double-conversion/diy-fp.cc

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#include "diy-fp.h"
#include "utils.h"

namespace double_conversion {

void DiyFp::Multiply(const DiyFp& other) {
  // Simply "emulates" a 128 bit multiplication.
  // However: the resulting number only contains 64 bits. The least
  // significant 64 bits are only used for rounding the most significant 64
  // bits.
  const uint64_t kM32 = 0xFFFFFFFFU;
  uint64_t a = f_ >> 32;
  uint64_t b = f_ & kM32;
  uint64_t c = other.f_ >> 32;
  uint64_t d = other.f_ & kM32;
  uint64_t ac = a * c;
  uint64_t bc = b * c;
  uint64_t ad = a * d;
  uint64_t bd = b * d;
  uint64_t tmp = (bd >> 32) + (ad & kM32) + (bc & kM32);
  // By adding 1U << 31 to tmp we round the final result.
  // Halfway cases will be round up.
  tmp += 1U << 31;
  uint64_t result_f = ac + (ad >> 32) + (bc >> 32) + (tmp >> 32);
  e_ += other.e_ + 64;
  f_ = result_f;
}

}  // namespace double_conversion