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Re: Option to make unsigned->signed conversion always well-defined?
On Fri, Oct 7, 2011 at 10:37 AM, Andrew Haley <aph@redhat.com> wrote:
> On 10/06/2011 07:24 PM, Ulf Magnusson wrote:
>> bool overflow_unsafe(unsigned int a, unsigned int b) {
>> ? ? const unsigned int sum = (int8_t)a + (int8_t)b;
>> ? ? return (int8_t)sum != sum;
>> }
>>
>> bool overflow_safe(unsigned int a, unsigned int b) {
>> ? ? const int sum = as_signed_8(a) + as_signed_8(b);
>> ? ? return as_signed_8(sum) != sum;
>> }
>
> I get this:
>
> overflow:
> ? ? ? ?sxtb ? ?r1, r1
> ? ? ? ?sxtab ? r1, r1, r0
> ? ? ? ?sxtb ? ?r0, r1
> ? ? ? ?subs ? ?r0, r0, r1
> ? ? ? ?it ? ? ?ne
> ? ? ? ?movne ? r0, #1
> ? ? ? ?bx ? ? ?lr
>
> It's not obvious to me how you'd improve on it.
>
> Andrew.
>
Nah, might be the best all-around version. Seems to generate the
nicest code when you make use of the sum afterwards as well.
/Ulf