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for (var k …) inside a function hangs forever when a closure in the body captures k #11052
Description
A for (var k = ...; k < n; k++) loop inside a function never terminates when any closure in the loop body captures k. The closure doesn't have to be called.
Reproduced on main f5cfbff88 (v0.5.1638), Linux x86_64, PERRY_NO_CACHE=1:
function f(n: any) { for (var k = 0; k < n; k++) { const r = () => k; } console.log(k); }
f(5);| variant | perry | node |
|---|---|---|
| closure created, never called (above) | hangs (killed by timeout 5, rc=124) |
5 |
read = () => k assigned each iteration, called after the loop |
hangs | 5 5 |
same with n: number |
hangs | 5 5 |
h += read() inside the loop |
hangs | 10 5 5 |
var k declared at module level, same loop in a function |
5 5 ✅ |
5 5 |
So the trigger is a function-scoped var induction variable that a closure captures. The loop uses the generic boxed counter in its IR (found while working on loop-counter representation). Presumably the increment and the k < n test are not reading and writing the same storage: one goes to the closure box and the other to a local copy. That is inferred, not verified.
Found while validating an unrelated loop-counter change, which hangs identically on both arms: the bug predates that change and the change does not admit this case.
Activity
proggeramlug commentedon Sep 22, 2026
A/B: this predates #11053, and it's a spin, not a park.
Same fixtures built with both compilers (
PERRY_NO_CACHE=1, Linux x86_64), each run for 3 s and then sampled:v3)read = () => k(v1)h += read()(v)f5cfbff881f1ef6c5fee7bb654007a784Every hung process was in state R at 100% CPU: 1 thread, no children, and 100 CPU ticks in the sampled second. So it's a busy loop, not a wait: the loop condition never becomes false. The first suspect is the counter's storage (the
k++and thek < ntest on different copies oncekis boxed for the closure), not the event loop or a lock.(B's binary was built from the pre-amend commit
4d1f97c3c;git diff 4d1f97c3c e8c3b8947touches only three campaign tooling files, no code.)https://claude.ai/code/session_01EQdCw7BN4AAnn2hAbNXg33
perf(codegen): version counted loops once at entry; no per-iteration …
perf(codegen): version counted loops once at entry; no per-iteration …
perf(codegen): version counted loops once at entry; no per-iteration …
perf(codegen): version counted loops once at entry; no per-iteration …