Re: [gentoo-amd64] gcc4 CFLAGS Was: gcc 4.1 upgrade - bad desktop interactivity anyone?

On Fri, Sep 15, 2006 at 04:47:14PM +0000, Duncan wrote:

> I'm unclear as to what "vectorization" means as used here.  My
> understanding of "vector" is as a synonym for "line", thus implying loop
> unrolling of some form or another, which will increase size.  As I
> explained in the grandparent, I believe such optimizations to be
> counterproductive on modern processors due to the extreme cost of cache
> misses as opposed to slight cycle inefficiencies.
>
> I am however aware that vectorization has a somewhat different meaning in
> programming terms than the above, but am not sufficiently educated on the
> topic to make an informed choice, so I've simply left gcc to go with its
> default choice given my overall stated intention of -Os.
>
> If you can sufficiently explain the concept to me such that I
> understand enough about it to feel comfortable going with other than the
> default (which means I can explain why I chose it and why it won't
> interfere with my overall strategy as outlined in the grandparent, or is
> worth it even if it does), I'd be very grateful! =8^)

Back in the day, vectorization was, I believe, a supercomputer SIMD
(single instruction multiple data) concept, where instruction operands
were pointers to data, so it would, for instance, add two arrays of
numbers to produce a third array.  Isn't this what the Altivec
instructions do?

I do not claim to know this is what gcc means, or even if the current
concept of vectorization has any relation to Yee Olde Concepte.

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