The prototypes for Streaming SIMD Extensions intrinsics are in the xmmintrin.h header file.
void _mm_pause(void)
The execution of the next instruction is delayed an implementation specific amount of time. The instruction does not modify the architectural state. This intrinsic provides especially significant performance gain and described in more detail below.
PAUSE Intrinsic
The PAUSE intrinsic is used in spin-wait loops with the processors implementing dynamic execution (especially out-of-order execution). In the spin-wait loop, PAUSE improves the speed at which the code detects the release of the lock. For dynamic scheduling, the PAUSE instruction reduces the penalty of exiting from the spin-loop.
Example of loop with the PAUSE instruction:
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spin_loop:pause cmp eax, A jne spin_loop |
In the above example, the program spins until memory location A matches the value in register eax. The code sequence that follows shows a test-and-test-and-set. In this example, the spin occurs only after the attempt to get a lock has failed.
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get_lock: mov eax, 1 xchg eax, A ; Try to get lock cmp eax, 0 ; Test if successful jne spin_loop |
Critical Section
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// critical_section code mov A, 0 ; Release lock jmp continue spin_loop: pause; // spin-loop hint cmp 0, A ; // check lock availability jne spin_loop jmp get_lock // continue: other code |
Note that the first branch is predicted to fall-through to the critical section in anticipation of successfully gaining access to the lock. It is highly recommended that all spin-wait loops include the PAUSE instruction. Since PAUSE is backwards compatible to all existing IA-32 processor generations, a test for processor type (a CPUID test) is not needed. All legacy processors will execute PAUSE as a NOP, but in processors which use the PAUSE as a hint there can be significant performance benefit.