This is a relatively new feature and there have been one or two ABI breaks since they thought it was done that required recompiling half the world. I think it's

This is a relatively new feature and there have been one or two ABI breaks since they thought it was done that required recompiling half the world. I think it's been solid for at least most of the last decade though so... stable enough? I'm not sure if there is an actual spec somewhere though, certainly not a proper one from a standards body. It's just "whatever GCC or MSVC do on that OS/CPU combo" and those two happen to have stopped changing the ABI every compiling release at some point. They're both at least based on the relatively well specified Itanium C++ ABI though so that helps.