Re: Alternative implementation of the generic __ffs

[dean gaudet]

On Fri, 18 Apr 2008, Joe Perches wrote:

> On Fri, 2008-04-18 at 18:11 -0700, dean gaudet wrote: 
> > have you benchmarked it?
> 
> I modified Alexander's benchmark:
> http://lkml.org/lkml/2008/4/18/267
> to include 32 and 64 bit variants called smallest.
> 
> On an old ARM:

i'm guessing the 32-bit constants suck :(

the code could be modified to use 16-bit constants only -- it would add 
some dependent operations though (to move the hot bit into the low 
16-bits).

-dean


> 
> $ gcc --version gcc (GCC) 3.4.6
> 
> $ cat /proc/cpuinfo 
> Processor	: Intel StrongARM-110 rev 4 (v4l)
> BogoMIPS	: 262.14
> Hardware	: Rebel-NetWinder
> Revision	: 57ff
> Serial		: 000000000000185c
> 
> $ gcc -Os -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3180 tics,  8379 tics
> New:            4280 tics,  8890 tics
> Smallest:       4027 tics,  7835 tics
> Empty loop:     1543 tics,  2260 tics
> 
> $ gcc -O2 -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3161 tics,  7843 tics
> New:            4778 tics,  8783 tics
> Smallest:       4408 tics,  7149 tics
> Empty loop:     1515 tics,  2140 tics
> 
> $ gcc -O3 -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3078 tics,  7692 tics
> New:            4714 tics,  8671 tics
> Smallest:       4344 tics,  7117 tics
> Empty loop:     1444 tics,  2024 tics
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