29.3.1. x86 exchange instructions
Intel 64 and IA-32 Architectures Software Developer’s Manuals Volume 1 7.3.1.2 "Exchange Instructions":
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userland/arch/x86_64/xadd.S XADD: exchange and add. This is how C `<atomic>`'s' `` is implemented in GCC 5.1. TODO: why is the exchange part needed?
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userland/arch/x86_64/xchg.S XCHG: exchange two values
TODO: concrete multi-thread GCC inline assembly examples of how all those instructions are normally used as synchronization primitives.