postincrement behaviour (differences between dmd and gdc)

[Dmitry Olshansky]

On 20.04.2011 13:53, Jonathan M Davis wrote:
>> This was partially discussed before some time ago -
>> http://www.digitalmars.com/d/archives/digitalmars/D/postincrement_behaviour
>> _differences_between_dmd_and_gdc_47334.html
>>
>>
>> The following statement has different behaviours in dmd and gdc.
>>
>> int x;
>> x = x++;
>>
>>
>> Where the backend for DMD interprets it as:
>>
>>    tmp = x;
>>    x++;
>>    x = tmp;
>>
>> And the backend for GDC interprets it as:
>>
>>    x = x;
>>    x++;
>>
>>
>> Skipping all undefined behaviour/which one is correct talk, an alternative
>> suggestion would be to treat such nonsensical code as an error - like
>> "Cannot modify the value of a variable twice in the same expression." - as
>> this kind of thing I would expect to be almost always a bug.
> Actually, I would have expected postincrement and preincrement to be identical
> in this case. It was my understanding that the entire expression to the right
> of the assignment operator was evaluated before the assignment was made, but I
> guess I could see how that could be open to interpretation with regards to the
> postincrement operator.
>
No, I think post-increment is done after the whole expression is 
evaluated, see the common C idiom:

*p++ = *q++;
it's always interpreted as:
*p = *q;
p++,q++; // or q++,p++;


> Regardless, I think that making this sort of case an error would make good
> sense.
>
Agreed, using variable as an L-value and post-incrementing it in the 
same expression is plain mistake.

> - Jonathan M Davis


-- 
Dmitry Olshansky