The compiler now can fully inline the co_switch, and with most registers being specified as clobbers and not saved explicitly, the compiler can choose to save only what it needs to (we don't have to defensively save everything).
Practically speaking, the co_switch calls are usually inlined, but the functions they're in don't seem to be that big and don't make direct use of r12..r15 too much anyway, so (push r12..r15, switch, pop r12..r15) is a common emit. But I see a miniscule FPS increase.
The waterbox system now uses host os facilities to track whether memory has been written to, to automatically choose what thing to savestate. This results in a large size decrease for some cores, like snes9x or gpgx (when running cartridge games). Doesn't do much for cores that were already memory efficient, or for bsnes because of libco compatibility issues; but those cores don't regress either.
Create an all new waterbox build environment:
WSL2 + Ubuntu 20.04 LTS (Other linuxes may work)
Musl libc with waterbox customizations
LLVM's libclang-rt, libunwind, libcxxabi, libcxx
Static linking to elf files
Compared with the old system, this is easier to set up a dev env for and easier to update in the future. The executables are larger but produce smaller savestates due to static linking. The modern toolchain means advanced library features and language features that sometimes appear in some upstream cores will be reusable.
nattthebear