first commit of the new libm!

[musl] / src / math / cbrtf.c

diff --git a/src/math/cbrtf.c b/src/math/cbrtf.c
new file mode 100644 (file)
index 0000000..4a984b1
--- /dev/null
+++ b/src/math/cbrtf.c
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+/* origin: FreeBSD /usr/src/lib/msun/src/s_cbrtf.c */
+/*
+ * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
+ * Debugged and optimized by Bruce D. Evans.
+ */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+/* cbrtf(x)
+ * Return cube root of x
+ */
+
+#include "libm.h"
+
+static const unsigned
+B1 = 709958130, /* B1 = (127-127.0/3-0.03306235651)*2**23 */
+B2 = 642849266; /* B2 = (127-127.0/3-24/3-0.03306235651)*2**23 */
+
+float cbrtf(float x)
+{
+       double r,T;
+       float t;
+       int32_t hx;
+       uint32_t sign;
+       uint32_t high;
+
+       GET_FLOAT_WORD(hx, x);
+       sign = hx & 0x80000000;
+       hx ^= sign;
+       if (hx >= 0x7f800000)  /* cbrt(NaN,INF) is itself */
+               return x + x;
+
+       /* rough cbrt to 5 bits */
+       if (hx < 0x00800000) {  /* zero or subnormal? */
+               if (hx == 0)
+                       return x;  /* cbrt(+-0) is itself */
+               SET_FLOAT_WORD(t, 0x4b800000);  /* set t = 2**24 */
+               t *= x;
+               GET_FLOAT_WORD(high, t);
+               SET_FLOAT_WORD(t, sign|((high&0x7fffffff)/3+B2));
+       } else
+               SET_FLOAT_WORD(t, sign|(hx/3+B1));
+
+       /*
+        * First step Newton iteration (solving t*t-x/t == 0) to 16 bits.  In
+        * double precision so that its terms can be arranged for efficiency
+        * without causing overflow or underflow.
+        */
+       T = t;
+       r = T*T*T;
+       T = T*((double)x+x+r)/(x+r+r);
+
+       /*
+        * Second step Newton iteration to 47 bits.  In double precision for
+        * efficiency and accuracy.
+        */
+       r = T*T*T;
+       T = T*((double)x+x+r)/(x+r+r);
+
+       /* rounding to 24 bits is perfect in round-to-nearest mode */
+       return T;
+}