How do I atomically swap 2 ints in C#?

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This is the likely implementation for Interlocked.Exchange() in the CLR, copied from the SSCLI20 source:

Note that UP in the function name means UniProcessor. This is not atomic on SMP / multi-core systems. This implementation will only be used by CLR on single-core systems.

FASTCALL_FUNC ExchangeUP,8
        _ASSERT_ALIGNED_4_X86 ecx
        mov     eax, [ecx]      ; attempted comparand
retry:
        cmpxchg [ecx], edx
        jne     retry1          ; predicted NOT taken
        retn
retry1:
        jmp     retry
FASTCALL_ENDFUNC ExchangeUP

It is superior to using XCHG because this code works without taking a bus lock. xchg has an implicit lock prefix, so unlike xadd or cmpxchg it simply can’t be omitted for single-core systems to still do the operation in one instruction to make it atomic with respect to interrupts (and thus other threads on uniprocessor).

The odd looking jumping code is an optimization in case branch prediction data is not available. Needless to say perhaps, trying to do a better job than what has been mulled over for many years by very good software engineers with generous helpings from the chip manufacturers is a tall task.

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