[bitc-dev] Re: Closure Conversion
swaroop at cs.jhu.edu
Fri Jul 8 16:34:59 EDT 2005
Jonathan S. Shapiro wrote:
> If the lambda does not escape then the closure can be stack-allocated.
> Closure conversion certainly should NOT be done blindly -- this
> transform should only be done for procedures that require it.
Determining whether a function needs to be closure converted is easy.
But, I think, to determine whether its closure can be stack allocated or
not is hard.
(define F (lambda (x) ((lambda (y) x) 2)))
are easy. The inner-lambda's closure can be safely stack allocated.
(define G (lambda (f) (let ((p 3)) (f (lambda (x) p)))))
the inner lambda is apparently not escaping downward. But it is not
clear whether its closure can be stack allocated or not.
For example, stack allocation is safe in cases like:
(define P (G (lambda (x) ())))
but not safe in cases like:
(define H (mutable (lambda (x) 5)))
(define I (lambda (m) (begin (set! H m) ())))
(define J (G I))
(define K ((immutable H) ())) ; K is now 3
This is why I had suggested that there should be some data-flow analysis
as in . Otherwise, I think the choices are:
i) Escaping will come to mean escaping upwards or downwards, and most
lambdas will have their closures heap allocated
ii) The language run-time will have to be very smart in order to
construct closures appropriately for each case.
Am I missing something?
 P. A. Steckler and M. Wand. Lightweight closure conversion.
(TOPLAS), pages 48--86, January 1997.
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