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[Bug middle-end/17886] variable rotate and long long rotate should be better optimized



------- Comment #21 from michael dot meissner at amd dot com  2005-10-04 20:46 -------
Subject: RE:  variable rotate and long long rotate
 should be better optimized

Sorry, I got mixed up as to who the original poster was.

SSE2 is harder to use because it deals with 128 bit items instead of 64
bit (unless you are in 64-bit and working on TImode values).
Ultimately, it is a matter whether it is important enough for somebody
to spend a week or two of work to use the multimedia instructions for
this case.  I suspect in most cases, it might be better to isolate the
code and use #ifdef's and builtin functions/asm's.

-----Original Message-----
From: ak at muc dot de [mailto:gcc-bugzilla@gcc.gnu.org] 
Sent: Tuesday, October 04, 2005 4:40 PM
To: Meissner, Michael
Subject: [Bug middle-end/17886] variable rotate and long long rotate
should be better optimized



------- Comment #20 from ak at muc dot de  2005-10-04 20:39 -------
Newer linux does that of course, although not always in older releases.

But even in user space it's not a good idea to use SSE2 unless you
really need
it because it increases the cost of the context switch and costs an
exception
each time first in a timeslice.

P.S.: I was the original poster, but the application wasn't a kernel but
I
doubt
it's a good idea to use SSE2.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17886


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