Re: [cxx-mem-model] compare_exchange implementation

[Andrew MacLeod <amacleod at redhat dot com>]

On 10/18/2011 06:07 PM, Jakub Jelinek wrote:
> On Tue, Oct 18, 2011 at 06:03:16PM -0400, Andrew MacLeod wrote:
> > Here's the last of the missing intrinsics.   compare_exchange is
> > slightly different than the others since it doesn't map directly to
> > an rtl pattern. Its format is :
> >
> >    bool __atomic_compare_exchange (T* mem, T* expected, T desired,
> > bool weak, memory_order success, memory_order failure)
> >
> > note that the expected parameter is a pointer as well.  In the case
> > where false is returned, the value of expected is updated.
> I think the __sync_* way here was much better (two intrinsics instead of
> one, one bool-ish and one returning val).  With mandating T*expected
> you force it to be addressable, probably until expansion time at which point
> it will be just forced into memory, which is highly undesirable.
>
> 	

Its impossible to implement a weak compare and swap unless you return 
both parameters in one operation.

the compare_exchange matches the atomic interface for c++, and if we 
can't resolve it with a lock free instruction sequence, we have to leave 
an external call with this format to a library, so I that why I provide 
this built-in.

Neither rth nor I like the addressable parameter, so thats why I left 
the rtl pattern for weak and strong compare and swap without that 
addressable argument, and let this builtin generate wrapper code around it.

You could provide an __atomic version of the bool and val routines with 
memory model.... we toyed with making the compare_and_swap return both 
values so we could implement a weak version, but getting 2 return values 
is not pretty.  It could be done with 2 separate built-ins that relate 
to each other, but thats not great either.

we may think of a more brilliant way later to do it under the covers, 
but at the moment, this gets us the external interface we're looking for.

Andrew