Issue #1580: use short float repr where possible.

 - incorporate and adapt David Gay's dtoa and strtod
   into the Python core
 - on platforms where we can use Gay's code (almost
   all!), repr(float) is based on the shortest
   sequence of decimal digits that rounds correctly.
 - add sys.float_repr_style attribute to indicate
   whether we're using Gay's code or not
 - add autoconf magic to detect and enable SSE2
   instructions on x86/gcc
 - slight change to repr and str:  repr switches
   to exponential notation at 1e16 instead of
   1e17, str switches at 1e11 instead of 1e12

5 files changed, 74 insertions, 0 deletions

diff --git a/Include/Python.h b/Include/Python.h
index 76b9503..5b45b38 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -118,6 +118,7 @@
 
 #include "pystrtod.h"
 #include "pystrcmp.h"
+#include "dtoa.h"
 
 /* _Py_Mangle is defined in compile.c */
 PyAPI_FUNC(PyObject*) _Py_Mangle(PyObject *p, PyObject *name);
diff --git a/Include/dtoa.h b/Include/dtoa.h
new file mode 100644
index 0000000..9b434b7
--- /dev/null
+++ b/Include/dtoa.h
@@ -0,0 +1,15 @@
+#ifndef PY_NO_SHORT_FLOAT_REPR
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+PyAPI_FUNC(double) _Py_dg_strtod(const char *str, char **ptr);
+PyAPI_FUNC(char *) _Py_dg_dtoa(double d, int mode, int ndigits,
+                        int *decpt, int *sign, char **rve);
+PyAPI_FUNC(void) _Py_dg_freedtoa(char *s);
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif
diff --git a/Include/pymacconfig.h b/Include/pymacconfig.h
index a8679af..7b20431 100644
--- a/Include/pymacconfig.h
+++ b/Include/pymacconfig.h
@@ -17,6 +17,9 @@
 # undef SIZEOF_VOID_P
 # undef SIZEOF__BOOL
 # undef WORDS_BIGENDIAN
+# undef DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754
+# undef DOUBLE_IS_BIG_ENDIAN_IEEE754
+# undef DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 
 #    undef VA_LIST_IS_ARRAY
 #    if defined(__LP64__) && defined(__x86_64__)
@@ -65,6 +68,9 @@
 
 #ifdef __BIG_ENDIAN__
 #define WORDS_BIGENDIAN 1
+#define DOUBLE_IS_BIG_ENDIAN_IEEE754
+#else
+#define DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 #endif /* __BIG_ENDIAN */
 
 
diff --git a/Include/pymath.h b/Include/pymath.h
index 6ad174d..8872e8a 100644
--- a/Include/pymath.h
+++ b/Include/pymath.h
@@ -92,6 +92,11 @@ PyAPI_FUNC(double) _Py_force_double(double);
 #  endif
 #endif
 
+#ifdef HAVE_GCC_ASM_FOR_X87
+PyAPI_FUNC(unsigned short) _Py_get_387controlword(void);
+PyAPI_FUNC(void) _Py_set_387controlword(unsigned short);
+#endif
+
 /* Py_IS_NAN(X)
  * Return 1 if float or double arg is a NaN, else 0.
  * Caution:
diff --git a/Include/pyport.h b/Include/pyport.h
index bf75d89..ed43569 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -465,6 +465,53 @@ extern "C" {
 			errno = 0;					\
 	} while(0)
 
+/* The functions _Py_dg_strtod and _Py_dg_dtoa in Python/dtoa.c require that
+   the FPU is using 53-bit precision.  Here are macros that force this.  See
+   Python/pystrtod.c for an example of their use. */
+
+#ifdef USING_X87_FPU
+#define _Py_SET_53BIT_PRECISION_HEADER				\
+	unsigned short old_387controlword, new_387controlword
+#define _Py_SET_53BIT_PRECISION_START					\
+	do {								\
+		old_387controlword = _Py_get_387controlword();		\
+		new_387controlword = (old_387controlword & ~0x0f00) | 0x0200; \
+		if (new_387controlword != old_387controlword)		\
+			_Py_set_387controlword(new_387controlword);	\
+	} while (0)
+#define _Py_SET_53BIT_PRECISION_END				\
+	if (new_387controlword != old_387controlword)		\
+		_Py_set_387controlword(old_387controlword)
+#else
+#define _Py_SET_53BIT_PRECISION_HEADER
+#define _Py_SET_53BIT_PRECISION_START
+#define _Py_SET_53BIT_PRECISION_END
+#endif
+
+/* If we can't guarantee 53-bit precision, don't use the code
+   in Python/dtoa.c, but fall back to standard code.  This
+   means that repr of a float will be long (17 sig digits).
+
+   Realistically, there are two things that could go wrong:
+
+   (1) doubles aren't IEEE 754 doubles, or
+   (2) we're on x86 with the rounding precision set to 64-bits
+       (extended precision), and we don't know how to change
+       the rounding precision.
+ */
+
+#if !defined(DOUBLE_IS_LITTLE_ENDIAN_IEEE754) && \
+    !defined(DOUBLE_IS_BIG_ENDIAN_IEEE754) && \
+    !defined(DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754)
+#define PY_NO_SHORT_FLOAT_REPR
+#endif
+
+/* double rounding is symptomatic of use of extended precision on x86 */
+#ifdef X87_DOUBLE_ROUNDING
+#define PY_NO_SHORT_FLOAT_REPR
+#endif
+
+
 /* Py_DEPRECATED(version)
  * Declare a variable, type, or function deprecated.
  * Usage: