[GR-24892] Fix and test struct nan packing

PullRequest: graalpython/1133

Author: timfel

graalpython/com.oracle.graal.python.cext/src/floatobject.c

@@ -16,6 +16,40 @@ typedef enum {
 static float_format_type double_format, float_format;
 static float_format_type detected_double_format, detected_float_format;
 
+__attribute__((constructor))
+static void init_formats(void) {
+#if SIZEOF_DOUBLE == 8
+    {
+        double x = 9006104071832581.0;
+        if (memcmp(&x, "\x43\x3f\xff\x01\x02\x03\x04\x05", 8) == 0)
+            detected_double_format = ieee_big_endian_format;
+        else if (memcmp(&x, "\x05\x04\x03\x02\x01\xff\x3f\x43", 8) == 0)
+            detected_double_format = ieee_little_endian_format;
+        else
+            detected_double_format = unknown_format;
+    }
+#else
+    detected_double_format = unknown_format;
+#endif
+
+#if SIZEOF_FLOAT == 4
+    {
+        float y = 16711938.0;
+        if (memcmp(&y, "\x4b\x7f\x01\x02", 4) == 0)
+            detected_float_format = ieee_big_endian_format;
+        else if (memcmp(&y, "\x02\x01\x7f\x4b", 4) == 0)
+            detected_float_format = ieee_little_endian_format;
+        else
+            detected_float_format = unknown_format;
+    }
+#else
+    detected_float_format = unknown_format;
+#endif
+
+    double_format = detected_double_format;
+    float_format = detected_float_format;
+}
+
 UPCALL_ID(PyFloat_AsDouble);
 double PyFloat_AsDouble(PyObject *op) {
     return ((double (*)(void*))_jls_PyFloat_AsDouble)(native_to_java(op));

graalpython/com.oracle.graal.python.test/src/tests/test_struct.py

@@ -140,3 +140,8 @@ def test_pack_unpack():
             _bytes = struct.pack(_fmt, val)
             _res = struct.unpack(_fmt, _bytes)[0]
             assert _res == res, "({}), {} != {}".format(_fmt, _res, res)
+
+
+def test_pack_nan():
+    import math
+    assert struct.pack('<d', math.nan) == b'\x00\x00\x00\x00\x00\x00\xf8\x7f'