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
That's also probably why we still have variable_test_bit
for test_bit too. It's best to be consistent with that - do you agree?
Or would you rather drop that too?
--
MST
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