"unhandled use" in vectorizing a dot product?

[Tim Prince]

Benjamin Redelings I wrote:


> 
> 2. Interestingly, the following is recognized WITHOUT -ffast-math:
> 
>   for(i=0;i<argc;i++)
>     f4[i] += f1[i]*f2[i]*f3[i];
> 
That's not a reduction; re-association from strict C standard order 
isn't required to vectorize it.
icc makes a similar distinction, this should vectorize when icc 
-fp-model source is set (as well as not set), for example.
> If I change this to the following, then it needs -ffast-math:
> 
>   for(i=0;i<argc;i++)
>     sum += f1[i]*f2[i]*f3[i];
> 
> This is essentially doing the first thing, plus also summing the f4[i].  
> I guess that is the problem?
Yes, vectorization involves at least 4 parallel sums (for float data 
type), adding the partials at the end, with numerically different result 
from the non-vector case (often, but not always, slightly more 
accurate).  Also, possibly varying slightly with alignment, and possibly 
  differing according to whether -msse3 is set.