Although I like the simplicity of eager rehashing, I'm not yet sold on the 
performance implications. On my usual (cd lisp && make compile-always) 
benchmark, the patch hurt user+system CPU time performance by 1.5%. Admittedly 
just one benchmark, but still....

Also, must we expose Vpdumper_hash_tables to Lisp? Surely it'd be better to keep 
it private to pdumper.c.

I'll CC: to Daniel to see whether he has any insights about improvements in this 
area.

PS. I ran that benchmark on my home desktop, an Intel Xeon E3-1225 v2 running 
Ubunto 18.04.2. To run it, I rebased your patch and also removed the 
no-longer-used PDUMPER_CHECK_REHASHING macro that my GCC complained about 
(wonder why that didn't happen for you?), resulting in the attached patch 
against current master 8ff09154a29a1151afb2902267ca35f89ebda73c.