100x Performance Regression from 3.1

[pvande]

Given this program:

x = 400_000.times.map do |i|
  { f: 100.times.map { i.to_f }, a: [i] * 100, s: "a" * 100 }
end

Since 3.1.0, the runtime has increased over 100x.

% time ./bin/mruby-3.1 main.rb
./bin/mruby-3.1 main.rb  10.04s user 0.31s system 97% cpu 10.622 total
% time ./bin/mruby-master main.rb
./bin/mruby-master main.rb  1013.02s user 1.32s system 97% cpu 17:15.59 total

(mruby-master was compiled from 12264e3 with CFLAGS="-D MRB_ARY_LENGTH_MAX=0".)

I'll see if I can't bisect where the regression was introduced.

[pvande]

0c47570 seems to be the commit that introduced this performance regression.

[matz]

It was caused by improper fix for #5791. I will address it soon.