This is complete, but after some extensive benchmarking on MacOS AArch64 the new ShortFixnum may introduce more instruction overhead than the size is worth. The bimorphic nature of having both LongFixnum and ShortFixnum does not appear to impact things much, but the ShortFixnum constantly widening and narrowing its values for reads and initialization of new instances adds enough overhead to a small loop to reduce performance. It also appears to interfere with escape analysis, both on HotSpot and Graal JIT.

Some loop benchmarks follow.

All shorts:

def foo
  i = 0
  while i < 32_000
    i+=1
    j = 0
    while j < 32_000
      j+=1
    end
  end
end
10.times {
  t = Time.now
  foo
  puts Time.now - t
}

Long and short with caching, HotSpot 25:

3.0774749999999997
2.830375
2.738893
2.7603999999999997
2.81643
2.75495
2.75328
2.7494810000000003
2.771321
2.801205

Long only with caching, HotSpot 25 (-Xfixnum.compact=false):

2.679109
2.154972
2.1185169999999998
2.0998430000000003
2.0842650000000003
2.088898
2.155581
2.0820969999999996
2.125191
2.12845

Long only without caching, HotSpot 25 (-Xfixnum.compact=false -Xfixnum.cache=false):

2.613342
2.065781
1.877827
1.8706520000000002
1.856989
1.899999
1.882702
1.8590119999999999
1.9631930000000002
1.873456

Same on HotSpot 26 (escape analysis seems to have improved? Enabling cache degrades this back to 25 speeds.):

3.782794
2.172116
1.229315
1.2248510000000001
1.231879
1.302273
1.234393
1.220123
1.233962
1.229031

Same on GraalVM 25 (escape analysis eliminating most objects):

0.17663399999999999
0.16367500000000001
0.14729299999999998
0.137191
0.142957
0.13831700000000002
0.143365
0.13711500000000001
0.14526699999999998
0.13666

On the other hand, an array of 1M short-ranged values goes from 37,391,992 bytes to 29,391,992 bytes, almost 22% less memory. That savings only applies to short-ranged integers but it still makes this a tough call since most loops will also be in short range.