first commit of the new libm!

[musl] / src / math / log10.c

diff --git a/src/math/log10.c b/src/math/log10.c
new file mode 100644 (file)
index 0000000..5422599
--- /dev/null
+++ b/src/math/log10.c
@@ -0,0 +1,84 @@
+/* origin: FreeBSD /usr/src/lib/msun/src/e_log10.c */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunSoft, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+/*
+ * Return the base 10 logarithm of x.  See e_log.c and k_log.h for most
+ * comments.
+ *
+ *    log10(x) = (f - 0.5*f*f + k_log1p(f)) / ln10 + k * log10(2)
+ * in not-quite-routine extra precision.
+ */
+
+#include "libm.h"
+#include "__log1p.h"
+
+static const double
+two54     = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
+ivln10hi  = 4.34294481878168880939e-01, /* 0x3fdbcb7b, 0x15200000 */
+ivln10lo  = 2.50829467116452752298e-11, /* 0x3dbb9438, 0xca9aadd5 */
+log10_2hi = 3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */
+log10_2lo = 3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */
+
+static const double zero = 0.0;
+
+double log10(double x)
+{
+       double f,hfsq,hi,lo,r,val_hi,val_lo,w,y,y2;
+       int32_t i,k,hx;
+       uint32_t lx;
+
+       EXTRACT_WORDS(hx, lx, x);
+
+       k = 0;
+       if (hx < 0x00100000) {  /* x < 2**-1022  */
+               if (((hx&0x7fffffff)|lx) == 0)
+                       return -two54/zero;  /* log(+-0)=-inf */
+               if (hx<0)
+                       return (x-x)/zero;   /* log(-#) = NaN */
+               /* subnormal number, scale up x */
+               k -= 54;
+               x *= two54;
+               GET_HIGH_WORD(hx, x);
+       }
+       if (hx >= 0x7ff00000)
+               return x+x;
+       if (hx == 0x3ff00000 && lx == 0)
+               return zero;  /* log(1) = +0 */
+       k += (hx>>20) - 1023;
+       hx &= 0x000fffff;
+       i = (hx+0x95f64)&0x100000;
+       SET_HIGH_WORD(x, hx|(i^0x3ff00000));  /* normalize x or x/2 */
+       k += i>>20;
+       y = (double)k;
+       f = x - 1.0;
+       hfsq = 0.5*f*f;
+       r = __log1p(f);
+
+       /* See log2.c for details. */
+       hi = f - hfsq;
+       SET_LOW_WORD(hi, 0);
+       lo = (f - hi) - hfsq + r;
+       val_hi = hi*ivln10hi;
+       y2 = y*log10_2hi;
+       val_lo = y*log10_2lo + (lo+hi)*ivln10lo + lo*ivln10hi;
+
+       /*
+        * Extra precision in for adding y*log10_2hi is not strictly needed
+        * since there is no very large cancellation near x = sqrt(2) or
+        * x = 1/sqrt(2), but we do it anyway since it costs little on CPUs
+        * with some parallelism and it reduces the error for many args.
+        */
+       w = y2 + val_hi;
+       val_lo += (y2 - w) + val_hi;
+       val_hi = w;
+
+       return val_lo + val_hi;
+}