[Python-checkins] r69465 - in python/branches/py3k: Include/pymath.h Misc/NEWS Python/pymath.c

[mark.dickinson]

Author: mark.dickinson
Date: Mon Feb  9 18:15:59 2009
New Revision: 69465

Log:
Merged revisions 69459 via svnmerge from 
svn+ssh://pythondev@svn.python.org/python/trunk

........
  r69459 | mark.dickinson | 2009-02-09 14:18:43 +0000 (Mon, 09 Feb 2009) | 3 lines
  
  Issue #4575: fix Py_IS_INFINITY macro to work correctly on x87 FPUs.
  It now forces its argument to double before testing for infinity.
........


Modified:
   python/branches/py3k/   (props changed)
   python/branches/py3k/Include/pymath.h
   python/branches/py3k/Misc/NEWS
   python/branches/py3k/Python/pymath.c

Modified: python/branches/py3k/Include/pymath.h
==============================================================================
--- python/branches/py3k/Include/pymath.h	(original)
+++ python/branches/py3k/Include/pymath.h	Mon Feb  9 18:15:59 2009
@@ -77,6 +77,21 @@
 #define Py_MATH_E 2.7182818284590452354
 #endif
 
+/* On x86, Py_FORCE_DOUBLE forces a floating-point number out of an x87 FPU
+   register and into a 64-bit memory location, rounding from extended
+   precision to double precision in the process.  On other platforms it does
+   nothing. */
+
+/* we take double rounding as evidence of x87 usage */
+#ifndef Py_FORCE_DOUBLE
+#  ifdef X87_DOUBLE_ROUNDING
+PyAPI_FUNC(double) _Py_force_double(double);
+#    define Py_FORCE_DOUBLE(X) (_Py_force_double(X))
+#  else
+#    define Py_FORCE_DOUBLE(X) (X)
+#  endif
+#endif
+
 /* Py_IS_NAN(X)
  * Return 1 if float or double arg is a NaN, else 0.
  * Caution:
@@ -101,14 +116,18 @@
  *    This implementation may set the underflow flag if |X| is very small;
  *    it really can't be implemented correctly (& easily) before C99.
  *    Override in pyconfig.h if you have a better spelling on your platform.
+ *  Py_FORCE_DOUBLE is used to avoid getting false negatives from a
+ *    non-infinite value v sitting in an 80-bit x87 register such that
+ *    v becomes infinite when spilled from the register to 64-bit memory.
  * Note: PC/pyconfig.h defines Py_IS_INFINITY as _isinf
  */
 #ifndef Py_IS_INFINITY
-#if defined HAVE_DECL_ISINF && HAVE_DECL_ISINF == 1
-#define Py_IS_INFINITY(X) isinf(X)
-#else
-#define Py_IS_INFINITY(X) ((X) && (X)*0.5 == (X))
-#endif
+#  if defined HAVE_DECL_ISINF && HAVE_DECL_ISINF == 1
+#    define Py_IS_INFINITY(X) isinf(X)
+#  else
+#    define Py_IS_INFINITY(X) ((X) &&                                   \
+                               (Py_FORCE_DOUBLE(X)*0.5 == Py_FORCE_DOUBLE(X)))
+#  endif
 #endif
 
 /* Py_IS_FINITE(X)

Modified: python/branches/py3k/Misc/NEWS
==============================================================================
--- python/branches/py3k/Misc/NEWS	(original)
+++ python/branches/py3k/Misc/NEWS	Mon Feb  9 18:15:59 2009
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #4575: Fix Py_IS_INFINITY macro to work correctly on x87 FPUs:
+  it now forces its argument to double before testing for infinity.
+
 - Issue #5137: Make len() correctly raise a TypeError when a __len__ method
   returns a non-number type.
 

Modified: python/branches/py3k/Python/pymath.c
==============================================================================
--- python/branches/py3k/Python/pymath.c	(original)
+++ python/branches/py3k/Python/pymath.c	Mon Feb  9 18:15:59 2009
@@ -1,5 +1,18 @@
 #include "Python.h"
 
+#ifdef X87_DOUBLE_ROUNDING
+/* On x86 platforms using an x87 FPU, this function is called from the
+   Py_FORCE_DOUBLE macro (defined in pymath.h) to force a floating-point
+   number out of an 80-bit x87 FPU register and into a 64-bit memory location,
+   thus rounding from extended precision to double precision. */
+double _Py_force_double(double x)
+{
+	volatile double y;
+	y = x;
+	return y;
+}
+#endif
+
 #ifndef HAVE_HYPOT
 double hypot(double x, double y)
 {