Robert Bradshaw wrote:
> On May 13, 2009, at 2:38 PM, Carl Witty wrote:
>
>> On Wed, May 13, 2009 at 1:49 PM, Dag Sverre Seljebotn
>> <[email protected]> wrote:
>>> Yes, but these are only used in temporaries which are presumably
>>> read just
>>> after. Wouldn't the compiler be able to optimize it? E.g.
>>>
>>> short a = ..., b = ..., c;
>>> long long long long int temp;
>>>
>>> temp = a + b;
>>> c = temp;
>>>
>>> It seems plausible that the compiler could decide that temp could
>>> be made
>>> a short (?). But it definitely needs investigation to see exactly
>>> which
>>> sort of situations the knowledge about the size is used in.
>> This doesn't work in more complicated situations.
>>
>> unsigned int a=..., b=..., c=..., d = ..., result;
>> unsigned long long temp1, temp2, temp3, temp4, temp5, temp6;
>>
>> temp1 = a+1;
>> temp2 = b+1;
>> temp3 = c+1;
>> temp4 = d+1;
>> temp5 = temp1*temp2;
>> temp6 = temp3*temp4;
>> result = temp5/temp6;
>>
>> And assume that int is 32 bits, long long is 64 bits.
>>
>> Here C semantics guarantee that temp5 and temp6 are computed to full
>> 64-bit resolution, and that the division is done in 64 bits (and then
>> truncated to 32 bits). But if the temporaries were unsigned int,
>> instead, then temp5 and temp6 would be truncated to 32 bits before
>> doing the division. So the C compiler can't possibly optimize temp5
>> and temp6 to unsigned int. (I believe it could optimize temp1 ..
>> temp4 to unsigned int, as long as it still used 32x32->64 multiplies
>> to compute temp5 and temp6.)
>>
>> I certainly have written C code that depends on the compiler exactly
>> following the C semantics for overflow on unsigned types. I don't
>> remember if I've written such code in Cython (probably not).
>
> I think this is a valid point too. In particular, with python
> division semantics, it must store the denominator in a temp in order
> to detect whether or not to raise a ZeroDivisionError.
What one could do is:
short temp_short
int temp_int
long long temp_longlong
switch (sizeof(footype)) {
case sizeof(short): temp_short = ....
case sizeof(int): temp_short = ....
case sizeof(long long): temp_short = ....
}
....
result... = ((sizeof(footype) == sizeof(short) ? temp_short : ...)
I sure hope I think of something better though :-)
Dag Sverre
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