From: Andrew Haley <aph@redhat.com>
To: "Moritz Strübe" <moritz.struebe@redheads.de>,
"Jakub Jelinek" <jakub@redhat.com>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: GCC turns &~ into | due to undefined bit-shift without warning
Date: Wed, 20 Mar 2019 17:36:00 -0000 [thread overview]
Message-ID: <7644f9cc-2fcd-c13c-a631-53c62c17333b@redhat.com> (raw)
In-Reply-To: <99e48024-6331-2ba6-272c-51f8cf9e9780@redheads.de>
On 3/20/19 2:08 PM, Moritz Strübe wrote:
>
> Ok, I played around a bit. Interestingly, if I set
> -fsanitize=udefined and -fsanitize-undefined-trap-on-error the
> compiler detects that it will always trap, and optimizes the code
> accordingly (the code after the trap is removed).* Which kind of
> brings me to David's argument: Shouldn't the compiler warn if there
> is undefined behavior it certainly knows of?
Maybe an example would help.
Consider this code:
for (int i = start; i < limit; i++) {
foo(i * 5);
}
Should GCC be entitled to turn it into
int limit_tmp = i * 5;
for (int i = start * 5; i < limit_tmp; i += 5) {
foo(i);
}
If you answered "Yes, GCC should be allowed to do this", would you
want a warning? And how many such warnings might there be in a typical
program?
--
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
next prev parent reply other threads:[~2019-03-20 17:36 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-11 8:49 Moritz Strübe
2019-03-11 9:14 ` Jakub Jelinek
2019-03-11 11:06 ` Moritz Strübe
2019-03-11 11:17 ` Jakub Jelinek
2019-03-20 14:08 ` Moritz Strübe
2019-03-20 14:26 ` Christophe Lyon
2019-03-20 15:39 ` Moritz Strübe
2019-03-20 15:49 ` Jakub Jelinek
2019-03-20 17:36 ` Andrew Haley [this message]
2019-03-21 8:17 ` Richard Biener
2019-03-21 8:25 ` Alexander Monakov
2019-03-21 8:35 ` Richard Biener
2019-03-21 8:54 ` Moritz Strübe
2019-03-21 9:52 ` Andrew Haley
2019-03-11 11:24 ` Vincent Lefevre
2019-03-11 12:51 ` David Brown
2019-03-12 15:40 ` Vincent Lefevre
2019-03-12 20:57 ` David Brown
2019-03-13 2:25 ` Vincent Lefevre
2019-03-13 10:18 ` David Brown
2019-03-26 22:51 ` Vincent Lefevre
2019-03-21 22:20 ` Allan Sandfeld Jensen
2019-03-21 22:31 ` Jakub Jelinek
2019-03-22 9:27 ` Allan Sandfeld Jensen
2019-03-22 9:50 ` Jakub Jelinek
2019-03-22 10:02 ` Andrew Haley
2019-03-22 10:20 ` Allan Sandfeld Jensen
2019-03-22 12:28 ` David Brown
2019-03-22 12:40 ` Jakub Jelinek
2019-03-22 13:38 ` Andrew Haley
2019-03-22 14:35 ` Allan Sandfeld Jensen
2019-03-22 22:08 ` Andrew Pinski
2019-03-22 22:38 ` Andrew Pinski
2019-03-22 23:42 ` Joseph Myers
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