[tialaramex]

Since they mention "a few decades ago" I'm guessing it's a very different mutex than the one you'd use today.

Today I believe all of these operating systems (except MacOS, LOL) have either futex or an equivalent technology (the Windows thing works on bytes rather than aligned 32-bit values, so that's cool) and so it seems like only contention could be more expensive as the whole point of a futex is that uncontended acquisition is a single CPU store and that's userspace, the kernel is nowhere near it.

[mmoskal]

macOS has futexes it just doesn't talk about them... https://crates.io/crates/ulock-sys

[tialaramex]

I was not aware. I wonder if anybody tried asking Apple? That's what happened for SRWLOCK in Windows. Mara asked Microsoft if they can promise it actually does what it seems like it does, formally, and they did document that promise so then Rust used SRWLOCK [today your Rust does not use SRWLOCK, this year it was replaced for performance reasons, which is timely because it turns out SRWLOCK is faulty and so "Don't use SRWLOCK" is the most practical fix in the medium term, awkward for poor C++ though as they used SRWLOCK too].

Apple are even less talkative than Microsoft, but it is possible that the reason libc++ uses this is that it's actually guaranteed and if properly asked Apple would say so. It would be nice for (newer) macOS to get the same promises as the mainstream platforms with respect to these primitives.