Re: Optimizations documentation

[Dorit Nuzman <DORIT at il dot ibm dot com>]

> Ira Rosen wrote:
> > Here is the link to the vectorizer's documentation:
> > http://gcc.gnu.org/projects/tree-ssa/vectorization.html
> >
> >
> >
> Thanks, but I take what it says there with some grains of salt.

?? please report any inaccuracies/outdated information you find, I'd like
to fix any such issues.

> Yes,
> -O3 implies -ftree-vectorize on x86_64, but I seem to have to specify
> the option on other targets.

tree-vectorize is enabled under -O3, for all targets. I don't know of any
target that disabled it. There may be however targets that require
additional flags to enable vectorization (like -maltivec for powerpc).
Specifically, on which targets does "-O3" behaves differently than "-O3
-ftree-vectorize"?

> Also, I have not understood the
> limitations of -freassociative-math.

I think this is a typo - should be "-fassociative-math" (I'll fix that;
here's the related thread:
http://gcc.gnu.org/ml/gcc-patches/2007-09/msg00183.html).

> Several gcc vectorization test cases currently throw FAIL on certain
> targets, but not on others.  I don't know if that is a significant issue.

please report specific testcases+targets... (there are already at least a
couple open PRs for failing testcases - 34168 and 34437 are the most recent
ones I think. there may be more).

> g++-4.3 seems well ahead of other compilers in ability to vectorize STL
> iterators:
> http://softwarecommunity.intel.com/Wiki/HighPerformanceComputing/688.htm
>

good to know.
I browsed over the posting above; do you have a summary table where one
could easily see which loops get vectorized by other compilers and not by
gcc (and vice versa)?

thanks,
dorit