Messages in this thread | | | Subject | Re: [PATCH 1/2] x86/bitops: implement __test_bit | From | yalin wang <> | Date | Mon, 31 Aug 2015 16:15:02 +0800 |
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> On Aug 31, 2015, at 15:59, Ingo Molnar <mingo@kernel.org> wrote: > > > * Michael S. Tsirkin <mst@redhat.com> wrote: > >> On Sun, Aug 30, 2015 at 11:13:20PM -0700, H. Peter Anvin wrote: >>> Presumably because gcc can't generate bt... whether or not it is worth it is another matter. >>> >>> On August 30, 2015 11:05:49 PM PDT, Ingo Molnar <mingo@kernel.org> wrote: >>>> >>>> * Michael S. Tsirkin <mst@redhat.com> wrote: >>>> >>>>> +static __always_inline int __constant_test_bit(long nr, const >>>> unsigned long *addr) >>>>> +{ >>>>> + return ((1UL << (nr & (BITS_PER_LONG-1))) & >>>>> + (addr[nr >> _BITOPS_LONG_SHIFT])) != 0; >>>>> +} >>>>> + >>>>> +static inline int __variable_test_bit(long nr, const unsigned long >>>> *addr) >>>>> +{ >>>>> + int oldbit; >>>>> + >>>>> + asm volatile("bt %2,%1\n\t" >>>>> + "sbb %0,%0" >>>>> + : "=r" (oldbit) >>>>> + : "m" (*addr), "Ir" (nr)); >>>>> + >>>>> + return oldbit; >>>>> +} >>>> >>>> Color me confused, why use assembly for this at all? >>>> >>>> Why not just use C for testing the bit (i.e. turn __constant_test_bit() >>>> into >>>> __test_bit()) - that would also allow the compiler to propagate the >>>> result, >>>> potentially more optimally than we can do it via SBB... >>>> >>>> Thanks, >>>> >>>> Ingo >> >> Exactly: >> >> >> Disassembly of section .text: >> >> 00000000 <__variable_test_bit>: >> __variable_test_bit(): >> 0: 8b 54 24 08 mov 0x8(%esp),%edx >> 4: 8b 44 24 04 mov 0x4(%esp),%eax >> 8: 0f a3 02 bt %eax,(%edx) >> b: 19 c0 sbb %eax,%eax >> d: c3 ret >> e: 66 90 xchg %ax,%ax >> >> 00000010 <__constant_test_bit>: >> __constant_test_bit(): >> 10: 8b 4c 24 04 mov 0x4(%esp),%ecx >> 14: 8b 44 24 08 mov 0x8(%esp),%eax >> 18: 89 ca mov %ecx,%edx >> 1a: c1 fa 04 sar $0x4,%edx >> 1d: 8b 04 90 mov (%eax,%edx,4),%eax >> 20: d3 e8 shr %cl,%eax >> 22: 83 e0 01 and $0x1,%eax >> 25: c3 ret > > But that's due to the forced interface of generating a return code. Please compare > it at an inlined usage site, where GCC is free to do the comparison directly and > use the result in flags. just curious : it seems __variable_test_bit() use less instructions, why not always use __variable_test_bit() , remove __constant_test_bit() version ?
Thanks
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