[Python-Dev] How can we use 48bit pointer safely?

March 30, 2018

      Hi,

As far as I know, most amd64 and arm64 systems use only 48bit address spaces.
(except [1])

[1] https://software.intel.com/sites/default/files/managed/2b/80/5-level_paging_...

It means there are some chance to compact some data structures.
I point two examples below.

My question is; can we use 48bit pointer safely?
It depends on CPU architecture & OS memory map.
Maybe, configure option which is available on only (amd64, amd64) *
(Linux, Windows, macOS)?

# Possible optimizations by 48bit pointer

## PyASCIIObject

[snip]
        unsigned int ready:1;
        /* Padding to ensure that PyUnicode_DATA() is always aligned to
           4 bytes (see issue #19537 on m68k). */
        unsigned int :24;
    } state;
    wchar_t *wstr;              /* wchar_t representation (null-terminated) */
} PyASCIIObject;

Currently, state is 8bit + 24bit padding.  I think we can pack state and wstr
in 64bit.

## PyDictKeyEntry

typedef struct {
    /* Cached hash code of me_key. */
    Py_hash_t me_hash;
    PyObject *me_key;
    PyObject *me_value; /* This field is only meaningful for combined tables */
} PyDictKeyEntry;

There are chance to compact it: Use only 32bit for hash and 48bit*2
for key and value.  CompactEntry may be 16byte instead of 24byte.

Regards,
-- 
INADA Naoki  <songofacandy@gmail.com>