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Since RAMCloud is written in C++, we should use asm rather than the uglier __asm__.

volatile vs __volatile__

The volatile keyword on asm statements keeps gcc from deleting those asm statements as useless code. You must inform gcc of all side effects of your asm statements. If your asm statements have important side effects that can't be expressed as output operands (or you don't use the output operands but still want the asm instructions to execute), you should use the volatile keyword. This keeps gcc from deleting the asm block as part of its optimization passes. From Extended Asm in the GCC manual:

The volatile keyword indicates that the instruction has important side-effects. GCC will not delete a volatile asm if it is reachable. (The instruction can still be deleted if GCC can prove that control-flow will never reach the location of the instruction.)

Volatile does not, however, prevent GCC from moving the asm instruction. From Extended Asm in the GCC manual:

Note that even a volatile asm instruction can be moved relative to other code, including across jump instructions.

The gcc manual is still unsatisfying in this regard. What can asm volatile statements be reordered with? If all the compiler knows is that running the asm is important, what does it assume about where the asm must run? The manual suggests it's not entirely conservative.

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