linux-kernel - Re: [PATCH 1/2] x86/bitops: implement __test

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Message-ID: <0779C35A-141F-4019-942A-CD3F861048A3@zytor.com>
Date:	Sun, 30 Aug 2015 23:13:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86/bitops: implement __test_bit

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@...nel.org> wrote:
>
>* Michael S. Tsirkin <mst@...hat.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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