fusl/src/math/ilogb.c

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

int ilogb(double x) {
  PRAGMA_STDC_FENV_ACCESS_ON
  union {
    double f;
    uint64_t i;
  } u = {x};
  uint64_t i = u.i;
  int e = i >> 52 & 0x7ff;

  if (!e) {
    i <<= 12;
    if (i == 0) {
      FORCE_EVAL(0 / 0.0f);
      return FP_ILOGB0;
    }
    /* subnormal x */
    for (e = -0x3ff; i >> 63 == 0; e--, i <<= 1)
      ;
    return e;
  }
  if (e == 0x7ff) {
    FORCE_EVAL(0 / 0.0f);
    return i << 12 ? FP_ILOGBNAN : INT_MAX;
  }
  return e - 0x3ff;
}