bigmerge
d231b7e5e7
El SDK CI - dev / build-and-test (pull_request) Failing after 10m21s
THE BUG. str_char_code() and str_slice() each called strlen() on every
invocation. The lexer walks source one character at a time, so every character
access rescanned the whole remaining input: O(n) per character over n
characters = O(n^2).
el_val_t str_char_code(el_val_t s, el_val_t i) {
...
int64_t n = (int64_t)strlen(str); // <- O(n), every call
if (idx < 0 || idx >= n) return 0;
return str[idx];
}
HOW IT WAS FOUND. Not by reading code — by sampling the running process, which
is the same method that resolved tonight's engram outage after four wrong
theories. A geometric sweep of synthetic sources showed wall-clock rising 3.0x,
3.0x, 4.0x, 4.14x per doubling (converging on 4x = quadratic), and a stack
sample put 779 of 779 samples inside lex(), every one bottoming out in
_platform_strlen via str_char_code and str_slice.
THE FIX. Remember the length instead of recomputing it. The subtlety is
INVALIDATION: El strings are arena-allocated, so a freed pointer can be reused
for a different string at the same address, and a naive pointer-keyed cache
would hand back a stale length and read past the end of the new string —
trading a performance bug for a memory-safety one. So entries carry a
generation, a hit requires pointer AND generation to match, and every path that
frees or mutates a runtime string bumps the generation: el_arena_pop,
seed_request_end, __str_set_char. Stale entries cannot be believed; they miss
and recompute.
MEASURED, same host, same inputs:
n(fns) before after
512 0.10s 0.01s
1024 0.37s 0.02s
2048 1.51s 0.03s 50x
the compiler's own 422 KB source concatenated (DESIGN.md's 3.58s case):
3.55s -> 0.03s 118x
The speedup GROWS with input size, which is the signature of removing a
complexity class rather than a constant factor. After the fix each doubling
adds ~0.01s: linear.
CORRECTNESS, verified rather than assumed:
- byte-identical output on every sweep input (n = 128..2048)
- byte-identical output on the 422 KB compiler concatenation
- byte-identical output on tests/runtime/string_test.el
- self-hosting fixpoint byte-identical
- new tests/runtime/str_cache_test.el: 17 assertions covering bounds, empty
strings, negative indices, slice clamping, distinct strings not sharing a
cached length, 1000 interleaved strings forcing cache-slot collisions, and
a grown string not reporting its old length. All pass.
This is the defect that made dist/soul.c a committed artifact: elc could not run
in CI because it needed 24 GB+ and minutes. It needs neither now.