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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/97976] Optimization relating to NULL pointer assumptions in gcc 9.1 Date: Wed, 25 Nov 2020 23:55:55 +0000 [thread overview] Message-ID: <bug-97976-4-WdNPRbiFQo@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-97976-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97976 --- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- 6.5.6 p8 covers it. You cannot increment or decrement a pointer past the end of an array (except the one past the end position). C++ has similar rules. GCC assumes there is no object at address zero, see the documentation for -fdelete-null-pointer-checks Since there is no object at address zero, the first possible byte that can be part of an array of bytes is address 1, and so decrementing a pointer to that byte would point to an element at index -1, which doesn't exist (this violates the "if the array is large enough" wording of 6.5.6 p8, because you've gone out of bounds of the array).
next prev parent reply other threads:[~2020-11-25 23:55 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-24 22:08 [Bug c++/97976] New: Optimization regression in 10.1 for lambda passed as template argument peter at int19h dot net 2020-11-24 22:14 ` [Bug c++/97976] " pinskia at gcc dot gnu.org 2020-11-24 22:24 ` peter at int19h dot net 2020-11-25 4:40 ` peter at int19h dot net 2020-11-25 8:25 ` [Bug c++/97976] Optimization relating to NULL pointer assumptions in gcc 9.1 rguenth at gcc dot gnu.org 2020-11-25 16:43 ` peter at int19h dot net 2020-11-25 18:18 ` redi at gcc dot gnu.org 2020-11-25 19:46 ` peter at int19h dot net 2020-11-25 23:55 ` redi at gcc dot gnu.org [this message] 2020-11-26 0:20 ` redi at gcc dot gnu.org 2020-11-26 1:37 ` peter at int19h dot net 2020-11-26 10:08 ` redi at gcc dot gnu.org 2020-11-26 10:09 ` redi at gcc dot gnu.org
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