From: Jakub Jelinek <jakub@redhat.com>
To: Borislav Petkov <bp@alien8.de>,
Richard Biener <rguenther@suse.de>,
linux-toolchains@vger.kernel.org, Michael Matz <matz@suse.de>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: older gccs and case labels producing integer constants
Date: Wed, 6 Apr 2022 12:04:17 +0200 [thread overview]
Message-ID: <Yk1lodskz34cmqJ1@tucnak> (raw)
In-Reply-To: <Yk1jC4oLnIklOTHI@tucnak>
On Wed, Apr 06, 2022 at 11:53:17AM +0200, Jakub Jelinek wrote:
> On Tue, Apr 05, 2022 at 12:36:58PM +0200, Borislav Petkov wrote:
> > On Tue, Apr 05, 2022 at 12:06:45PM +0200, Richard Biener wrote:
> > > Wird auch mit gcc 11 rejected. Kanns sein dass mit gcc 7 andere
> > > compiler flags genommen werden?
> >
> > Found it:
> >
> > $ gcc -fsanitize=shift -c switch.c
> > switch.c: In function ‘foo’:
> > switch.c:10:7: error: case label does not reduce to an integer constant
> > case (((0xfc08) << 16) | (0x0101)):;
> >
> > $ gcc --version
> > gcc (SUSE Linux) 7.4.1 20190905 [gcc-7-branch revision 275407]
> > Copyright (C) 2017 Free Software Foundation, Inc.
> >
> > Something not fully backported?
>
> That is rejected with -fsanitize=shift even on current trunk (in C, C++ is
> fine).
> C++ constexpr code has cases for ubsan builtins and internal functions,
> but C just doesn't handle those apparently.
But I think the error is actually correct.
In C99 and later, for signed left shift the rule for x << y is that
there is UB if (similarly to all C family) if y is negative or greater or
equal to precision of promoted x, but for C99 also when
((unsigned_typeof_x) x >> (precision_of_x - 1 - y)) != 0.
That is the case above, 0xfc08 is signed int and 0xfc08 << 16 is
0xfc080000 where (0xfc08 >> 15) is 1 and so it is UB.
In C99 and later you need:
case (int)(((0xfc08U) << 16) | (0x0101)):;
or so.
Note, C++ has different rules (and C++20 and later only has the
y non-negative and less than precision requirement and nothing else).
Jakub
next prev parent reply other threads:[~2022-04-06 13:27 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-05 9:50 older gccs and case labels producing integer constants Borislav Petkov
2022-04-05 9:58 ` Richard Biener
2022-04-05 10:04 ` Borislav Petkov
2022-04-05 10:06 ` Richard Biener
2022-04-05 10:36 ` Borislav Petkov
2022-04-05 10:45 ` Borislav Petkov
2022-04-05 11:41 ` Richard Biener
2022-04-07 15:16 ` Borislav Petkov
2022-04-05 11:37 ` Richard Biener
2022-04-06 9:53 ` Jakub Jelinek
2022-04-06 10:04 ` Jakub Jelinek [this message]
2022-04-05 12:23 ` Peter Zijlstra
2022-04-05 12:39 ` Michael Matz
2022-04-05 12:53 ` Richard Biener
2022-04-05 13:04 ` Borislav Petkov
2022-04-06 10:13 ` Jakub Jelinek
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=Yk1lodskz34cmqJ1@tucnak \
--to=jakub@redhat.com \
--cc=bp@alien8.de \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-toolchains@vger.kernel.org \
--cc=matz@suse.de \
--cc=rguenther@suse.de \
/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 NNTP newsgroup(s).