Question

Visual Studio produces the following machine code when _InterlockedIncrement is used:

; 40   :        _InterlockedIncrement(&framecounter);

  00078 b8 00 00 00 00   mov     eax, OFFSET ?framecounter@@3JA ; framecounter
  0007d b9 01 00 00 00   mov     ecx, 1
  00082 f0 0f c1 08  lock     xadd   DWORD PTR [eax], ecx

If I would be writing this i would use just lock inc DWORD PTR [eax] instead of mov and xadd

Is there a valid reason why Microsoft preferred xadd and using 2 instructions instead of 1?

Answer 1

Because _InterlockedIncrement also returns the new value.

You can’t do that with lock inc DWORD PTR [eax], because now neither the old nor the new value are anywhere to be found. Except in memory, but if you do an other read, clearly it won’t be atomic (the increment itself would be, but you could get a value back that has nothing to do with what happened at the time of the increment).

Returning the value makes _InterlockedIncrement more useful.