Re: Caller/Callee saved Registers

One additional point about caller vs. callee save which I'm surprised no
one has mentioned: callee-save makes it rather more difficult to implement
tail-call optimization. Recall that full tail-call (as opposed to mere
self tail-recursion) optimization avoids saving any state (including a
return address) whenenever the last instruction in a procedure is a call
to another procedure. This is a very valuable optimization for recursive
programs, but to implement it on a machine with callee-save registers
typically requires a second entry point to the procedure which avoids the
(unnecessary) saves during a tail-call. I know of no C-based systems
whose calling conventions support multiple entry points to a procedure, so
unless the C calling conventions for a system don't require callee-saves,
your C compiler can't implement full tail-call on it.


(I hope the above paragraph wasn't too opaque.)


Bart Massey
bart@cs.uoregon.edu
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