l...@gnu.org (Ludovic Courtès) writes:
> Andy Wingo <wi...@pobox.com> skribis:
>
>> +;;; Commentary:
>> +;;;
>> +;;; A pass to adapt expressions to the arities of their continuations,
>> +;;; and to rewrite some tail expressions as primcalls to "return".
>> +;;;
>> +;;; Code:
>
> [...]
>
>> + (define (adapt-exp nvals k exp)
>> + (match nvals
>> + (0
>> + (rewrite-cps-term (lookup-cont k conts)
>> + (($ $ktail)
>> + ,(let-gensyms (kvoid kunspec unspec)
>> + (build-cps-term
>> + ($letk* ((kunspec #f ($kargs (unspec) (unspec)
>> + ($continue k
>> + ($primcall 'return (unspec)))))
>> + (kvoid #f ($kargs () ()
>> + ($continue kunspec ($void)))))
>> + ($continue kvoid ,exp)))))
>
> [...]
>
>> + (let ((drop-result
>> + (lambda (kseq)
>> + (let-gensyms (k* drop)
>> + (build-cps-term
>> + ($letk ((k* #f ($kargs ('drop) (drop)
>> + ($continue kseq ($values ())))))
>> + ($continue k* ,exp)))))))
>
> Are ($void) and ($values ()) both equivalent to *unspecified*?
No. ($void) passes a single value (*unspecified*), to the continuation.
($values ()) passes zero values to the continuation, like (values).
>> + (rewrite-cps-term (lookup-cont k conts)
>> + (($ $ktail)
>> + ,(rewrite-cps-term exp
>> + (($var sym)
>> + ($continue ktail ($primcall 'return (sym))))
>> + (_
>> + ,(let-gensyms (k* v)
>> + (build-cps-term
>> + ($letk ((k* #f ($kargs (v) (v)
>> + ($continue k
>> + ($primcall 'return (v))))))
>> + ($continue k* ,exp)))))))
>> + (($ $ktrunc ($ $arity () () #f () #f) kseq)
>> + ,(drop-result kseq))
>> + (($ $kargs () () _)
>> + ,(drop-result k))
>> + (_
>> + ($continue k ,exp)))))))
>
> Perhaps that happens in the Tree-IL->CPS pass, but is there/could there
> be an effect analysis when truncating multiple-value returns?
>
> Such that (f (values 1 2 3)) reduces to (f 1).
I guess this is a question for Andy, but some experimentation suggests
that this transformation is already done by an earlier optimization pass
in tree-il.
Regards,
Mark
