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

On Sun, Jan 17, 2016 at 6:38 AM, Andreas Seltenreich <seltenreich(at)gmx(dot)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.

--
Robert Haas
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