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authorMatthias Benkard <mulk@minimulk.mst-plus>2008-10-02 16:54:05 +0200
committerMatthias Benkard <mulk@minimulk.mst-plus>2008-10-02 16:54:05 +0200
commita4a3054ddeeecbf7ec3b087b2db69bea192ebc8a (patch)
treeaedf0098f11a14d329a8085f158378764d182257 /JOURNAL
parentf805cbe8ac26baa444827e42e74a95bd473669a8 (diff)
New journal entry.
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@@ -18,6 +18,47 @@ Context: [[http://matthias.benkard.de/toilet][The Toilet Lisp Project]].
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+* 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