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From: rth@gcc.gnu.org To: 136659@bugs.debian.org, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org, lists@lina.inka.de, nobody@gcc.gnu.org, rth@gcc.gnu.org Subject: Re: target/9080: optimisation on sparc leads to unalligned access in memcpy Date: Sun, 26 Jan 2003 10:13:00 -0000 [thread overview] Message-ID: <20030126101338.2759.qmail@sources.redhat.com> (raw) Synopsis: optimisation on sparc leads to unalligned access in memcpy Responsible-Changed-From-To: unassigned->rth Responsible-Changed-By: rth Responsible-Changed-When: Sun Jan 26 10:13:38 2003 Responsible-Changed-Why: . State-Changed-From-To: open->closed State-Changed-By: rth State-Changed-When: Sun Jan 26 10:13:38 2003 State-Changed-Why: I'm fairly sure this has been discussed before, but I can't find the reference right away. The problem is known, and I consider the input program incorrect. The problem is that alignof(struct iphdr) == 4, and yet the given pointer to that structure is only 2 byte aligned. This violates section 6.3.2.3/7, to wit: If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined. GCC uses this clause to infer from the type of the pointer the alignment that it *must* have. There are two solutions for this piece of code. One, use packed data structures, so that they are 1 byte aligned. In this case something like struct iphdr_u { struct iphdr h __attribute__((packed)); }; followed by &tmp_iphdr->h.saddr Two, don't cast to struct iphdr at all. Instead use offsetof to find the proper displacement into the buffer. http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9080
next reply other threads:[~2003-01-26 10:13 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-01-26 10:13 rth [this message] -- strict thread matches above, loose matches on Subject: below -- 2002-12-28 5:56 Matthias Klose
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