Proposal: provide cas and barriers symbols even without -threaded

Ryan Newton rrnewton at
Thu Aug 1 18:32:00 CEST 2013

On Sun, Jul 21, 2013 at 3:32 AM, Carter Schonwald <
carter.schonwald at> wrote:

> ok, could you add those comments (about additional operations to consider)
> to the ticket?

Sure.  Just did that.

> relatedly: if we want these atomic ops to use the sequential analogues
> when we're not using the threaded run time system, does that mean
> we need to have a symbol / constant variable exposed in the RTS we link
> in, so that the inline code branches on a linktime constant value / symbol
> (something like "isThreadedRTS:: Bool", )  or some sort of analogue
> thereof?

I think it will take some care to mimic the semantics perfectly.  Why not
just leave the real atomic ops even in non-threaded mode, at least at
first?  Later we can optimize it if we find that people are using
concurrent data structures heavily in non-threaded mode ;-).

> one nice thing about doing such, is that if at some point link time
> optimization is added, the branch would go away! On the other hand, it
> could be argued that the cost of the call to the CAS primops in their
> current form isn't that much more expensive than such a branch.

Indeed, I'm much more concerned about performance in the threaded case and
making sure they're correct.
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