[gentoo-amd64] Re: FW: GCC4 and -ftree-vectorize

["Duncan" <1i5t5.duncan@cox.net>]

"Duncan" <1i5t5.duncan@cox.net> posted efqqvt$sqh$6@sea.gmane.org,
excerpted below, on  Mon, 02 Oct 2006 10:51:41 +0000:

> I'll probably have a bit more to say about it after checking the
> tinyurls, which at this point I don't even know where they go yet, let
> alone their content.
> 
> <quote from Ryan Hill>
> 
> Duncan wrote:
> 
>> Could you point me at some info on this one (-ftree-vectorize)?  It
>> came up on the amd64 list a week or so ago[]
> 
> http://tinyurl.com/l75we

OK, this one's the gcc bugtracker list of -ftree-vectorize bugs.  Lots of
reports, many of them reduced to skeleton programs with just the
triggering code. Of note, /most/ of the issues involve x86/ppc and/or size
increasing loop optimizations that aren't of interest to those whose
strategy targets -Os in any case. For example, one of the triggers seems
to be -funswitch-loops, enabled by default with -O3, another is
-funroll-loops, neither of which (again) are of interest to those
targeting size for cache efficiency among other reasons.

> See http://tinyurl.com/rt3aa for some real-world examples.

This one's the Gentoo bugtracker -ftree-vectorize related bug list. 
Again, most are x86/ppc and/or involve flags folks interested in -Os
won't be worrying about.

Looks like on amd64 it may be less trouble than -combine is, and Flameeyes
said he has been using it for awhile, so it looks like it'll be standard
in my cflags now, to be removed only where it seems to cause problems,
which shouldn't be that often, hopefully.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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