[Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too)

["rakdver at kam dot mff dot cuni dot cz" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #3 from rakdver at kam dot mff dot cuni dot cz  2010-07-24 20:41 -------
Subject: Re:  Redundant prefetches for some
        loops (vectorizer produced ones too)

> > The direct reason is that prefetching could not differentiate the base
> > addresses
> > of the vectorized load and store (of a[i]):
> > *vect_pa.6_24
> > *vect_pa.19_37
> 
> Here is a testcase which shows the same issue without the vectorizer (compile
> -O2 -fprefetch-loop-arrays -march=amdfam10 -fno-tree-ccp -fno-tree-vrp
> -fno-tree-dominator-opts):
> float *f();
> float aa[1024];
> float bb[1024];
> void foo(int beta)
> {
>   int i;
>   float *a = aa, *a1 = aa, *b = bb;
>   for(i=0; i<1024; i++)
> {
>      *a = *a1 + beta * *b;
> a++; a1++; b++;
> }
> }

I am not sure that this issue should be addressed in the prefetching pass; as
this
example shows, we already have three other passes that deal with it ordinarily.
Perhaps adjusting the vectorizer code generation or scheduling copy propagation
after vectorizer would be better.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021