I’d also argue

   > decent amount of stuff that CAN be done in _start before you run main

It’s all optional. Before I found the Linux kernel’s nolibc [0], I wrote a minimal C runtime based on bunch of stuff I read [1]. Only thing you really need to do is align the stack (at least on x86, amd64). You don’t get argc/argv/envp but main will still work.

[0] https://github.com/torvalds/linux/tree/master/tools/include/...

[1] https://github.com/lpsantil/rt0/blob/master/src/lib/00_start...