bitmanipulation.h

#ifndef SIMDJSON_FALLBACK_BITMANIPULATION_H
#define SIMDJSON_FALLBACK_BITMANIPULATION_H

#include "simdjson.h"
#include <limits>

namespace simdjson {
namespace fallback {

#if defined(_MSC_VER) && !defined(_M_ARM64) && !defined(_M_X64)
static inline unsigned char _BitScanForward64(unsigned long* ret, uint64_t x) {
  unsigned long x0 = (unsigned long)x, top, bottom;
  _BitScanForward(&top, (unsigned long)(x >> 32));
  _BitScanForward(&bottom, x0);
  *ret = x0 ? bottom : 32 + top;
  return x != 0;
}
static unsigned char _BitScanReverse64(unsigned long* ret, uint64_t x) {
  unsigned long x1 = (unsigned long)(x >> 32), top, bottom;
  _BitScanReverse(&top, x1);
  _BitScanReverse(&bottom, (unsigned long)x);
  *ret = x1 ? top + 32 : bottom;
  return x != 0;
}
#endif

// We sometimes call trailing_zero on inputs that are zero,
// but the algorithms do not end up using the returned value.
// Sadly, sanitizers are not smart enough to figure it out.
NO_SANITIZE_UNDEFINED
really_inline int trailing_zeroes(uint64_t input_num) {
#ifdef _MSC_VER
  unsigned long ret;
  // Search the mask data from least significant bit (LSB) 
  // to the most significant bit (MSB) for a set bit (1).
  _BitScanForward64(&ret, input_num);
  return (int)ret;
#else // _MSC_VER
  return __builtin_ctzll(input_num);
#endif // _MSC_VER
}

/* result might be undefined when input_num is zero */
really_inline uint64_t clear_lowest_bit(uint64_t input_num) {
  return input_num & (input_num-1);
}

/* result might be undefined when input_num is zero */
really_inline int leading_zeroes(uint64_t input_num) {
#ifdef _MSC_VER
  unsigned long leading_zero = 0;
  // Search the mask data from most significant bit (MSB) 
  // to least significant bit (LSB) for a set bit (1).
  if (_BitScanReverse64(&leading_zero, input_num))
    return (int)(63 - leading_zero);
  else
    return 64;
#else
  return __builtin_clzll(input_num);
#endif// _MSC_VER
}

really_inline bool add_overflow(uint64_t value1, uint64_t value2, uint64_t *result) {
  *result = value1 + value2;
  return *result < value1;
}

really_inline bool mul_overflow(uint64_t value1, uint64_t value2, uint64_t *result) {
  *result = value1 * value2;
  // TODO there must be a faster way
  return value2 > 0 && value1 > std::numeric_limits<uint64_t>::max() / value2;
}

} // namespace fallback
} // namespace simdjson

#endif // SIMDJSON_FALLBACK_BITMANIPULATION_H