gcc-4.3 considers unaligned accesses on X86 as undefined

gcc-4.3 considers unaligned accesses on X86 as undefined

[Török Edwin]

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

Re: gcc-4.3 considers unaligned accesses on X86 as undefined

[H. Peter Anvin]

Török Edwin wrote:
> 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.
> 

Generating vectorized code in the kernel is death anyway, so I don't 
think the change in alignment is an issue.  We CANNOT ALLOW vectorized 
code in the kernel under any circumstances (well, except when surrounded 
by the appropriate protection constructs.)

	-hpa

Re: gcc-4.3 considers unaligned accesses on X86 as undefined

[Alan Cox]

On Tue, 25 Mar 2008 22:51:09 +0200
Török Edwin <edwintorok@gmail.com> wrote:

> 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 

FPU/MMX/SSE are not available or usable for the kernel anyway fortunately
in this case.

Alan

Re: gcc-4.3 considers unaligned accesses on X86 as undefined

[Andi Kleen]

Török Edwin <edwintorok@gmail.com> writes:

> 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)

The 2.6 kernel disables SSE and MMX code generation explicitely.
2.4 should be also safe unless someone misguided recompiles it with -O3.

-Andi