Messages in this thread | | | Date | Sun, 9 Mar 2008 21:10:16 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Change x86 to use generic find_next_bit |
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* 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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