[Bug libstdc++/80721] Sorting/Merging of free EH-emergency buffer may wrong or uncomplete

["meisenmann dot lba at fh-salzburg dot ac dot at" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80721

--- Comment #2 from Markus Eisenmann <meisenmann.lba@fh-salzburg.ac.at> ---
Hi!

(In reply to Richard Biener from comment #1)
> Confirmed.  Isn't it enough to add, after
> 
>       else if (reinterpret_cast <char *> (e) + sz
>                == reinterpret_cast <char *> (first_free_entry))
>         {
> ...
> 
> a
> 
>       else if (reinterpret_cast <char *> (e)
>                < reinterpret_cast <char *> (first_free_entry))
>         {
>           // First is right of us, replace the head.
>           free_entry *f = reinterpret_cast <free_entry *> (e);
>           new (f) free_entry;
>           f->next = first_free_entry;
>           first_free_entry = f;
>         }
> 
> ?  That's a much less intrusive (and hard to review) fix.

Okay, a less intrusive fix for issue a) [set in front, if free-list is empty
or starts with a non merge-able block on a higher address]

Instead of (sorry, not fully formatted as unified diff), my suggestion would:

       allocated_entry *e = reinterpret_cast <allocated_entry *>
        (reinterpret_cast <char *> (data) - offsetof (allocated_entry, data));
       std::size_t sz = e->size;
-      if (!first_free_entry)
+      if (!first_free_entry
+          || (reinterpret_cast <char *> (e) + sz
+              < reinterpret_cast <char *> (first_free_entry)))
        {
          // If the free list is empty just put the entry there.
          free_entry *f = reinterpret_cast <free_entry *> (e);
          new (f) free_entry;
          f->size = sz;
-         f->next = NULL;
+         f->next = first_free_entry;
          first_free_entry = f;
        }
      else if (reinterpret_cast <char *> (e) + sz

I.e., set in front if first_free_entry = null or has to be first, because non
merge-able and a "right" item.
Note: Following Merging with head will be "is-as-is";

About issue b) - additional merging with direct right follower - I have to
think a little about ...

Best regards,
Markus