[PATCH] malloc: Rewrite dumped heap for compatibility in __malloc_set_state

[Florian Weimer]

This will allow us to change many aspects of the malloc implementation 
while preserving compatibility with existing Emacs binaries.

As a result, existing Emacs binaries will have a larger RSS, and Emacs 
needs a few more milliseconds to start.  This overhead is specific to 
Emacs (and will go away once Emacs switches to its internal malloc).

The new checks to make free and realloc compatible with the dumped heap 
are confined to the mmap paths, which are already quite slow due to the 
munmap overhead.

This patch weakens some security checks, but only for heap pointers in 
the dumped main arena.  By default, this area is empty, so those checks 
are as effective as before.

I tested this on ppc (32-bit) with an additional patch which changes 
malloc alignment, and on x86_64.  In both cases, I was able to run 
pre-existing Emacs binaries.

Florian

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