On 11/30/2009 05:37 PM, Tim Prince wrote:
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.
Thank you! I don't know how long it would have taken me to notice this.
Even better, I see that it can vectorize loops that marginalize
simultaneously multiple three numbers as well.
for(int i=0;i<16;i++)
sum += f1[i] * f2[i] * f3[i]?
Hmm... the point about -fassociative-math is a good point, presuming
that SSE math handles NaNs and Infs.
Also, perhaps the documentation should explicitly say somewhere that
vectorization can depend on flags like this. "unhandled use in
statement" certainly doesn't point the user to an idea of how to fix
this :-P
-BenRI
