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    SubjectRe: [PATCH] x86: Change x86 to use generic find_next_bit

    On Thu, 13 Mar 2008 18:14:24 +0530, "Aneesh Kumar K.V"
    <> said:
    > On Sun, Mar 09, 2008 at 09:01:04PM +0100, Alexander van Heukelum wrote:
    > > x86: Change x86 to use the generic find_next_bit implementation
    > >
    > > The versions with inline assembly are in fact slower on the machines I
    > > tested them on (in userspace) (Athlon XP 2800+, p4-like Xeon 2.8GHz, AMD
    > > Opteron 270). The i386-version needed a fix similar to 06024f21 to avoid
    > > crashing the benchmark.
    > >
    > > Benchmark using: gcc -fomit-frame-pointer -Os. For each bitmap size
    > > 1...512, for each possible bitmap with one bit set, for each possible
    > > offset: find the position of the first bit starting at offset. If you
    > > follow ;). Times include setup of the bitmap and checking of the
    > > results.
    > >
    > > Athlon Xeon Opteron 32/64bit
    > > x86-specific: 0m3.692s 0m2.820s 0m3.196s / 0m2.480s
    > > generic: 0m2.622s 0m1.662s 0m2.100s / 0m1.572s
    > >
    > > If the bitmap size is not a multiple of BITS_PER_LONG, and no set
    > > (cleared) bit is found, find_next_bit (find_next_zero_bit) returns a
    > > value outside of the range [0,size]. The generic version always returns
    > > exactly size. The generic version also uses unsigned long everywhere,
    > > while the x86 versions use a mishmash of int, unsigned (int), long and
    > > unsigned long.
    > This problem is observed on x86_64 and powerpc also.

    I'm not entirely sure if it is a problem. In some cases it
    certainly is an inconvenience, though ;). I mentioned the
    difference between the old and generic versions, because
    of the possibility of dependence of this behaviour.

    Indeed I see for example (in fs/ext4/mballoc.c).

    bit = mb_find_next_zero_bit(bitmap_bh->b_data, end, bit);
    if (bit >= end)
    next = mb_find_next_bit(bitmap_bh->b_data, end, bit);
    if (next > end)
    next = end;
    free += next - bit;

    So here it needed to adjust the value.

    > We need a long
    > aligned address for test_bit, set_bit find_bit etc. In ext4 we have
    > to make sure we align the address passed to
    > ext4_test_bit
    > ext4_set_bit
    > ext4_find_next_zero_bit
    > ext4_find_next_bit
    > fs/ext4/mballoc.c have some examples.

    This is a different 'problem'. find_next_bit works on arrays of long,
    while the bitmaps in ext4_find_next_bit are of type void * and seem
    not to have any alignment restrictions. ext4 implements wrappers
    around find_next_bit to solve that 'problem'.

    The question that arises is: do we want find_first_bit, find_next_bit.
    etc. to always return a value in the range [0, size], or do we want
    to allow implementations that return [0, size-1] if there is a bit
    found and something else (roundup(size,bitsperlong) or ulongmax, for
    example if, none were found?

    The current x86_64 versions of find_first_bit and find_next_bit return
    roundup(size,bitsperlong) if no bits were found, but on the other hand
    I guess most bitmaps are a multiple of bitsperlong bits in size, which
    hides the difference.


    > -aneesh
    Alexander van Heukelum

    -- - Or how I learned to stop worrying and
    love email again

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