gh-145633: Remove support for ancient ARM platforms with mixed-endian doubles (#145634)

* Drop DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754 macro.
* Use DOUBLE_IS_BIG/LITTLE_ENDIAN_IEEE754 to detect endianness of
  float/doubles.
* Drop "unknown_format" code path in PyFloat_Pack/Unpack*().

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: skirpichev

Doc/c-api/float.rst

@@ -224,11 +224,6 @@ endian processor, or ``0`` on little endian processor.
 Return value: ``0`` if all is OK, ``-1`` if error (and an exception is set,
 most likely :exc:`OverflowError`).
 
-There are two problems on non-IEEE platforms:
-
-* What this does is undefined if *x* is a NaN or infinity.
-* ``-0.0`` and ``+0.0`` produce the same bytes string.
-
 .. c:function:: int PyFloat_Pack2(double x, char *p, int le)
 
    Pack a C double as the IEEE 754 binary16 half-precision format.
@@ -256,9 +251,6 @@ Return value: The unpacked double.  On error, this is ``-1.0`` and
 :c:func:`PyErr_Occurred` is true (and an exception is set, most likely
 :exc:`OverflowError`).
 
-Note that on a non-IEEE platform this will refuse to unpack a bytes string that
-represents a NaN or infinity.
-
 .. c:function:: double PyFloat_Unpack2(const char *p, int le)
 
    Unpack the IEEE 754 binary16 half-precision format as a C double.

Include/internal/pycore_floatobject.h

@@ -12,7 +12,6 @@ extern "C" {
 
 /* runtime lifecycle */
 
-extern void _PyFloat_InitState(PyInterpreterState *);
 extern PyStatus _PyFloat_InitTypes(PyInterpreterState *);
 extern void _PyFloat_FiniType(PyInterpreterState *);
 
@@ -42,6 +41,15 @@ extern double _Py_parse_inf_or_nan(const char *p, char **endptr);
 
 extern int _Py_convert_int_to_double(PyObject **v, double *dbl);
 
+/* Should match endianness of the platform in most (all?) cases. */
+
+#ifdef DOUBLE_IS_BIG_ENDIAN_IEEE754
+#  define _PY_FLOAT_BIG_ENDIAN 1
+#  define _PY_FLOAT_LITTLE_ENDIAN 0
+#else
+#  define _PY_FLOAT_BIG_ENDIAN 0
+#  define _PY_FLOAT_LITTLE_ENDIAN 1
+#endif
 
 #ifdef __cplusplus
 }

Include/internal/pycore_pymath.h

@@ -182,8 +182,7 @@ extern void _Py_set_387controlword(unsigned short);
 //     (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)
+    !defined(DOUBLE_IS_BIG_ENDIAN_IEEE754)
 #  define _PY_SHORT_FLOAT_REPR 0
 #endif
 

Include/internal/pycore_runtime_init.h

@@ -13,7 +13,7 @@ extern "C" {
 #include "pycore_debug_offsets.h"  // _Py_DebugOffsets_INIT()
 #include "pycore_dtoa.h"          // _dtoa_state_INIT()
 #include "pycore_faulthandler.h"  // _faulthandler_runtime_state_INIT
-#include "pycore_floatobject.h"   // _py_float_format_unknown
+#include "pycore_floatobject.h"   // _py_float_format_*
 #include "pycore_function.h"
 #include "pycore_hamt.h"          // _PyHamt_BitmapNode_Type
 #include "pycore_import.h"        // IMPORTS_INIT
@@ -84,10 +84,6 @@ extern PyTypeObject _PyExc_MemoryError;
         .stoptheworld = { \
             .is_global = 1, \
         }, \
-        .float_state = { \
-            .float_format = _py_float_format_unknown, \
-            .double_format = _py_float_format_unknown, \
-        }, \
         .types = { \
             .next_version_tag = _Py_TYPE_VERSION_NEXT, \
         }, \
@@ -233,4 +229,4 @@ extern PyTypeObject _PyExc_MemoryError;
 #ifdef __cplusplus
 }
 #endif
-#endif /* !Py_INTERNAL_RUNTIME_INIT_H */
+#endif /* !Py_INTERNAL_RUNTIME_INIT_H */

Include/internal/pycore_runtime_structs.h

@@ -35,17 +35,6 @@ struct _pymem_allocators {
     PyObjectArenaAllocator obj_arena;
 };
 
-enum _py_float_format_type {
-    _py_float_format_unknown,
-    _py_float_format_ieee_big_endian,
-    _py_float_format_ieee_little_endian,
-};
-
-struct _Py_float_runtime_state {
-    enum _py_float_format_type float_format;
-    enum _py_float_format_type double_format;
-};
-
 struct pyhash_runtime_state {
     struct {
 #ifndef MS_WINDOWS
@@ -270,7 +259,6 @@ struct pyruntimestate {
     } audit_hooks;
 
     struct _py_object_runtime_state object_state;
-    struct _Py_float_runtime_state float_state;
     struct _Py_unicode_runtime_state unicode_state;
     struct _types_runtime_state types;
     struct _Py_time_runtime_state time;

Include/pymacconfig.h

@@ -18,7 +18,6 @@
 #undef SIZEOF_UINTPTR_T
 #undef SIZEOF_PTHREAD_T
 #undef WORDS_BIGENDIAN
-#undef DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754
 #undef DOUBLE_IS_BIG_ENDIAN_IEEE754
 #undef DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 #undef HAVE_GCC_ASM_FOR_X87

Misc/NEWS.d/next/Build/2026-03-08-06-18-26.gh-issue-145633.Ogu-RF.rst

@@ -0,0 +1,3 @@
+Remove support for ancient ARM platforms (ARMv4L and ARMv5L OABI boards),
+using mixed-endian representation
+for doubles.  Patch by Sergey B Kirpichev.