Check UTF-8 validity for all constant strings on compile time

[mvorisek]

Description

Currently the UTF-8 string validity is checked on demand and cached the string if not interned.

However:

• when a string is interned, such flag cannot be cached on runtime (at least not due TS)
• it is not stored back to the source script cache/opcache, ie. the validity is checked on every request at least once
• when a string is created from unvalidated source before checking, the validation cannot be cached
• currently only "valid UTF-8" flag is cached , but when a string is "invalid UTF-8", nothing is cached at all

This is a feature request to:
a) check UTF-8 validity on compile time on every const string
b) store a flag if UTF-8 validity was checked or not

Thanks to #10436 the UTF-8 validity check is very fast and the compile time impact should be minimal.

[iluuu1994]

2b9acd3

We currently have a large performance problem when implementing lexers
working on UTF-8 strings in PHP. This kind of code tends to perform a
large number of matches at different offsets on a single string. This
is generally fast. However, if /u mode is used, the full string will
be UTF-8 validated on each match. This results in quadratic runtime.

This patch fixes the issue by adding a IS_STR_VALID_UTF8 flag, which
is set when we have determined that the string is valid UTF8 and
further validation is skipped.

A limitation of this approach is that we can't set the flag for interned
strings. I think this is not a problem for this use-case which will
generally work on dynamic data. If we want to use this flag for other
purposes as well (mbstring?) then it might be worthwhile to UTF-8 validate
strings during interning. But right now this doesn't seem useful.

As we do now use this flag in mbstring I suppose this makes sense. Although I still suspect that mbstring primarily operates on dynamic and thus non-interned strings.

[youkidearitai]

Is also that related #10409 ? That thread talk about mb_check_encoding adding flag IS_STR_VALID_UTF8 when UTF-8 string is valid, but it isn't still worked.

[iluuu1994]

@youkidearitai Yes, string that appear in your source code would be marked as valid UTF-8 after implementing this. Thanks for pointing out that thread, updating the flag atomically is something I didn't consider.