> On Aug 31, 2015, at 15:59, Ingo Molnar <mi...@kernel.org> wrote:
>
>
> * Michael S. Tsirkin <m...@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 <mi...@kernel.org> wrote:
>>>>
>>>> * Michael S. Tsirkin <m...@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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