add float_repr_style to sys module and enable test_strtod

from_cpython/Include/pyport.h

@@ -458,3 +458,28 @@ typedef PY_LONG_LONG            Py_intptr_t;
         else if (errno == ERANGE)                                       \
             errno = 0;                                                  \
     } while(0)
+
+/* 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.  If
+   we're seeing double rounding, and we don't have any mechanism available for
+   changing the FPU rounding precision, then don't use Python/dtoa.c. */
+#if defined(X87_DOUBLE_ROUNDING) && !defined(HAVE_PY_SET_53BIT_PRECISION)
+#define PY_NO_SHORT_FLOAT_REPR
+#endif

from_cpython/Lib/test/test_strtod.py

-# expected: fail
 # Tests for the correctly-rounded string -> float conversions
 # introduced in Python 2.7 and 3.1.
 

src/runtime/builtin_modules/sys.cpp

@@ -721,6 +721,14 @@ void setupSys() {
 #ifdef Py_USING_UNICODE
     SET_SYS_FROM_STRING("maxunicode", PyInt_FromLong(PyUnicode_GetMax()));
 #endif
+
+/* float repr style: 0.03 (short) vs 0.029999999999999999 (legacy) */
+#ifndef PY_NO_SHORT_FLOAT_REPR
+    SET_SYS_FROM_STRING("float_repr_style", PyString_FromString("short"));
+#else
+    SET_SYS_FROM_STRING("float_repr_style", PyString_FromString("legacy"));
+#endif
+
     sys_flags_cls->tp_mro = BoxedTuple::create({ sys_flags_cls, object_cls });
     sys_flags_cls->freeze();
 

test/CPYTHON_TEST_NOTES.md

@@ -198,7 +198,6 @@ test_sqlite             [unknown]
 test_ssl                [unknown]
 test_startfile          [unknown]
 test_str                memory leak?
-test_strtod             [unknown]
 test_structmembers      [unknown]
 test_struct             [unknown]
 test_subprocess         exit code 141 [sigpipe?], no error message