On 03 August 2007 18:49, Joe Buck wrote:
> On Fri, Aug 03, 2007 at 06:45:55PM +0100, Dave Korn wrote:
>> 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?
>
> The standard makes the same guarantee about (char *). This is not
> "exploiting a hole"; it is standard C that you can access anything
> as (char *).
Well, the compiler's not doing anything invalid by failing to optimise a
constant expression, so everything's all still valid and compliant. Shame it
can't recognize the cast case same as the union case and optimise them the
same though.
cheers,
DaveK
--
Can't think of a witty .sigline today....
