emit-loop compiled every loop/recur to a self-recursive local closure called once
per iteration — relying on Janet TCO for stack safety but paying a fn frame + arg
bind each iteration. The jolt-5vsp spike localized the whole ~1.43x
jolt-over-hand-Janet gap on compute loops to exactly this.
Lower instead to a Janet `while` + state vars: the loop bindings become vars
carried across iterations, a recur writes them and raises a continue flag, and a
non-recur tail value falls out through a result var. recur-name routing in
emit-recur picks the while-set lowering for loops and leaves the fn-arity self-call
path untouched.
The one subtlety is closure capture: Janet closures capture vars BY REFERENCE, so
a closure built in the body over a shared mutable loop var would see the final
value ([3 3 3]) instead of its iteration's ([0 1 2]). Each iteration rebinds the
loop names into a fresh immutable `let` before running the body, which restores
per-iteration capture. recur reads those immutable bindings and writes the state
vars, so cross-referencing args (swap, fib) need no temps.
mandelbrot 218 -> 164 ms (~11.2x JVM, from 15x). fib is unaffected — it's
fn-arity recursion, not a loop. Regression spec in control-flow-spec covers
closure capture, no-clobber recur, nested loops, sequential init, recur through
let, and that fn-arity recur still works. Gate green (conformance x3, full suite).
Note: validating this after a rebuild needs JOLT_NO_DEPS_CACHE=1 — the deps-image
cache keys on the version string, not build identity, so it served stale codegen
(filed separately).
Co-authored-by: Yogthos <yogthos@gmail.com>
e2b607f0f3