Benjamin Redelings I wrote:
I noticed that (in gcc 4.5 as of Oct-18) the following code is not
vectorized:
float sum=0;
int i;
for(i=0;i<16;i++)
sum += f1[i]*f2[i];
The error is "unhandled use in statement"
However, the web page at
http://gcc.gnu.org/projects/tree-ssa/vectorization.html says:
"Detection and vectorization of special idioms, such as dot-product and
widening-summation: Incorporated into GCC 4.2."
Can you tell me if I am missing something? Is the web page correct?
I haven't seen vector sum or dot product reduction except with the use
of -ffast-math. At one time, it was said that -fassociative-math also
should permit it. It's more effective with -mtune=barcelona
(particularly for CPUs introduced the last 2 years).
With those options, gcc/g++/gfortran are fairly good at dot product
vectorization, both traditional code such as you show, and
dot_product/inner_product.
In my opinion, it's unfortunate not having an option to enable this
optimization independent of riskier ones.
