From c97c9096ed05c89041b2aab9578489ca634c9375 Mon Sep 17 00:00:00 2001
From: Mark Dickinson <dickinsm@gmail.com>
Date: Mon, 9 Feb 2009 14:18:43 +0000
Subject: Issue #4575: fix Py_IS_INFINITY macro to work correctly on x87 FPUs.
 It now forces its argument to double before testing for infinity.

---
 Include/pymath.h | 29 ++++++++++++++++++++++++-----
 Misc/NEWS        |  3 +++
 Python/pymath.c  | 13 +++++++++++++
 3 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/Include/pymath.h b/Include/pymath.h
index 0631b93..6ad174d 100644
--- a/Include/pymath.h
+++ b/Include/pymath.h
@@ -77,6 +77,21 @@ extern double copysign(double, double);
 #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 @@ extern double copysign(double, double);
  *    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)
diff --git a/Misc/NEWS b/Misc/NEWS
index c926347..9f56bb8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 1
 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 #4978: Passing keyword arguments as unicode strings is now allowed.
 
 - Issue 1242657: the __len__() and __length_hint__() calls in several tools
diff --git a/Python/pymath.c b/Python/pymath.c
index 5cf61ab..5d09b4c 100644
--- a/Python/pymath.c
+++ b/Python/pymath.c
@@ -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)
 {
-- 
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