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
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 @@
# 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 @@
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: