Fix minor ldexp issues

* doc/lispref/numbers.texi (Float Basics): Improve ldexp documentation.
* src/floatfns.c (Fldexp): Require 2 args.  Avoid undefined behavior
if the exponent is out of 'int' range.  Improve documentation.
Fixes: bug#20185

4 files changed, 25 insertions, 8 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index a546306f9e4..9b1bbb357a5 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,7 @@
+2015-03-24  Paul Eggert  <eggert@cs.ucla.edu>
+
+        * numbers.texi (Float Basics): Improve ldexp documentation.
+
 2015-03-23  Eli Zaretskii  <eliz@gnu.org>
 
         * commands.texi (Event Input Misc): Fix incorrect usage of @code.
diff --git a/doc/lispref/numbers.texi b/doc/lispref/numbers.texi
index 8d1d3a798eb..7b4a0a6d407 100644
--- a/doc/lispref/numbers.texi
+++ b/doc/lispref/numbers.texi
@@ -265,9 +265,15 @@ If @var{x} is a NaN, then @var{s} is also a NaN@.
 If @var{x} is zero, then @var{e} is 0.
 @end defun
 
-@defun ldexp sig &optional exp
-This function returns a floating-point number corresponding to the
-significand @var{sig} and exponent @var{exp}.
+@defun ldexp s e
+Given a numeric significand @var{s} and an integer exponent @var{e},
+this function returns the floating point number
+@ifnottex
+@var{s} * 2**@var{e}.
+@end ifnottex
+@tex
+@math{s 2^e}.
+@end tex
 @end defun
 
 @defun copysign x1 x2
diff --git a/src/ChangeLog b/src/ChangeLog
index 815c117308b..23f125c567d 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,10 @@
+2015-03-24  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Fix minor ldexp issues
+        * floatfns.c (Fldexp): Require 2 args.  Avoid undefined behavior
+        if the exponent is out of 'int' range.  Improve documentation.
+        Fixes: bug#20185
+
 2015-03-24  Daniel Colascione  <dancol@dancol.org>
 
         * process.c (Fprocess_running_child_p): Return number identifier of
diff --git a/src/floatfns.c b/src/floatfns.c
index c68b9bd3a65..072e85776b5 100644
--- a/src/floatfns.c
+++ b/src/floatfns.c
@@ -185,14 +185,14 @@ If X is zero, both parts (SGNFCAND and EXP) are zero.  */)
   return Fcons (make_float (sgnfcand), make_number (exponent));
 }
 
-DEFUN ("ldexp", Fldexp, Sldexp, 1, 2, 0,
-       doc: /* Construct number X from significand SGNFCAND and exponent EXP.
-Returns the floating point value resulting from multiplying SGNFCAND
-(the significand) by 2 raised to the power of EXP (the exponent).   */)
+DEFUN ("ldexp", Fldexp, Sldexp, 2, 2, 0,
+       doc: /* Return X * 2**EXP, as a floating point number.
+EXP must be an integer.   */)
   (Lisp_Object sgnfcand, Lisp_Object exponent)
 {
   CHECK_NUMBER (exponent);
-  return make_float (ldexp (XFLOATINT (sgnfcand), XINT (exponent)));
+  int e = min (max (INT_MIN, XINT (exponent)), INT_MAX);
+  return make_float (ldexp (XFLOATINT (sgnfcand), e));
 }
 
 DEFUN ("exp", Fexp, Sexp, 1, 1, 0,