Re: [PATCH] Improve spinlock inline assembly for x86.

On January 18, 2016 7:27:59 PM GMT+01:00, Robert Haas <robertmhaas@gmail.com> wrote:
>On Sun, Jan 17, 2016 at 6:38 AM, Andreas Seltenreich
><seltenreich@gmx.de> wrote:
>> I'm currently experimenting with just-in-time compilation using
>libfirm.
>> While discussing issues with its developers, it was pointed out to me
>> that our spinlock inline assembly is less than optimal.  Attached is
>a
>> patch that addresses this.
>>
>> ,----
>> | Remove the LOCK prefix from the XCHG instruction.  Locking is
>implicit
>> | with XCHG and the prefix wastes a byte.  Also remove the "cc"
>register
>> | from the clobber list as the XCHG instruction does not modify any
>flags.
>> |
>> | Reported by Christoph Mallon.
>> `----
>
>I did a Google search and everybody seems to agree that the LOCK
>prefix is redundant.  I found a source agreeing with the idea that it
>doesn't clobber registers, too:
>
>http://www.oopweb.com/Assembly/Documents/ArtOfAssembly/Volume/Chapter_6/CH06-1.html#HEADING1-85
>
>So I guess it would be safe to change this.  Scares me slightly,
>though.

Clobbers IIRC are implicit on x86 anyway. Rather doubt that the space for the prefix makes any sorry of practical
difference,but there indeed seems no reason to have it.
 

Andres

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