I did the same for tinycc here https://github.com/mingodad/tinycc and used Netbeans IDE that has great refactoring options for C/C++/Java.

Benchmarking the reentrant result showed it to be around 5% slower.

Now I'm trying to redo it again but this time scripting the refactoring using sparse https://github.com/lucvoo/sparse to parse and using it's error messages with with line/column to guide the refactoring, I already got an initial script that performs some initial transformations and is repeatable, but more work need to be done, mainly enhance/extend the info that sparse provide while parsing the code.

Why not use thread_local instead of passing a param everywhere? What's the drawback there?

they could adopt mold as a linker

Ive always struggled to understand the need to have linker

Like, you could easily write your compiler to do not have to rely on such machinery

Meanwhile they add complexity and decrease quality of error messages (in cpp)