youshi10 at u.washington.edu
Fri Mar 16 03:25:45 UTC 2007
Gary Kline wrote:
> On Wed, Mar 14, 2007 at 08:19:49PM -0700, youshi10 at u.washington.edu wrote:
>> On Thu, 15 Mar 2007, Danny Pansters wrote:
>>> On Thursday 15 March 2007 02:16, Gary Kline wrote:
>>>> Two quick one for kernel and/or compiler wizards: first, is
>>>> a 400Mz processor considered a 586 (for my KERNELCONF file)?
>>> Think its 686 (but really, leaving 486 and 586 in isn't going to slow down
>>> booting or anything!) I always say: Use GENERIC unless you have a good
>>> not to.
>>>> Second, is it safe to do a buildworld with -O3? If there are
>>> No. It's not supported if things break.
>>>> stability concerns, I'll go with the default when I rebuild my
>>>> 6.2 systems.
>>> The defaults should be fine. Also, like I said consider just using GENERIC
>>> load the odd kmod if needed. Generally it's less headache and equal
>>>> thanks in advance,
>> As Dan and Gary said -O3 isn't supported, and in many cases that "level of
>> optimization" gets filtered out while compiling sections of FreeBSD.
>> Besides, I've compiled stuff with -O3 and various optimizations in Gentoo
>> Linux before, and let me say that it caused a great deal of headaches...
>> that's why I stick with -O2 now, because it's better to have something in
>> executable shape and a bit slower (arguably because some optimizations slow
>> things down) than it is to have something run fast and break all the time.
>> Some food for thought :).
> --Food for thought and a chuckle too! (not to mention that
> it's waaay early, the chickens are still snoring, and I've
> only had *one* cup of joe)... I've done some investigation
> with optimizing my own code, usually < 1000 lines, and haven't
> seen much gain between -O2 and -O3. Loop-unrolling may be
> different; one trick that compiler hackers at supercomputer
> companies use by default in to unroll small loops. Cray is
> one example. Soooo, to get any real gain is going to mean
> going thru the most freq used tools (*grep, find, ls) and
> hand-tweak. Might buy 5 - 7%.
> have a good one,
No problem. -funroll-loops might not buy you too much other than a few
less instructions overall but I'm not sure how intelligent gcc is at
unrolling loops. It seemed like there was a difference between
optimizations in the 4.x branch compared to the 3.4.x sub branch. They
made a lot of improvements in the 4.x branch though.. it's just that
some of those improvements broke code, so that's probably why FreeBSD
doesn't have gcc-4.x in the base system.
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