Re: [PATCH 1/2] x86/bitops: implement __test_bit

[yalin wang <yalin.wang2010@gmail.com>]

> 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