From: Alexander Monakov <amonakov@ispras.ru>
To: DJ Delorie <dj@redhat.com>
Cc: Andreas Schwab <schwab@linux-m68k.org>, libc-alpha@sourceware.org
Subject: Re: [patch] Use unsigned constants for ICMP6 filters [BZ #22489]
Date: Sun, 26 Apr 2020 14:25:56 +0300 (MSK) [thread overview]
Message-ID: <alpine.LNX.2.20.13.2004261416540.31260@monopod.intra.ispras.ru> (raw)
In-Reply-To: <xnsggr9rap.fsf@greed.delorie.com>
Computing 1<<31 causes undefined behavior (signed overflow), you get
a warning with -Wshift-overflow=2 and a runtime error with UBSan,
i.e. with -fsanitize=undefined.
I'd say the situation with -Wsign-conversion warning is more subtle
than your initial mail seemed to imply, normally gcc suppresses warnings
for code originating in system headers unless -Wsystem-headers is also
specified. Here the warning appears where user code expands a macro
defined in a system header, maybe this situation is not properly
handled for -Wsign-conversion in GCC.
In summary, you need 1u to avoid causing undefined behavior if
a shift left by 31 is possible in practice, and it also happens to
suppress GCC's -Wsign-conversion warning (while there's
nothing undefined about the conversion itself).
Alexander
next prev parent reply other threads:[~2020-04-26 11:25 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-24 21:35 DJ Delorie
2020-04-25 8:53 ` Andreas Schwab
2020-04-25 17:15 ` DJ Delorie
2020-04-25 17:48 ` Andreas Schwab
2020-04-25 21:23 ` DJ Delorie
2020-04-25 21:30 ` Andreas Schwab
2020-04-25 21:41 ` DJ Delorie
2020-04-26 11:25 ` Alexander Monakov [this message]
2020-04-26 11:56 ` Florian Weimer
2020-04-30 20:47 ` DJ Delorie
2020-05-03 20:23 ` Florian Weimer
2020-05-08 20:10 ` DJ Delorie
2020-05-08 20:22 ` Florian Weimer
2020-05-11 21:09 ` DJ Delorie
2020-05-12 7:35 ` Andreas Schwab
2020-04-26 8:03 ` Florian Weimer
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