math: new log2

[musl] / src / math / exp2.c

diff --git a/src/math/exp2.c b/src/math/exp2.c
index 8e25228..e14adba 100644 (file)
--- a/src/math/exp2.c
+++ b/src/math/exp2.c
@@ -305,7 +305,7 @@ static const double tbl[TBLSIZE * 2] = {
  * Method: (accurate tables)
  *
  *   Reduce x:
- *     x = 2**k + y, for integer k and |y| <= 1/2.
+ *     x = k + y, for integer k and |y| <= 1/2.
  *     Thus we have exp2(x) = 2**k * exp2(y).
  *
  *   Reduce y:
@@ -330,41 +330,41 @@ static const double tbl[TBLSIZE * 2] = {
  */
 double exp2(double x)
 {
-       double r, t, z;
-       uint32_t hx, ix, i0;
+       double_t r, t, z;
+       uint32_t ix, i0;
+       union {double f; uint64_t i;} u = {x};
        union {uint32_t u; int32_t i;} k;
 
        /* Filter out exceptional cases. */
-       GET_HIGH_WORD(hx, x);
-       ix = hx & 0x7fffffff;
-       if (ix >= 0x40900000) {        /* |x| >= 1024 */
-               if (ix >= 0x7ff00000) {
-                       GET_LOW_WORD(ix, x);
-                       if (hx == 0xfff00000 && ix == 0) /* -inf */
-                               return 0;
-                       return x;
-               }
-               if (x >= 1024) {
-                       STRICT_ASSIGN(double, x, x * 0x1p1023);
+       ix = u.i>>32 & 0x7fffffff;
+       if (ix >= 0x408ff000) {  /* |x| >= 1022 or nan */
+               if (ix >= 0x40900000 && u.i>>63 == 0) {  /* x >= 1024 or nan */
+                       /* overflow */
+                       x *= 0x1p1023;
                        return x;
                }
-               if (x <= -1075) {
-                       STRICT_ASSIGN(double, x, 0x1p-1000*0x1p-1000);
-                       return x;
+               if (ix >= 0x7ff00000)  /* -inf or -nan */
+                       return -1/x;
+               if (u.i>>63) {  /* x <= -1022 */
+                       /* underflow */
+                       if (x <= -1075 || x - 0x1p52 + 0x1p52 != x)
+                               FORCE_EVAL((float)(-0x1p-149/x));
+                       if (x <= -1075)
+                               return 0;
                }
        } else if (ix < 0x3c900000) {  /* |x| < 0x1p-54 */
                return 1.0 + x;
        }
 
        /* Reduce x, computing z, i0, and k. */
-       STRICT_ASSIGN(double, t, x + redux);
-       GET_LOW_WORD(i0, t);
+       u.f = x + redux;
+       i0 = u.i;
        i0 += TBLSIZE / 2;
        k.u = i0 / TBLSIZE * TBLSIZE;
        k.i /= TBLSIZE;
        i0 %= TBLSIZE;
-       t -= redux;
-       z = x - t;
+       u.f -= redux;
+       z = x - u.f;
 
        /* Compute r = exp2(y) = exp2t[i0] * p(z - eps[i]). */
        t = tbl[2*i0];       /* exp2t[i0] */