Re: gcc4.0 autovect-branch

[Daniel Berlin <dberlin at dberlin dot org>]

On Mon, 20 Dec 2004, Villemin Ryusuke wrote:

> Thank you for your reply!
> I was expecting an output like icc, so I checked the assembler code file 
> ("-S") but didn t see any vector instruction...

we've meant to add a flag to output info like icc does when compiling, and 
we have the info to do so, it's just nobody actually went and did it :)

> It was because I declared my arrays local to the foo function:
>
> foo () {
> float a[256], b[256], c[256];
> int i;
> for (i=0; i<256; i++){
>  a[i] = b[i] + c[i];
> }
> }
>
> the vect output is :
> loop at vecto.c:9: not vectorized: unsupported data-type
You never use the results, so we delete the computations when they are 
local.

Thus, the reason it's not vectorizing is because there is nothing left to 
vectorize.
Your loop looks like this by that point:

foo () {
int i;
for (i = 0; i < 256; i++) {
}
}

If you add another loop or something that uses the value of a[i], so that 
it's actually needed, you'll see we vectorize the loop.

for example, add

  for (i=0; i<256; i++)
        printf ("%f\n", a[i]);

and we'll vectorize your loop.