a 0 return value before returning
> result_out_of_range, occurred, otherwise assume it's a denormal
> number.
> ---
> libstdc++-v3/src/c++17/floating_from_chars.cc | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
> index a25ac5ce3aa..939c751f861 100644
> --- a/libstdc++-v3/src/c++17/floating_from_chars.cc
> +++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
> @@ -637,8 +637,13 @@ namespace
> {
> if (__builtin_isinf(tmpval)) // overflow
> ec = errc::result_out_of_range;
> - else // underflow (LWG 3081 wants to set value = tmpval here)
> + else if (tmpval == 0) // underflow (LWG 3081 wants to set value = tmpval here)
> ec = errc::result_out_of_range;
> + else // denormal value
> + {
> + value = tmpval;
> + ec = errc();
> + }
> }
> else if (n)
> {
> --
> 2.38.1.381.gc03801e19c
>
prev parent reply other threads:[~2022-11-02 11:20 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-01 21:30 Patrick Palka
2022-11-02 11:20 ` Jonathan Wakely [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CACb0b4=OE5Pzzzr51YiY-tmxx11OfYqf0cAmMA2r_PaLvV8nTg@mail.gmail.com' \
--to=jwakely@redhat.com \
--cc=gcc-patches@gcc.gnu.org \
--cc=jakub@redhat.com \
--cc=libstdc++@gcc.gnu.org \
--cc=ppalka@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).