linux-kernel - gcc-4.3 considers unaligned accesses on X86 as undefined

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Message-ID: <47E965BD.7010604@gmail.com>
Date:	Tue, 25 Mar 2008 22:51:09 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	"Thomas Gleixner mingo@...hat.com" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: gcc-4.3 considers unaligned accesses on X86 as undefined

Hello x86 architecture maintainers,

GCC-4.3 now considers that it is undefined behaviour to access memory
through an int*  that is not aligned to sizeof(int).
At -O3 it generates vectorized code that  _relies_  on the fact that
pointers are always aligned (unless you use packed attributes, etc.),
and the resulting code crashes if the pointer is unaligned.  (-O3 -msse
on 32-bit, and simply -O3 on 64-bit since -msse is default)
See this gcc bugreport: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35653
[I do not really agree with this sudden change, because unaligned
accesses have always been possible on x86, but the C99 standard does say
it is undefined behaviour ...]

I thought to inform you of this change in gcc's behaviour, because
include/asm-x86/unaligned.h is no longer safe in the above context,
especially that it is being used in a loop:
http://lxr.linux.no/linux/net/bluetooth/bnep/core.c#L153

P.S.: I only compile my kernels with -O2, so I don't know if it actually
crashes or not at -O3.

Best regards,
--Edwin

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