[Sbcl-commits] master: Better commentary for %INSTANCE-LENGTH in the cross compiler.

[Douglas K. <sn...@us...>]

The branch "master" has been updated in SBCL:
       via  c82cc623c8af54cb309de7b77da9519950947298 (commit)
      from  c857378f7efb569018843ac505fdead1fe53701f (commit)

- Log -----------------------------------------------------------------
commit c82cc623c8af54cb309de7b77da9519950947298
Author: Douglas Katzman <do...@go...>
Date:   Sun Oct 19 00:50:20 2014 -0400

    Better commentary for %INSTANCE-LENGTH in the cross compiler.
---
 src/code/defbangstruct.lisp |   15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/code/defbangstruct.lisp b/src/code/defbangstruct.lisp
index c3ac50b..2f5ee3f 100644
--- a/src/code/defbangstruct.lisp
+++ b/src/code/defbangstruct.lisp
@@ -186,10 +186,17 @@
   (defun %instance-length (instance)
     (aver (or (typep instance 'structure!object)
               (xc-dumpable-structure-instance-p instance)))
-    ;; This seems strangely incompatible- INSTANCE-LENGTH should return an odd number
-    ;; so that including the header word (not counted), it comes out even.
-    ;; Except that on the xc host you can't access a slot beyond the layout-length,
-    ;; whereas on the target you can if there is a slack word - it reads as NIL.
+    ;; INSTANCE-LENGTH tells you how many data words the backend is able to
+    ;; physically access in this structure. Since every structure occupies
+    ;; an even number of words, the storage slots comprise an odd number
+    ;; of words after subtracting 1 for the header.
+    ;; And in fact the fasl dumper / loader do write and read potentially
+    ;; one cell beyond the instance's LAYOUT-LENGTH if it was not odd.
+    ;; I'm not sure whether that is a good or bad thing.
+    ;; But be that as it may, in the cross-compiler you must not access
+    ;; more cells than there are in the declared structure because there
+    ;; is no lower level storage that you can peek at.
+    ;; So INSTANCE-LENGTH is exactly the same as LAYOUT-LENGTH on the host.
     (layout-length (classoid-layout (find-classoid (type-of instance)))))
   (defun %instance-ref (instance index)
     (aver (or (typep instance 'structure!object)

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