Messages in this thread | | | From | "Alexander van Heukelum" <> | Subject | Re: Alternative implementation of the generic __ffs | Date | Mon, 21 Apr 2008 13:43:16 +0200 |
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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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