* yalin wang <yalin.wang2...@gmail.com> wrote:
>
> > 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 ?
It's an artifact of testing it in isolation.
For constant bit positions GCC is able to do a fairly good job:
ffffffff8103d2a0 <vmx_get_rflags>:
ffffffff8103d2a0: f6 87 4a 02 00 00 08 testb $0x8,0x24a(%rdi)
...
ffffffff8103d2ab: 75 39 jne ffffffff8103d2e6
<vmx_get_rflags+0x46>
with just 2 instructions: a TESTB plus using the flag result in a JNE.
Using variable_test_bit() forces the result into a register, which is
suboptimal.
Thanks,
Ingo
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