Investigate using a faster Regex library

[bigbluejay9]

Spring uses the std::regex library to perform regular expression operations.

std::regex is known to be quite slow https://github.com/HFTrader/regex-performance#results with potentially no path forward to improve its performance stack overflow, llvm discussion.

Spring has minimal dependencies on the std::regex library, which is exposed through https://github.com/beyond-all-reason/spring/blob/BAR105/rts/System/SpringRegex.h by re-exporting the std::regex* symbols.

Investigate replacing std::regex with a more performant library. Doing this when there are few dependencies should make the work somewhat more straightforward.

[bigbluejay9]

CTRE, as the name implies, only supports compile time regular expressions. Spring mostly uses compile time constant regular expressions, with the key notable exception of globbing: https://github.com/beyond-all-reason/spring/blob/d01542dbad7627dc9a00b3d78e7fee3af7755e30/rts/System/FileSystem/FileSystem.cpp#L40, https://github.com/beyond-all-reason/spring/blob/d01542dbad7627dc9a00b3d78e7fee3af7755e30/rts/System/FileSystem/FileFilter.cpp#L130, https://github.com/beyond-all-reason/spring/blob/d01542dbad7627dc9a00b3d78e7fee3af7755e30/rts/System/FileSystem/FileSystemAbstraction.h#L126.

To use CTRE, we'd have to add support for globbing without regexes - either manually, or pulling in another library, e.g. https://github.com/p-ranav/glob and replacing calls to the real filesystem with calls to VFS.

[bigbluejay9]

Other alternative regex libraries:

• boost::regex supports runtime expressions with ok performance (it is the basis of std::regex). However, I'm not sure how allergic Spring is to boost.
• hsan is x86 only
• oniguruma C API, seems quite usable.
• RE2 brings in the absl library

[p2004a]

Is there any heavy regex usage that justifies the cost of additional dependency?

[bigbluejay9]

Not that significant, but it is the source of some slowness during loading. ~300ms from sampling (~4% of total load time).

Mainly just investigating what it'll take to switch regex libraries. Even if we don't decide to it might be good to have a trail of what was previously tried.

[Beherith]

I think the engine depended on boost at some point, because even the mention of it gives me ptsd.

[lhog]

I like the CTRE + https://github.com/p-ranav/glob idea.