Add log2 support and make log10 obsolete for consistency.

* configure.ac (log2): Check for this function.
* doc/lispref/numbers.texi (Math Functions): Remove obsolete function log10.
* lisp/subr.el (log10): Move here from C code, and declare as obsolete.
All uses of (log10 X) replaced with (log X 10).
* src/floatfns.c (Flog) [HAVE_LOG2]: Use log2 if available and if the
base is 2; this is more accurate.
(Flog10): Move to Lisp (marked obsolete there).

1 files changed, 10 insertions, 15 deletions

diff --git a/src/floatfns.c b/src/floatfns.c
index dd6d3dfe582..f3d0936f888 100644
--- a/src/floatfns.c
+++ b/src/floatfns.c
@@ -25,7 +25,8 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 /* C89 requires only the following math.h functions, and Emacs omits
    the starred functions since we haven't found a use for them:
    acos, asin, atan, atan2, ceil, cos, *cosh, exp, fabs, floor, fmod,
-   frexp, ldexp, log, log10, *modf, pow, sin, *sinh, sqrt, tan, *tanh.
+   frexp, ldexp, log, log10 [via (log X 10)], *modf, pow, sin, *sinh,
+   sqrt, tan, *tanh.
 
    C99 and C11 require the following math.h functions in addition to
    the C89 functions.  Of these, Emacs currently exports only the
@@ -33,10 +34,10 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
    acosh, atanh, cbrt, *copysign, erf, erfc, exp2, expm1, fdim, fma,
    fmax, fmin, fpclassify, hypot, ilogb, isfinite, isgreater,
    isgreaterequal, isinf, isless, islessequal, islessgreater, *isnan,
-   isnormal, isunordered, lgamma, log1p, log2, *logb (approximately),
-   lrint/llrint, lround/llround, nan, nearbyint, nextafter,
-   nexttoward, remainder, remquo, *rint, round, scalbln, scalbn,
-   signbit, tgamma, trunc.
+   isnormal, isunordered, lgamma, log1p, *log2 [via (log X 2)], *logb
+   (approximately), lrint/llrint, lround/llround, nan, nearbyint,
+   nextafter, nexttoward, remainder, remquo, *rint, round, scalbln,
+   scalbn, signbit, tgamma, trunc.
  */
 
 #include <config.h>
@@ -252,21 +253,16 @@ If the optional argument BASE is given, return log ARG using that base.  */)
 
       if (b == 10.0)
         d = log10 (d);
+#if HAVE_LOG2
+      else if (b == 2.0)
+        d = log2 (d);
+#endif
       else
         d = log (d) / log (b);
     }
   return make_float (d);
 }
 
-DEFUN ("log10", Flog10, Slog10, 1, 1, 0,
-       doc: /* Return the logarithm base 10 of ARG.  */)
-  (Lisp_Object arg)
-{
-  double d = extract_float (arg);
-  d = log10 (d);
-  return make_float (d);
-}
-
 DEFUN ("sqrt", Fsqrt, Ssqrt, 1, 1, 0,
        doc: /* Return the square root of ARG.  */)
   (Lisp_Object arg)
@@ -564,7 +560,6 @@ syms_of_floatfns (void)
   defsubr (&Sexp);
   defsubr (&Sexpt);
   defsubr (&Slog);
-  defsubr (&Slog10);
   defsubr (&Ssqrt);
 
   defsubr (&Sabs);