[Python-Dev] Tagged integers

[Jeff Epler]

On Wed, Jul 14, 2004 at 02:41:19AM -0400, James Y Knight wrote:
> - Thus, no code can directly access object fields ->ob_refcnt, or 
> ->ob_type. Introduced Py_GETTYPE/Py_GETREF macros and search&replaced 
> code in python to use them. These macros check the tag bits, and do the 
> right thing if it is tagged data.

If this looks like a promising direction, Py_GETTYPE and Py_GETREF
macros could be added today, in 2.5 or 2.6 ob_refcnt and ob_type would
be removed/renamed, and 2.6 or 2.7 would see the first actual use of
tagged types.  Sure, you couldn't compile a 2.4 extension on 2.6, but
will it matter?

By the way, I don't know what optimizers actually do, but given
    PyObject *spam(PyObject *o) {
        Py_GETTYPE(o); junk; Py_GETREF(o); junk;
        Py_INCREF(o); junk; Py_DECREF(o); junk;
    }
the compiler is free to notice that "o" can't change (its address is never
taken), so the "is it a tagged object" test can be done once, not 4 times.

I assume that tagged integers could be turned off at compile time, for
the platforms where they don't work for one reason or another.

I'm showing my ignorance here, but what other types are traditionally
tagged, and do any of those translate well to Python?

I suppose you could tag some singleton objects, like None, True, False,
and (), but those are already shared.

Tagging length-1 strings and unicode strings seems like it might have its
advantages, though the length-0 and length-1 strings, length-0 unicode
strings, and first 256 length-1 unicode strings are already shared too.

I can't see much use for a 21-bit-mantissa floating-point type, and
I can't see how you could use tagging for any other types in Python.

Jeff
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