Ensure properly aligned data pointers

hpy/devel/src/runtime/ctx_type.c

@@ -4,6 +4,31 @@
 #include "hpy.h"
 #include "hpy/runtime/ctx_type.h"
 
+/* Since C11, there is type 'max_align_t' which is defined as: 'max_align_t' is
+   a type whose alignment requirement is at least as strict (as large) as that
+   of every scalar type.
+   Also, pointers returned by allocation functions such as malloc are suitably
+   aligned for any object, which means they are aligned at least as strictly as
+   'max_align_t'.
+   We use sizeof(max_align_t) to align pure HPy type data pointers.
+ */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#define MAX_ALIGN_SIZE sizeof(max_align_t)
+#else
+/* Before C11, we do not have 'max_align_t' but it is usually synonymous with
+   the largest scalar type, which is 'long double' on most platforms. So, this
+   is best effort.
+ */
+#define MAX_ALIGN_SIZE sizeof(long double)
+#endif
+
+/* Macro ALIGN_SIZE aligns a given size to a multiple of the provided
+   alignment. We apply this macro to the inherited basicsize such that the
+   appended data will have an aligned data pointer. For example, if we would
+   get a inherited basicsize of 11 and the aligment is 16 bytes, then
+   ALIGN_SIZE(16, 11) == 16. Picking the right alignment is crucial! */
+#define ALIGN_SIZE(_align, _size) ((HPy_ssize_t)((((uintptr_t)(_size)) - 1u + (_align)) & -(_align)))
+
 #ifdef HPY_UNIVERSAL_ABI
    // for _h2py and _py2h
 #  include "handles.h"
@@ -959,6 +984,7 @@ ctx_Type_FromSpec(HPyContext *ctx, HPyType_Spec *hpyspec,
                 base_member_offset = compute_largest_basicsize(bases);
                 if (base_member_offset < 0)
                     return HPy_NULL;
+                base_member_offset = ALIGN_SIZE(MAX_ALIGN_SIZE, base_member_offset);
                 if (base_member_offset > INT_MAX)
                     Py_FatalError("basicsize is too large");
             }