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| -rw-r--r-- | JOURNAL | 41 |
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@@ -18,6 +18,47 @@ Context: [[http://matthias.benkard.de/toilet][The Toilet Lisp Project]]. ---- +* 2008-10-02, 16:38:15 CEST + +** Multiple-Value Returns + +Multiple-value returns are now implemented in the most C-friendly way that I +can imagine. The way they work is: + + - Every compiled function gets an additional argument that points to a cell + which the caller has allocated. The cell may be set to point to a list + (i.e., either =nil= or an instance of =MLKCons=) by the callee, in which + case the caller will use the list as the list of return values. + + - The argument that points to the return value list cell may be =NULL=, in + which case the callee must assume a single-return convention. + + - Return-value setting may be delegated to a subform by simply passing the + pointer on. + + - The compiler always passes =NULL= as the list pointer when it neither uses + nor delegates the return values. + + - The compiler occasionally sets the =llvm:Value= that represents the list + pointer to =NULL=. It thereby communicates to itself that no + multiple-return value handling is needed in the current lexical context. + This should speed some things up. + + - A function implemented in C may choose to simply ignore the list pointer. + Similarly, it may choose to always pass a list pointer of =NULL= to callees + if it does not care about return values other than the primary one. + + - A function that does not return multiple values simply ignores the list + pointer, which is guaranteed to contain a value that the caller can + identify as signifying that no multiple-value return has occurred. + In the current implementation, this role is filled by the + =MLKEndOfArgumentsMarker=. + + - Regardless of multiple-value returns happening or not, the primary value of + a function call is always its normal return value. When there is no + primary value, =nil= is to be used. + + * 2008-08-11, 02:09:21 CEST ** Function Calling, 2nd Take |