Re: cmpxchg and x86 flags output

From: H. Peter Anvin
Date: Tue Jun 21 2016 - 13:31:10 EST


On 06/21/16 02:06, David Howells wrote:
>
> However, there's probably not a great deal of difference to be had if the
> inline asm codes the appropriate instruction in each case for something like
> x86*. The emitted code ought to look the same. The second biggest win for
> the intriniscs, I think, is the ability to ask the CMPXCHG instruction whether
> it actually did anything rather than comparing the result. I added two
> variants, one that only returned the yes/no and one that passed back the value
> as well as the yes/no.
>

Right, and we want that either way. The API change that you are
proposing is definitely what we want; the specifics of the x86
implementation is sort of orthogonal.

-hpa