Messages in this thread | | | Date | Sun, 20 Apr 2008 15:31:46 +0300 | From | Matti Aarnio <> | Subject | Re: Alternative implementation of the generic __ffs |
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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. ARM architecture v5 introduced a CLZ instruction -- Count Leading Zeroes.
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
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