On Jun 17, 2011, at 5:26 AM, Richard Guenther wrote:
> On Fri, Jun 17, 2011 at 11:14 AM, Bingfeng Mei <[email protected]> wrote:
>> Hi,
>> I noticed that GCC converts short arithmetic to unsigned short.
>>
>> short foo2 (short a, short b)
>> {
>> return a - b;
>> }
>>
>> In .gimple file:
>>
>> foo2 (short int a, short int b)
>> {
>> short int D.3347;
>> short unsigned int a.0;
>> short unsigned int b.1;
>> short unsigned int D.3350;
>>
>> a.0 = (short unsigned int) a;
>> b.1 = (short unsigned int) b;
>> D.3350 = a.0 - b.1;
>> D.3347 = (short int) D.3350;
>> return D.3347;
>> }
>>
>> Is this for some C standard conformance, or optimization purpose?
>> This doesn't happen with int type.
>
> GCC optimizes (short)((int)a - (int)b) (which is what your source code example
> does) to (short)((unsigned short)a - (unsigned short)b) because it
> cannot shorten it to use signed short as the operation may overflow in
> that type which would result in undefined behavior. unsigned types
> do not suffer from this problem.
But how did it end up with ...(int)a ... when the source declares a as short?
paul
