Insert locations at the end of a FOR loop.

[ltratt]

The intuition here is that if we've gone around one iteration of a for loop, we're more likely to close a "full, proper" iteration, whereas if we have the location on entry, we're likely to hit the "nothing to do" case. This is -- from memory! -- the same thing that PyPy does.

There is a trade-off here: it means every time we execute a loop we do one iteration in the interpreter. Probably because of that, benchmarks are mixed, but IMHO show a small improvement. b15:

storage/lua/1000        4.95% faster
richards/lua/100        4.27% faster
sieve/lua/3000          2.12% faster
cd/lua/250              1.19% faster
bounce/lua/1500         1.84% slower
knucleotide/lua/        3.97% slower
permute/lua/1000        4.30% slower

b16:

binarytrees/lua/15      4.93% faster
storage/lua/1000        3.98% faster
queens/lua/1000         3.44% faster
cd/lua/250              2.24% faster
spectralnorm/lua/1000   1.54% faster
json/lua/100            3.04% slower
knucleotide/lua/        5.87% slower
HashIds/lua/6000        6.13% slower
nbody/lua/250000        13.60% slower

nbody is very nondeterministic so it can be hard to draw conclusions; that said, it does seem on b16 to have meaningfully slowed down. On b15, the slowdown is within the margin of noise, though on the edge of it: I think it may well have slowed down, but by perhaps 5-8%. So whether this holds on other machines is a bit unclear.

[vext01]

Happy to follow your lead on this one.