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

[Ingo Molnar]

* Michael S. Tsirkin <mst@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > >* Michael S. Tsirkin <mst@xxxxxxxxxx> 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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