Re: Proposal: block-based vector allocator

[Dmitry Antipov]

On 06/01/2012 09:44 AM, Paul Eggert wrote:

> > On non-USE_LSB_TAG system, roundup_size is orthogonal to pointer
> > values (roundup_size can't be less than sizeof (Lisp_Object), of course).
>
> Ah, OK, then we can make roundup_size be the equivalent of
> COMMON_MULTIPLE (sizeof (Lisp_Object), defined USE_LSB_TAG ? 8 : 1).
>
> > For example, 32-bit system (non-wide-int, i.e. sizeof (Lisp_Object) == 4)
> > without USE_LSB_TAG is expected to work with roundup_size = 4, but I can't
> > test this and have no ideas why it's worth trying at all.
>
> It would waste less memory due to internal fragmentation, which
> would be a win.  I don't see the downside -- perhaps you could
> explain?

I expect very negligible win here. But I'll try to reconfigure with
--enable-use-lisp-union-type and see what happens.

BTW, is this configuration usable? When I'm trying to configure with
this option, I still get USE_LSB_TAG - but the comment in lisp.h says
they're incompatible. I suppose we shouldn't enforce USE_LSB_TAG
if USE_LISP_UNION_TYPE is defined, i.e.:

=== modified file 'src/lisp.h'
--- src/lisp.h  2012-05-30 19:23:37 +0000
+++ src/lisp.h  2012-06-01 08:58:16 +0000
@@ -201,9 +201,10 @@
 # endif
 #endif

-/* Let's USE_LSB_TAG on systems where we know malloc returns mult-of-8.  */
+/* If USE_LISP_UNION_TYPE is not explicitly enabled, let's USE_LSB_TAG
+   on systems where we know malloc returns mult-of-8.  */
 #if (defined GNU_MALLOC || defined DOUG_LEA_MALLOC || defined __GLIBC__ \
-     || defined DARWIN_OS || defined __sun)
+     || defined DARWIN_OS || defined __sun) && !defined USE_LISP_UNION_TYPE
 /* We also need to be able to specify mult-of-8 alignment on static vars.  */
 # if defined DECL_ALIGN
 /* On hosts where pointers-as-ints do not exceed VAL_MAX,

Dmitry