Leopold Toetsch <[EMAIL PROTECTED]> writes: > Piers Cawley <[EMAIL PROTECTED]> wrote: >> Leopold Toetsch <[EMAIL PROTECTED]> writes: > >>> Matt Fowles <[EMAIL PROTECTED]> wrote: >>> >>>> Thanks for the clear explanation. I did not realize that S registers >>>> could switch pointers, that does make things a little harder. I have >>>> a recommendation for a possible hybrid solution. Incur the cost of >>>> spilling I,S,N registers heavily. Restore the state of P register. >>> >>> My conclusion was that with the copying approach I,S,N registers are >>> unusable. > >> But you only need to copy when the frame you're restoring is a full >> continuation > > Yes. With the effect that semantics of I,S,N (i.e. value registers) > suddenly changes. > >> I'd submit that, in the vast majority of cases you're not going to be >> dealing with full continuations, and on the occasions when you are the >> programmer using them will be aware of the cost and will be willing to >> pay it. > > *If* the programmer is aware of the fact that a subroutine can return > multiple times, he can annotate the source so that a correct CFG is > created that prevents register reusing alltogether. The problem is > gone in the first place. > > *If* that's not true, you'd get the effect that suddenly I,S,N registers > restore to some older values which makes this registers de facto > unusable.
Further to my last response. If you have things set up so that you can return multiple times from the same function invocation then the return continuation should have been replaced with a full continuation before the first return, so even the first return will use copying semantics, and the registers will always be restored to the state they were in when the function was first called, which is absolutely the right thing to do.
