* 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.
Thanks,
Ingo
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