[cpp-threads] Failed compare-and-swap

[Boehm, Hans]

> From:  Lawrence Crowl
> 
> On 8/1/07, Herb Sutter <hsutter at microsoft.com> wrote:
> > It seems like there are two legitimate choices:
> >
> >   - CAS returns the old value, in which case a failed CAS should at 
> > least by default have acquire semantics because it's a read.
> >
> >   - CAS returns only a success/fail boolean, in which case a failed 
> > CAS need not have any ordering semantics.
> >
> > Does that make sense? It seems we can happily choose 
> either, but the 
> > choice affects the return type.
> 
> The current design returns a boolean and also modifies the 
> reference parameter.
> 
> I contend that for the spurious-fail CAS that we have, any 
> reasonable code will need to re-read the value in the case of 
> failure, and so separating the read from the CAS is 
> ultimately pointless.
I think that's often, but not always true.  I think I've written CAS
uses that check for a specific constant value, and are not preceded by a
read at all, e.g. if I need to store something in an empty slot, and it
really doesn't matter which of N slots I use.

Hans