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
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