Correct Fast-Path Semantics

When implementing algorithmic optimizations (fast paths, binary search vs linear, prefetch/slot routing, realloc/compaction shortcuts), require semantic correctness and invariant preservation before performance.

Apply this standard:

• Guard fast paths with exactness conditions (range, precision, rounding model). If the proof/assumption doesn’t hold, fall back to the reference/slow path.
• Match externally observable semantics exactly (e.g., parse success vs failure, and endptr/“consumed input” behavior). Never accept partial parses just because a value (like nan) was detected.
• Use overflow checks that match the actual arithmetic (multiplication vs addition). Validate the intermediate that can overflow, not just a later result.
• After possible realloc/relayout, re-apply any derived indexes/maps that depend on the old address/layout (e.g., tree/rax nodes pointing at listpack memory).
• Defer micro-optimizations if correctness is the priority and the difference is unlikely to matter at RC time; choose the simpler correct algorithm first.

Example (parsing correctness guard):

// Rule: only accept nan(...) if the parse consumed the expected characters.
if (parsed && isnan(result)) {
    if (endptr) *endptr = (char*)eptr;
    // But also ensure the parse covered the full grammar for the token.
    // If eptr != pend for an otherwise-invalid/partial nan(...), reject.
}

Example (overflow check matching operation):

// If overflow risk is in multiplication, check multiplication, not addition.
if (emission_interval_us > LLONG_MAX / num_requests) {
    addReplyError(c, "overflow");
    return;
}