Optimizing synctex_edit by hand-write string-to-integer conversion

[user202729]

Proof of concept: https://github.com/user202729/luatex/commit/005c6702489d3a06dd3eca15ab4dbe8a7c544f0c

Admittedly this is ugly, but this part is very time-critical in the code.

In my testing, this change alone improves 17% performance (after applying the optimization described in #69 )

What do you think?

Caveat of the quick implementation above:

• technically the special int value -2^31 will trigger undefined behavior, but the output will be correct regardless
• tab and newline are not skipped, only spaces are skipped

[user202729]

@jlaurens Thanks for the quick fix, although the new code has the issue that -INT_MIN (as written in the source code) will overflow and evaluate to INT_MIN. Maybe better to just put INT_MAX in both cases?

[user202729]

Speaking of which I think there are two issues with the synctex_merge_strings function:

• if(dest != strncpy(dest,temp,size)) this check does nothing, strncpy always return dest.
• result is obtained from malloc(size), but if we include the null terminator, we need to malloc(size+1) instead.

• Miscellaneous: on my machine, the compiler raises a warning like this

  ../synctex_parser_utils.c:370:52: warning: ‘strncpy’ output truncated before terminating nul copying
   as many bytes from a string as its length [-Wstringop-truncation]
    370 |                                         if(dest != strncpy(dest,temp,size)) {
        |                                                    ^~~~~~~~~~~~~~~~~~~~~~~

  maybe it's better to make the intent explicit by using memcpy instead. (I believe memcpy should be faster as well because it doesn't need to check for null byte)