Re: [RFC, PATCH] shrink struct vectorlike_header #2

[Dmitry Antipov <dmantipov@yandex.ru>]

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On 11/08/2012 06:45 PM, Dmitry Antipov wrote:

> On 11/08/2012 06:03 PM, Stefan Monnier wrote:
>
>>> struct Lisp_Vector
>>>    {
>>>       struct vectorlike_header header;
>>>       union
>>>         {
>>>            Lisp_Object contents[1];
>>>            struct Lisp_Vector *next;
>>>         } u;
>>>    };
>>
>> Yes, that'd be better.
>
> This will be a quite large patch, with a lot of small changes,
> and I don't want to mix them with the core logical changes.
> So if others votes for such a change, I would rather like to
> do it separately.

Argh, there is another solution (comes into my head immediately
after commit, of course).

Dmitry



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=== modified file 'src/alloc.c'
--- src/alloc.c	2012-11-08 14:10:28 +0000
+++ src/alloc.c	2012-11-08 15:05:02 +0000
@@ -2611,16 +2611,18 @@
 
 #define VINDEX(nbytes) (((nbytes) - VBLOCK_BYTES_MIN) / roundup_size)
 
-/* When V is on the free list, first word after header is used as a pointer
-   to next vector on the free list.  It might be done in a better way with:
-
-   (*(struct Lisp_Vector **)&(v->contents[0]))
-
-   but this breaks GCC's strict-aliasing rules (which looks more relaxed
-   for char and void pointers).  */
-
-#define NEXT_IN_FREE_LIST(v)				\
-  (*(struct Lisp_Vector **)((char *) v + header_size))
+/* This is a pseudo vectorlike object used to represent any
+   block-allocated vectorlike object on the free list.  */
+
+struct Lisp_Vectorlike_Free
+{
+  struct vectorlike_header header;
+  struct Lisp_Vector *next;
+};
+
+/* When V is on the free list, it's always treated as Lisp_Vectorlike_Free.  */
+
+#define NEXT_IN_FREE_LIST(v) ((struct Lisp_Vectorlike_Free *) v)->next
 
 /* Common shortcut to setup vector on a free list.  */