Author: Anthony Cozzie
Date: 13:35:38 12/17/03
Go up one level in this thread
On December 17, 2003 at 15:06:14, Dieter Buerssner wrote:
>On December 17, 2003 at 14:51:38, Anthony Cozzie wrote:
>
>
>>suppose we make a slight change:
>>
>>void vectoradd(double *a, double *b, double *c, int len)
>>{
>> for(int i = 0; i < len; i++)
>> c[i+1] = b[i] + a[i];
>>}
>>
>>now the compiler cannot unroll the loop unless it knows that there is no
>>aliasing.
>
>Hmmm, I think the compiler can still unroll the loop. For example like this
>(untested, but you get the idea):
>
>void vectoradd(double *a, double *b, double *c, int len)
>{
> int l2 = len/2;
> for (i=0; i<l2; i+=2)
> {
> c[i+1] = b[i] + a[i];
> c[i+2] = b[i+1] + a[i+1];
> }
> /* And now handle odd numbers ... */
>}
Oops. I was thinking vectorizer. Note that you still can't do the two adds in
parallel.
>What the compiler cannot do (in your first and in your second source snippet)
>for example: He cannot load a[i] and a[i+1] into two registers, same for b, and
>then do two adds with the four registers. It could do this when the pointers
>were restrict.
>
>A better example may be,
>
>void copy_pv(pv_t *dest, pv_t *src, size_t n)
>{
> int i;
> for (i=0; i<n; i++)
> dest[i] = src[i];
>}
>
>Now assume pv_t is unsigned char, and n is long. An obvious optimization would
>be to copy the elements word wise (for example 4 chars on typical hardware). The
>compiler cannot do this optimization. You might call
>
> copy_pv(pv_array, pv_array+1, m);
>
>and that would go wrong. So this optimization must not be done. However, when
>the pointers are restricted (and then, you never can call copy_pv like above),
>the compiler can do this optimization. For example it will be secure, when you
>typically only use calls like
>
> copy_pv(pv1_array, pv2_array, m);
>
>where pv1_array and pv2_array are different objects (no overlap).
>
>Regards,
>Dieter
very true. Of course, anyone who writes code that uses aliasing heavily should
be smacked with a wet towel :)
anthony
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