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Subject: Re: Alternative implementation of the generic __ffs
From: Joe Perches <joe@perches.com>
To: Alexander van Heukelum <heukelum@fastmail.fm>
Cc: dean gaudet <dean@arctic.org>, Harvey Harrison <harvey.harrison@gmail.com>,
       Alexander van Heukelum <heukelum@mailshack.com>,
       Ingo Molnar <mingo@elte.hu>, Andi Kleen <andi@firstfloor.org>,
       LKML <linux-kernel@vger.kernel.org>
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Date: Sat, 19 Apr 2008 11:17:01 -0700
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On Sat, 2008-04-19 at 14:10 +0200, Alexander van Heukelum wrote:
> I've added that to the benchmark, which you can now find here:
> http://heukelum.fastmail.fm/ffs/.

retested on arm:

$ gcc -Os -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3170 tics,  8326 tics
New:            4214 tics,  8793 tics
Smallest:       4023 tics,  7733 tics
Small const:    3442 tics,  6188 tics
Empty loop:     1517 tics,  2243 tics

$ gcc -O2 -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3172 tics,  7832 tics
New:            4805 tics,  8790 tics
Smallest:       4405 tics,  7154 tics
Small const:    3442 tics,  5612 tics
Empty loop:     1516 tics,  2145 tics

$ gcc -O3 -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3080 tics,  7709 tics
New:            4723 tics,  8656 tics
Smallest:       4333 tics,  7121 tics
Small const:    3379 tics,  5483 tics
Empty loop:     1447 tics,  2016 tics

>  Testing the same with 
> "return x4 + x3 + x2 + x1 + x0;" as the last line would be
> interesting too.

Adding is slower:

$ gcc -Os -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3152 tics,  8310 tics
New:            4214 tics,  8789 tics
Smallest:       4024 tics,  7737 tics
Small const:    3538 tics,  6295 tics
Empty loop:     1517 tics,  2243 tics

$ gcc -O2 -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3184 tics,  7849 tics
New:            4790 tics,  8814 tics
Smallest:       4406 tics,  7161 tics
Small const:    3538 tics,  5806 tics
Empty loop:     1521 tics,  2153 tics

$ gcc -O3 -fomit-frame-pointer ffs.c
$ ./a.out
Original:       3091 tics,  7694 tics
New:            4718 tics,  8656 tics
Smallest:       4333 tics,  7124 tics
Small const:    3467 tics,  5687 tics
Empty loop:     1445 tics,  2066 tics