Re: Caller/Callee saved Registers

bart@cs.uoregon.edu (Barton Christopher Massey) writes:
>[I admit callee-save allows tail recursion, but may lead to excess
> spills and restores of registers that are needed but which must be
> saved across the recursive calls.]


Typical caller/callee-save divisions leave a "modest" number of free
caller-save registers (e.g. 10) for the body of each procedure, so only
"larger" routines (those that need many registers) need to save and
restore callee-save regs.


Further, since tail-recursion can be transformed in to iteration, spills
and restores of registers can be placed outside of the iteration, e.g.


f: ... ! normal entry
spill
f': ... ! tail-recursion loop entry
body
bn.cnd f' ! iteration/recursion
restore
return


I believe this is what GNU CC does in some cases (it is thus able to
inline recursive calls). In other cases the code at f' allocates stack
space (but is less general than f) and in some cases it uses the standard
calling convention.


;-D on ( Have your registers been SAVED? ) Pardo
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