Hi :) On Sat 27 Apr 2024 19:04, Simon Tournier <zimon.touto...@gmail.com> writes:
> In Guile module (ice-9 vlist), one reads: > > ;; Asserting that something is a vlist is actually a win if your next > ;; step is to call record accessors, because that causes CSE to > ;; eliminate the type checks in those accessors. > ;; > (define-inlinable (assert-vlist val) > (unless (vlist? val) > (throw 'wrong-type-arg > #f > "Not a vlist: ~S" > (list val) > (list val)))) > > [...] > > (define (vlist-head vlist) > "Return the head of VLIST." > (assert-vlist vlist) > (let ((base (vlist-base vlist)) > (offset (vlist-offset vlist))) > (block-ref (block-content base) offset))) > > > Other said, the argument ’vlist’ is “type-checked” with ’assert-vlist’ > and thus that is exploited by Guile compiler, if I understand correctly > the comment. > > The first question is: is it still correct? Because this module had > been implemented before many Guile compiler improvements. No, the comment is incorrect. The type check on whatever accessor is called first (unspecified in scheme; probably we should just bite the bullet and do predictable left-to-right semantics, as racket does) will dominate the rest and eliminate those checks. The assert-type is unnecessary. To see this, do ,optimize-cps at the repl, and count the number of e.g. struct? checks with and without the assert-vlist. There is only one, either way. (A type check is a heap-object? check, then struct?, then get the vtable, then check against the global variable <vlist>. All of these duplicates get eliminated.) > PS: Raining day and weird pastime… diving into Guile source code. ;-) :) Cheers Andy
