src/gromacs/random/seed.cpp

   1 /*
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  35
  36 #include "gmxpre.h"
  37
  38 #include "seed.h"
  39
  40 #include <time.h>
  41
  42 #include "gromacs/utility/basedefinitions.h"
  43 #include "gromacs/utility/fatalerror.h"
  44
  45 namespace gmx
  46 {
  47
  48
  49 /*! \brief Check if the RDRAND random device functioning correctly
  50  *
  51  * Due to a bug in AMD Ryzen microcode, RDRAND may always return -1 (0xFFFFFFFF).
  52  * To avoid that, fall back to using PRNG instead of RDRAND if this happens.
  53  *
  54  * \returns The result of the checks.
  55  */
  56 static bool checkIfRandomDeviceIsFunctional()
  57 {
  58     std::random_device rd;
  59
  60     uint64_t randomNumber1 = static_cast<uint64_t>(rd());
  61     uint64_t randomNumber2 = static_cast<uint64_t>(rd());
  62
  63     // Due to a bug in AMD Ryzen microcode, RDRAND may always return -1 (0xFFFFFFFF).
  64     // To avoid that, fall back to using PRNG instead of RDRAND if this happens.
  65     if (randomNumber1 == 0xFFFFFFFF && randomNumber2 == 0xFFFFFFFF)
  66     {
  67         if (debug)
  68         {
  69             fprintf(debug,
  70                     "Hardware random number generator (RDRAND) returned -1 (0xFFFFFFFF) twice in\n"
  71                     "a row. This may be due to a known bug in AMD Ryzen microcode.");
  72         }
  73         return false;
  74     }
  75     return true;
  76 }
  77
  78 /*! \brief Get the next pure or pseudo-random number
  79  *
  80  * Returns the next random number taken from the hardware generator or from PRNG.
  81  *
  82  * \param[in] gen  Pseudo-random/random numbers generator/device to use.
  83  *
  84  * \return Random or pseudo-random number.
  85  */
  86 template<typename GeneratorType>
  87 static uint64_t makeRandomSeedInternal(GeneratorType& gen)
  88 {
  89     constexpr std::size_t resultBits = std::numeric_limits<uint64_t>::digits;
  90     constexpr std::size_t numBitsInRandomNumber =
  91             std::numeric_limits<typename GeneratorType::result_type>::digits;
  92
  93     uint64_t result = static_cast<uint64_t>(gen());
  94     // This is needed so that compiler understands that what follows is a dead branch
  95     // and not complains about shift count larger than number of bits in the result.
  96     constexpr std::size_t shiftCount = (resultBits < numBitsInRandomNumber) ? numBitsInRandomNumber : 0;
  97     for (std::size_t bits = numBitsInRandomNumber; bits < resultBits; bits += numBitsInRandomNumber)
  98     {
  99         result = (result << shiftCount) | static_cast<uint64_t>(gen());
 100     }
 101     return result;
 102 }
 103
 104 uint64_t makeRandomSeed()
 105 {
 106     bool useRdrand = checkIfRandomDeviceIsFunctional();
 107     if (useRdrand)
 108     {
 109         std::random_device rd;
 110         return makeRandomSeedInternal(rd);
 111     }
 112     else
 113     {
 114         std::mt19937_64 prng(time(nullptr));
 115         return makeRandomSeedInternal(prng);
 116     }
 117 }
 118
 119
 120 } // namespace gmx