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Subject: C optimization: int vs char

Author: Travers Waker

Date: 05:38:43 10/10/01


Hi.

I recently read that using word-sized variables for a particular machine (e.g. a
32-bit int on an Intel PC CPU) yields faster code than using variables that are
not the size of a word.

For example, I understood the article to be saying that if you needed an index,
i,  to access an array, A[256], then it would yield faster code to define:

int i;

instead of:

unsigned char i;

even though the unsigend char is big enough to be able to index all the elements
of the array.  I find this strange, especially since it seems to me that you
waste space in the memory caches when the most sinificant 3 bytes of the 32-bit
int are always going to be zero's.

Can someone explain to me why using word-sized variables should be faster?

Thanks

Travers



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