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

* Alexander van Heukelum <heukelum@mailshack.com> 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

ok, that's rather convincing.

the generic version in lib/find_next_bit.c is open-coded C which gcc can
optimize pretty nicely.

the hand-coded assembly versions in arch/x86/lib/bitops_32.c mostly use
the special x86 'bit search forward' (BSF) instruction - which i know
from the days when the scheduler relied on it has some non-trivial setup
costs. So especially when there's _small_ bitmasks involved, it's more
expensive.

> 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.

i'm not surprised that the hand-coded assembly versions had a bug ...

[ this means we have to test it quite carefully though, as lots of code
only ever gets tested on x86 so code could have built dependency on
the buggy behavior. ]

> Using the generic version does give a slightly bigger kernel, though.
>
> defconfig: text data bss dec hex filename
> x86-specific: 4738555 481232 626688 5846475 5935cb vmlinux (32 bit)
> generic: 4738621 481232 626688 5846541 59360d vmlinux (32 bit)
> x86-specific: 5392395 846568 724424 6963387 6a40bb vmlinux (64 bit)
> generic: 5392458 846568 724424 6963450 6a40fa vmlinux (64 bit)

i'd not worry about that too much. Have you tried to build with:

CONFIG_CC_OPTIMIZE_FOR_SIZE=y
CONFIG_OPTIMIZE_INLINING=y

(the latter only available in x86.git)

> Patch is against -x86#testing. It compiles.

i've picked it up into x86.git, lets see how it goes in practice.

Ingo


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