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Subject: Re: [PATCH 1/2] x86/bitops: implement __test_bit
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Sun, 30 Aug 2015 23:13:20 -0700
To: Ingo Molnar <mingo@kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>
CC: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Rusty Russell <rusty@rustcorp.com.au>,
        Linus Torvalds <torvalds@linux-foundation.org>
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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@kernel.org> wrote:
>
>* Michael S. Tsirkin <mst@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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