Subject: Re: bug#38753: 27.0.60; cl--random-state uncontrolled growth due to bignums

From: Christopher Wellons <[email-masked]>
Date: Sun, 29 Dec 2019 09:27:18 -0500

> Any idea how easy it would be to fix either of them, or both?

The reason I found that bug is because I was experimenting with 
addressing exactly that problem:

https://github.com/skeeto/lcg128

It's got the right features, including support for arbitrary limits (per 
your example), but I'm not satisfied with the performance. For arbitrary 
limits it uses the same rejection algorithm as Python — generate (logb 
lim) bits and reject if out of range — and cl-random could be updated to 
use the same technique. The LCG around 6x slower than cl-random in the 
common case (generating small numbers). Bignums aren't ideal for this 
since every operation requires an allocation, and the LCG throws away 
half of the work (the upper half of the multiplication result).

The lagged Fibonacci generator really does hit a sweet spot for Emacs 
Lisp, where, before bignums, even 32-bit integers weren't a reliable 
option. So it fits within Emacs' worst fixnum limitations and requires 
few bytecode instructions to generate a number.

The random built-in ought to simply be pcg32 across all platforms:

http://www.pcg-random.org/download.html

Currently it's whatever crummy host-provided PRNG it happens to find at 
compile time. It's certainly possible to implement pcg32 using Emacs 27 
bignums, but it would be even slower than my LCG, especially on 32-bit 
platforms. In C it's very, very fast.

If the pcg32 state, just one 64-bit integer, was an optional parameter, 
then cl-random could build on it. However, the state would nearly always 
be a bignum, even on 64-bit platforms, and that would really slow it 
down. I haven't yet thought of any obviously good options for this.