bug#13783: simplify data_start configuration for Emacs

[Paul Eggert]

On 02/22/2013 05:05 AM, Eli Zaretskii wrote:

> Could you please explain more about what this is intended to
> accomplish?

Mostly, it's simplification.  There's a lot of cruft in there, about
linking Emacs specially and so forth, that was needed way back when we
made pure space read-only but is now no longer needed.

> are we removing the data-start thing on all platforms where GCPRO is
> a no-op, i.e. those which support USE_LSB_TAG?

USE_LSB_TAG is a different thing.  The patch omits data_start on hosts
that use the system malloc, since data_start is now needed only on
hosts where GNU Emacs supplies malloc.

USE_LSB_TAG is more closely related to DATA_SEG_BITS, but that
is not changed by this patch.

> Should platforms that HAVE_DATA_START initialize data_start to some
> value?  Should _all_ platforms do that?  If not, which ones should?

No, data_start's contents are irrelevant.  Only its address matters.
On GNU hosts the linker supplies the address automatically;
on non-GNU hosts we approximate it by declaring a
variable whose contents don't matter.

>> -  extern POINTER (*real_morecore) (ptrdiff_t);
>> +  extern void *(*real_morecore) (ptrdiff_t);
>
> This needs corresponding changes in ralloc.c, I think, at least for
> consistency if nothing else.

Done in trunk bzr 111859, which simplifies the attached patch.
Revised patch attached, relative to trunk bzr 111859.

>> -  data_start = (int) start_of_data ();
>> +  data_start = (int) get_data_start ();
>...
> This part is wrong: unexcoff.c is used only by the MSDOS build

Thanks for catching that.  I'll fix that as follows;
this fix is contained in the attached patch.

--- src/unexcoff.c      2013-02-22 08:11:05 +0000
+++ src/unexcoff.c      2013-02-22 19:39:27 +0000
@@ -99,7 +99,7 @@
 
 #include <sys/file.h>
 
-extern unsigned char *get_data_start (void);
+extern int etext[];
 
 static long block_copy_start;          /* Old executable start point */
 static struct filehdr f_hdr;           /* File header */
@@ -168,7 +168,7 @@
   pagemask = getpagesize () - 1;
 
   /* Adjust text/data boundary. */
-  data_start = (int) get_data_start ();
+  data_start = (int) (etext + 1);
   data_start = ADDR_CORRECT (data_start);
   data_start = data_start & ~pagemask; /* (Down) to page boundary. */

data_start.txt
Description: Text document