bug#36447: 27.0.50; New "Unknown keyword" errors

[Stefan Monnier]

I think we should get Daniel's opinion on this.


        Stefan


> On Sat, Jul 6, 2019 at 6:45 AM Eli Zaretskii <eliz@gnu.org> wrote:
>> > > Indeed. I'm attaching a proof of concept that we can simply freeze the
>> > > hash tables when dumping and thaw them when loading a dump, which
>> > > includes rehashing. Do you happen to know why it wasn't done that way?
>> >
>> > I'd guess it was to shorten the startup time by doing this rehashing 
>> > lazily.
>>
>> The function pdumper-stats with show the time it took to load Emacs,
>> so the effect of this on the load time can be measured
>
> I'm measuring it directly, and it's more than I expected: about a
> millisecond, for 4,300 hash table entries. What we can't easily
> measure is how much the lazy rehashing code would slow us down anyway.
>
> For comparison, the entire time stored in pdumper-stats is 15 ms here.
>
> I don't think that's significant, because we'd probably end up
> rehashing most of the large hash tables anyway. We're saving some 250
> KB of space in the pdmp image, which was previously used for redundant
> information. (I'm surprised it's that much, but I guess pdumper
> relocations are fairly large?)
>
> I'm attaching a revised patch, which uses vectors rather than consed
> lists for both the key_and_value vector, avoiding a copy in the common
> case where there is more than one hash table entry, and for the list
> of hash tables. It still contains debugging/timing code.
>
> charset.c currently assumes hash table entries will stay at the same
> index in Vcharset_hash_table. I think that works okay in practice,
> because we don't shrink or reorder hash tables, but it was still a bit
> of a nasty surprise.
>
> This concept appears to work: modify pdumper to special-case hash
> tables and freeze/thaw them properly. You probably shouldn't dump hash
> tables with complicated user-defined hash functions.
>
> Both PURE_P and pdumper_object_p fail to distinguish between tables
> that were pure or impure before being dumped.
>
> This also fixes the bug that (hash-table-count dumped-hash-table) will
> return a negative number if no previous access to the hash table has
> happened, but of course we can fix that directly...
>
> Of course, we're still modifying purecopied information.