OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to
  memory bus
 #ifdef OPENSSL_CPUID_OBJ
 size_t OPENSSL_instrument_bus(int *vector, size_t num);
 size_t OPENSSL_instrument_bus2(int *vector, size_t num, size_t max);
 #endif
It was empirically found that timings of references to primary memory are
  subject to irregular, apparently non-deterministic variations. The subroutines
  in question instrument these references for purposes of gathering randomness
  for random number generator. In order to make it bus-bound a 'flush cache
  line' instruction is used between probes. In addition probes are added to
  vector elements in atomic or interlocked manner, which should
  contribute additional noise on multi-processor systems. This also means that
  vector[num] should be zeroed upon invocation (if you want to retrieve
  actual probe values).
OPENSSL_instrument_bus() performs num probes and
    records the number of oscillator cycles every probe took.
OPENSSL_instrument_bus2() on the other hand
    accumulates consecutive probes with the same value, i.e. in a way it
    records duration of periods when probe values appeared deterministic. The
    subroutine performs at most max probes in attempt to fill the
    vector[num], with max value of 0 meaning "as many as it
    takes."
Return value of 0 indicates that CPU is not capable of performing the benchmark,
  either because oscillator counter or 'flush cache line' is not available on
  current platform. For reference, on x86 'flush cache line' was introduced with
  the SSE2 extensions.
Otherwise number of recorded values is returned.
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Licensed under the OpenSSL license (the "License"). You
    may not use this file except in compliance with the License. You can obtain
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