gh-110190: Fix ctypes structs with array on Arm (#112604)

Set MAX_STRUCT_SIZE to 32 in stgdict.c when on Arm platforms.
This because on Arm platforms structs with at most 4 elements of any
floating point type values can be passed through registers. If the type
is double the maximum size of the struct is 32 bytes.
On x86-64 Linux, it's maximum 16 bytes hence we need to differentiate.

Modules/_ctypes/_ctypes_test.c

@@ -150,6 +150,42 @@ _testfunc_array_in_struct2a(Test3B in)
     return result;
 }
 
+/*
+ * See gh-110190. structs containing arrays of up to four floating point types
+ * (max 32 bytes) are passed in registers on Arm.
+ */
+
+typedef struct {
+    double data[4];
+} Test3C;
+
+EXPORT(Test3C)
+_testfunc_array_in_struct_set_defaults_3C(void)
+{
+    Test3C s;
+    s.data[0] = 1.0;
+    s.data[1] = 2.0;
+    s.data[2] = 3.0;
+    s.data[3] = 4.0;
+    return s;
+}
+
+typedef struct {
+    double data[5];
+} Test3D;
+
+EXPORT(Test3D)
+_testfunc_array_in_struct_set_defaults_3D(void)
+{
+    Test3D s;
+    s.data[0] = 1.0;
+    s.data[1] = 2.0;
+    s.data[2] = 3.0;
+    s.data[3] = 4.0;
+    s.data[4] = 5.0;
+    return s;
+}
+
 typedef union {
     long a_long;
     struct {

Modules/_ctypes/stgdict.c

@@ -697,29 +697,43 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
     stgdict->align = total_align;
     stgdict->length = len;      /* ADD ffi_ofs? */
 
-#define MAX_STRUCT_SIZE 16
+/*
+ * On Arm platforms, structs with at most 4 elements of any floating point
+ * type values can be passed through registers. If the type is double the
+ * maximum size of the struct is 32 bytes.
+ * By Arm platforms it is meant both 32 and 64-bit.
+*/
+#if defined(__aarch64__) || defined(__arm__)
+#  define MAX_STRUCT_SIZE 32
+#else
+#  define MAX_STRUCT_SIZE 16
+#endif
 
     if (arrays_seen && (size <= MAX_STRUCT_SIZE)) {
         /*
-         * See bpo-22273. Arrays are normally treated as pointers, which is
-         * fine when an array name is being passed as parameter, but not when
-         * passing structures by value that contain arrays. On 64-bit Linux,
-         * small structures passed by value are passed in registers, and in
-         * order to do this, libffi needs to know the true type of the array
-         * members of structs. Treating them as pointers breaks things.
+         * See bpo-22273 and gh-110190. Arrays are normally treated as
+         * pointers, which is fine when an array name is being passed as
+         * parameter, but not when passing structures by value that contain
+         * arrays.
+         * On x86_64 Linux and Arm platforms, small structures passed by
+         * value are passed in registers, and in order to do this, libffi needs
+         * to know the true type of the array members of structs. Treating them
+         * as pointers breaks things.
          *
-         * By small structures, we mean ones that are 16 bytes or less. In that
-         * case, there can't be more than 16 elements after unrolling arrays,
-         * as we (will) disallow bitfields. So we can collect the true ffi_type
-         * values in a fixed-size local array on the stack and, if any arrays
-         * were seen, replace the ffi_type_pointer.elements with a more
-         * accurate set, to allow libffi to marshal them into registers
-         * correctly. It means one more loop over the fields, but if we got
-         * here, the structure is small, so there aren't too many of those.
+         * By small structures, we mean ones that are 16 bytes or less on
+         * x86-64 and 32 bytes or less on Arm. In that case, there can't be
+         * more than 16 or 32 elements after unrolling arrays, as we (will)
+         * disallow bitfields. So we can collect the true ffi_type values in
+         * a fixed-size local array on the stack and, if any arrays were seen,
+         * replace the ffi_type_pointer.elements with a more accurate set,
+         * to allow libffi to marshal them into registers correctly.
+         * It means one more loop over the fields, but if we got here,
+         * the structure is small, so there aren't too many of those.
          *
-         * Although the passing in registers is specific to 64-bit Linux, the
-         * array-in-struct vs. pointer problem is general. But we restrict the
-         * type transformation to small structs nonetheless.
+         * Although the passing in registers is specific to x86_64 Linux
+         * and Arm platforms, the array-in-struct vs. pointer problem is
+         * general. But we restrict the type transformation to small structs
+         * nonetheless.
          *
          * Note that although a union may be small in terms of memory usage, it
          * could contain many overlapping declarations of arrays, e.g.
@@ -745,6 +759,9 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
          *     struct { uint_32 e1; uint_32 e2; ... uint_32 e_4; } f6;
          * }
          *
+         * The same principle applies for a struct 32 bytes in size like in
+         * the case of Arm platforms.
+         *
          * So the struct/union needs setting up as follows: all non-array
          * elements copied across as is, and all array elements replaced with
          * an equivalent struct which has as many fields as the array has