summaryrefslogtreecommitdiff
path: root/src/math/cbrtf.c
diff options
context:
space:
mode:
Diffstat (limited to 'src/math/cbrtf.c')
-rw-r--r--src/math/cbrtf.c69
1 files changed, 69 insertions, 0 deletions
diff --git a/src/math/cbrtf.c b/src/math/cbrtf.c
new file mode 100644
index 00000000..4a984b10
--- /dev/null
+++ b/src/math/cbrtf.c
@@ -0,0 +1,69 @@
+/* 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;
+}