Re: [PATCH] x86 rwsem optimization extreme

H. Peter Anvin
2010-02-17T22:30:32Z

On 02/17/2010 02:10 PM, Linus Torvalds wrote:
>
>
> On Wed, 17 Feb 2010, Zachary Amsden wrote:
>>
>> The x86 instruction set provides the ability to add an additional
>> bit into addition or subtraction by using the carry flag.
>> It also provides instructions to directly set or clear the
>> carry flag. By forcibly setting the carry flag, we can then
>> represent one particular 64-bit constant, namely
>>
>> 0xffffffff + 1 = 0x100000000
>>
>> using only 32-bit values. In particular we can optimize the rwsem
>> write lock release by noting it is of exactly this form.
>
> Don't do this.
>
> Just shift the constants down by two, and suddenly you don't need any
> clever tricks, because all the constants fit in 32 bits anyway,
> regardless of sign issues.
>

Why bother at all? I thought it mattered when I saw __downgrade_write()
as an inline, but in fact it is only ever used inside the
downgrade_write() out-of-line function, so we're talking about saving
*five bytes* across the whole kernel in the best case. I vote for
leaving it the way it is, and get the very slight extra readability.
There is no point in moving bits around, either.

-hpa