In std::hint there's a spin_loop function with the following definition in its documentation:

Emits a machine instruction to signal the processor that it is running in a busy-wait spin-loop (“spin lock”).

Upon receiving the spin-loop signal the processor can optimize its behavior by, for example, saving power or switching hyper-threads.

Depending on the target architecture, this compiles to either:

• _mm_pause, A.K.A. the pause intrinsic on x86
• yield instruction on 32-bit arm
• ISB SY on 64-bit arm (aarch64)

That last one has got my head spinning a little bit (