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PyGC_Head structure size depends on the Python version, sizes of 64-bit:
* 2.7: 32 bytes
* 3.6, 3.7: 24 bytes
* 3.8 (master): 16 bytes
bpo-36618 "clang expects memory aligned on 16 bytes, but pymalloc aligns to 8 bytes" should be even worse on 3.7: 24 is not aligned on 16. I don't understand why nobody saw this alignment issue previously. Maybe clang only became stricer about 16 bytes alignment recently?
2.7:
typedef union _gc_head {
 struct {
 union _gc_head *gc_next;
 union _gc_head *gc_prev;
 Py_ssize_t gc_refs;
 } gc;
 double dummy; /* Force at least 8-byte alignment. */
 char dummy_padding[sizeof(union _gc_head_old)];
} PyGC_Head;
3.7:
typedef union _gc_head {
 struct {
 union _gc_head *gc_next;
 union _gc_head *gc_prev;
 Py_ssize_t gc_refs;
 } gc;
 double dummy; /* force worst-case alignment */
} PyGC_Head;
3.8:
typedef struct {
 // Pointer to next object in the list.
 // 0 means the object is not tracked
 uintptr_t _gc_next;
 // Pointer to previous object in the list.
 // Lowest two bits are used for flags documented later.
 uintptr_t _gc_prev;
} PyGC_Head;
In 3.8, the union used to ensure alignment on a C double is gone.