CAS for SAP-REF-*

[Nicolas Hafner]

Currently there are no CAS expanders for SAP-REF-* functions, meaning one cannot CAS foreign memory.

This would be valuable for interacting with foreign code that requires atomic synchronisation.

This could then be used by the CFFI backend as well to allow doing things like

   (sb-ext:compare-and-swap (cffi:mem-ref ...) ...)

[Andrew Berkley (ajberkley)]

It's almost there:

(let* ((addr (cffi:foreign-alloc :int32 :initial-element 1234))
                (starting-value (cffi:mem-ref addr :int32)))
           (print starting-value)
           (sb-int::%primitive sb-vm::signed-sap-cas-32 (cffi:mem-aptr addr :int32) 0
                               starting-value (random 32))
           (print (cffi:mem-ref addr :int32)))

[Douglas Katzman (dougk)]

that doesn't really count. It's ad-hoc vop, not for external use, only works on x86-64, and only supports 32-bit int. It needs to support 1,2,4,8 bytes and work all on architectures.