mirror of
https://github.com/vercel-labs/scriptc.git
synced 2026-11-01 07:33:12 +08:00
Chris Tate
5d8804db38
`for (let i = 0; i < s.length; i++) if (s.charCodeAt(i) === 32) n++` over a text with any non-ASCII character failed the loop version's ASCII guard and ran the checked loop. Each read tested the ASCII bit, compared the string with the runtime's published scan receiver, tested the bounds, loaded the units pointer, and kept the opaque fallback. The guard now also accepts a version's one string receiver when it is not ASCII. One runtime read publishes the string as the scan receiver (ScrStrScan), whose UTF-16 units the runtime holds once a scan has read it enough. The guard then tests the accesses against the published unit count, and a third copy of the loop reads each unit with one 16-bit load from the units pointer taken in the guard. The guard takes that path only while the runtime publishes the receiver itself, so the length it took is the receiver's UTF-16 length on both fast paths. Both fast copies read it for the receiver's `length`, which a while loop's condition otherwise asks the runtime for on every iteration. The runtime drops a string's units when the string is released or appended to in place, or when its index entry is evicted because another non-ASCII string is indexed. The receiver is a binding the loop does not write, so the first two cannot happen while the loop runs. A units copy is emitted only for a loop that uses unit-safe operations (string-unit-versions.ts): numbers, locals, records and class fields, typed arrays, number arrays, Math, the receiver's charCodeAt and length, and calls of functions made of the same operations that read no string. Tests of `s.charAt(i)` against one-unit literals now become charCodeAt compares in the IR pass (char-unit-locals.ts), so a version sees one kind of read.