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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106868] [12/13 Regression] Bogus -Wdangling-pointer warning with -O1
Date: Mon, 12 Sep 2022 16:18:00 +0000	[thread overview]
Message-ID: <bug-106868-4-D7l3EdodEF@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-106868-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106868

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed.
> 
> <bb 2> [local count: 1073741824]:
> alloc (&q);
> q.0_1 = q;
> *p_4(D) = q.0_1;
> q ={v} {CLOBBER(eol)};
> a_8 = __builtin_memcpy (q.0_1, "", 1);
> *a_8 = 0;
> return;
...
> we somehow confuse q.0_1 = q; as assigning the address of the object 'q'.

The reason for the false positive is plain to see in the IL: the memcpy call is
passed a copy of the clobbered q.  It then returns another copy of the same q
which is then used to dereference whatever the pointer points to.  The warning
is due to the (known) mismatch between how the optimizers and the warning
interpret clobbers: (IIUC) the optimizers treat it as the value of the assigned
variable alone becoming indeterminate, while the warning as all copies of it
becoming so.

  parent reply	other threads:[~2022-09-12 16:18 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-07  8:47 [Bug c/106868] New: " medhefgo at web dot de
2022-09-07  9:05 ` [Bug tree-optimization/106868] [12/13 Regression] " rguenth at gcc dot gnu.org
2022-09-12 16:18 ` msebor at gcc dot gnu.org [this message]
2022-09-12 17:04 ` msebor at gcc dot gnu.org
2022-12-05  9:41 ` rguenth at gcc dot gnu.org
2022-12-05 13:56 ` cvs-commit at gcc dot gnu.org
2022-12-05 14:06 ` rguenth at gcc dot gnu.org
2022-12-12 11:20 ` [Bug tree-optimization/106868] [12 " cvs-commit at gcc dot gnu.org
2022-12-12 11:20 ` rguenth at gcc dot gnu.org

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