This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
RE: ARM constant folding bug?
On 03 August 2007 18:35, Nathan Froyd wrote:
> On Fri, Aug 03, 2007 at 06:24:06PM +0100, Paul Brook wrote:
>> On Friday 03 August 2007, Jonathan S. Shapiro wrote:
>>> Then it seems very curious that the constant folding should fail on this
>>> platform. Any idea what may be going on here?
>>
>> You're exploiting a hole in the C aliasing rules by accessing a 32-bit int
>> as type char. I tested several compilers (4.2, 4.1 and 3.4 x86, 4.2 m68k
>> and 4.2 arm) and the only one that eliminated the comparison was 3.4-x86.
>
> FWIW, rewriting it with the "obvious" union approach seems to give the
> desired results on 4.2 arm with and without -mbig-endian.
Shouldn't it also work by changing the (char *) cast to (unsigned char *),
since IIUIC the standard guarantees you're allowed to access any object as an
array of unsigned char and not get into aliasing difficulties?
cheers,
DaveK
--
Can't think of a witty .sigline today....