func detectUTF8(s string) (valid, require bool) {
	for i := 0; i < len(s); {
		r, size := utf8.DecodeRuneInString(s[i:])
		i += size
		// Officially, ZIP uses CP-437, but many readers use the system's
		// local character encoding. Most encoding are compatible with a large
		// subset of CP-437, which itself is ASCII-like.
		//
		// Forbid 0x7e and 0x5c since EUC-KR and Shift-JIS replace those

// At the end of the archive, Next returns the error io.EOF.
//
// If Next encounters a non-local name (as defined by [filepath.IsLocal])
// and the GODEBUG environment variable contains `tarinsecurepath=0`,
// Next returns the header with an [ErrInsecurePath] error.
// A future version of Go may introduce this behavior by default.
// Programs that want to accept non-local names can ignore
// the [ErrInsecurePath] error and use the returned header.

For example, on essentially all CPUs, it is valid to rewrite
</p>

<pre>
n := 0
for i := 0; i < m; i++ {
	n += *shared
}
</pre>

into:

<pre>
n := 0
local := *shared
for i := 0; i < m; i++ {
	n += local
}
</pre>

<p>
provided it can be proved that <code>*shared</code> will not fault on access,
because the potential added read will not affect any existing concurrent reads or writes.