emacs and large-address awareness under recent snapshots

Corinna Vinschen corinna-cygwin@cygwin.com
Tue Aug 9 15:22:00 GMT 2011

On Aug  9 10:23, Ken Brown wrote:
> On 8/9/2011 10:12 AM, Ken Brown wrote:
> >On 8/9/2011 7:19 AM, Ken Brown wrote:
> >>(gdb) thread 1
> >>[Switching to thread 1 (Thread 19828.0x447c)]
> >>#0  0x00622ee0 in morecore_nolock (size=1052672) at gmalloc.c:703
> >>703           while ((__malloc_size_t) BLOCK ((char *) result + size)>
> >>newsize);
> >>(gdb) p /x size
> >>$1 = 0x101000
> >>(gdb) p /x heapsize
> >>$2 = 0x80000
> >>(gdb) p result
> >>$3 = (void *) 0x807d0000
> >>(gdb) p newsize
> >>$4 = 0
> >>(gdb) p _heapbase
> >>$5 = 0x816000 "\202"
> >>(gdb) p _heapinfo
> >>$6 = (malloc_info *) 0x80060000
> >>
> >>Is _heapbase the problem?  This is initialized to _heapinfo at the first
> >>call of malloc and is never changed.  _heapinfo presumably points into
> >>the static heap at that point.  (_heapinfo is later changed as a result
> >>of realloc.)  This low value of _heapbase is used in the BLOCK macro.
> >
> >Here's what I think is happening.  When temacs.exe is running during the
> >build process (see my explanation of this earlier in the thread),
> >malloc_init is called and _heapbase is set.  At this point, temacs is
> >using its own static buffer as the heap, and _heapbase gets the value
> >0x816000.  This gets dumped as initialized data into emacs.exe, as does
> >the value __malloc_initialized = 1.  Now when emacs.exe is run, it sees
> >that malloc has already been initialized, so _heapbase retains its
> >value, which is no longer appropriate.  All code relying on the BLOCK
> >macro is now invalid.
> >
> >AFAICS, this has always been wrong.  But the error didn't have dramatic
> >consequences until the heap was put into high memory.
> >
> >I'm not sure what's the best way to fix this (assuming my analysis is
> >right).  Would it be enough to set __malloc_initialized to 0 before
> >dumping?  That would force emacs to reinitialize and get the correct
> >value of _heapbase.
> No, that's too simple-minded.  I just tried it, and emacs aborted.
> This seems like a mess.

What happens if you remove the Cygwin-specific call to bss_sbrk in
__default_morecore?  In theory that should also break, as long as
temacs isn't also build large address aware.  The only difference,
_heapbase = 0x20000000.  But if temacs gets build with large address
awareness set, _heapbase should become 0x80000000.

However, whatever you do, it will not really work.  Keep in mind that
the large address awareness only makes sense (and has any effect!) on
systems which provide a large address area.

To me the bottom line here is, that emacs is doing the wrong thing.
There are a couple of assumptions how a system maintains memory, which
are just not valid on all systems.  The malloc initialization and the
assignment of the heapbase (the first call to sbrk(0)) should happen
in emacs every time it starts.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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