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SubjectRe: Alternative implementation of the generic __ffs
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On Sun, 20 Apr 2008 15:31:46 +0300, "Matti Aarnio"
<matti.aarnio@zmailer.org> said:
> On Sun, Apr 20, 2008 at 10:42:21AM +0200, Alexander van Heukelum wrote:
> > On Sat, 19 Apr 2008 20:06:57 -0700, "Joe Perches" <joe@perches.com>
> > said:
> > > On Sun, 2008-04-20 at 01:29 +0300, Matti Aarnio wrote:
> > > > I am curious, why not take the code already in glibc ffs() for ARM ?
> > > > That is, if the ffs() is all that important detail in kernel ?
> >
> > Hi,
> >
> > The glibc version is based on a table-lookup. This makes it
> > behave differently in hot and cold cache situations. That's
> > fine if __ffs is used in tight loops, but in the kernel such
> > use of __ffs is avoided because it might be slow. I added it
> > to the benchmark, but it would need testing for the cold
> > cache case too.
> >
> > As for the importance of __ffs in the kernel: as far as I
> > know the hot-spots in the kernel using __ffs are the
> > schedular (sched_find_first_bit) and the cpu mask walking
> > code (for_each_cpu_mask).
>
> Perhaps those hot-spots would benefit from more broadly
> accelerable algorithms.

That would be a possibility too ;). The advantages of bitmaps
are that they are so compact and so easy to understand.

> ARM architecture v5 introduced
> a CLZ instruction -- Count Leading Zeroes.

Yeah, if such an instruction exist, the arch should provide
optimized versions for the bit functions. The interest here
was to provide a (beter) generic version in pure C for
architectures without such instructions.

Inline assembly versions are indeed provided in
asm-arm/bitops.h for ARM5+.

Greetings,
Alexander

> Well, gcc's __builtin_ffs() for ARM Arch5 and up (including
> XScale) does things in a bit more interesting way:
>
> http://mail-index.netbsd.org/port-arm/2002/08/20/0001.html
>
> $ cat try.c
> int foo(int i)
> {
> return __builtin_ffs(i);
> }
> $ arm-gp2x-linux-gcc -S -O -march=armv5 try.c
> $ more try.s
> .file "try.c"
> .text
> .align 2
> .global foo
> .type foo, %function
> foo:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> @ lr needed for prologue
> rsb r3, r0, #0
> and r3, r3, r0
> clz r3, r3
> rsb r0, r3, #32
> bx lr
> .size foo, .-foo
> .ident "GCC: (GNU) 4.1.2 (Fedora GP2X 4.1.2-8.fc9)"
>
>
> > Greetings,
> > Alexander
>
> /Matti Aarnio
--
Alexander van Heukelum
heukelum@fastmail.fm

--
http://www.fastmail.fm - I mean, what is it about a decent email service?



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