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Malte UblandClaude Opus 5.5
714ef5aa09
Index loops over arrays left the counter a double in several common shapes, and every element access at the counter tested the array's hole-free prefix on each pass: - A syntactic proof (ir/array-length-stability.ts) shows that a loop never changes an array's length: the body runs no user code, never rebinds the array or lets it escape, and only resizes local arrays that no other binding can hold. Element writes at a counter kept below the length are accepted. - Counted loops take a stable `a.length` as their limit, and an element access at the counter makes an unversioned counted loop worthwhile, so strided (`i += 2`) and descending (`i = a.length - 1; i >= 0`) loops get an integer counter. - Array lengths bound while-loop and for-loop counters by their range, which needs no stability proof. - A loop whose array length is stable and whose indexed arrays keep their storage runs as two copies. One test before the loop (the length is at most 2^31 - 1 and at most each indexed array's `filled` bound) selects a fast copy whose element accesses at the counter skip the prefix test and whose counter is an int32. LLVM can vectorize that copy. - Loop counters whose ranges fit in 32 bits convert to double from their low half, so an inlined callee's index conversion folds away. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>