Re: MPS signals and Emacs

[Paul Eggert]

On 2024-04-22 00:49, Gerd Möllmann wrote:
> There are ambigupus and there are exact referenes. An object references
> from at least one ambiguos reference cannot be moved, others can.

It's not merely a question of ambiguous vs exact references. It's also a 
question of hashing.

For example, the Emacs Lisp internals hash references with low-level 
functions like sxhash_eq's call to XHASH, to implement functions like 
make-hash-table. If exact references are moved, hashing them won't work 
because their XHASH values will change. Does the MPS branch use exact 
references for any Lisp object visible to the Elisp programmer? If so, 
won't there be subtle bugs with make-hash-table and the like? Or does 
the MPS branch work around this issue by rebuilding every hash table 
after a GC?

Also, does the MPS branch have similar workarounds for every other place 
the C code uses XHASH, XLI, XPNTR, etc.? For example, suppose GC occurs 
during sort_overlays, so that compare_overlays (which uses XLI) is no 
longer a total ordering, which means qsort has undefined behavior?