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    SubjectRe: Alternative implementation of the generic __ffs
    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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