[issue14422] Pack PyASCIIObject fields to reduce memory consumption of pure ASCII strings

[STINNER Victor]

New submission from STINNER Victor <victor.stinner at gmail.com>:

It is possible to reduce PyASCIIObject.state to 8 bits instead of 32, move it to the end (exchange wstr and state) of the structure and pack the structure. As a result, the structure size is reduced by 3 bytes (state type changes from int to char).

I expect a low or not overhead on performances because only PyASCIIObject.state field is affected and this field size is 8 bits.

See also the issue #14419 which relies on memory alignment (of the ASCII string data) to optimize the ASCII decoder. If I understand correctly, my patch disables the possibility of this optimization.

--

Example on Linux 32 bits:

$ cat x.c 
#include <Python.h>

int main()
{
    printf("sizeof(PyASCIIObject)=%u bytes\n", sizeof(PyASCIIObject));
    printf("sizeof(PyCompactUnicodeObject)=%u bytes\n", sizeof(PyCompactUnicodeObject));
    printf("sizeof(PyUnicodeObject)=%u bytes\n", sizeof(PyUnicodeObject));
    return 0;
}

# unpatched
$ gcc -I Include/ -I . x.c -o x && ./x
sizeof(PyASCIIObject)=24 bytes
sizeof(PyCompactUnicodeObject)=36 bytes
sizeof(PyUnicodeObject)=40 bytes

# pack the 3 structures
$ gcc -I Include/ -I . x.c -o x && ./x
sizeof(PyASCIIObject)=21 bytes
sizeof(PyCompactUnicodeObject)=33 bytes
sizeof(PyUnicodeObject)=37 bytes

--

We might also pack PyCompactUnicodeObject and PyUnicodeObject but it would have a bad impact on performances because utf8_length, utf8, wstr_length and data would not be aligned anymore.

----------
components: Interpreter Core
files: pack_pyasciiobject.patch
keywords: patch
messages: 156905
nosy: haypo, loewis, pitrou, storchaka
priority: normal
severity: normal
status: open
title: Pack PyASCIIObject fields to reduce memory consumption of pure ASCII strings
versions: Python 3.3
Added file: http://bugs.python.org/file25037/pack_pyasciiobject.patch

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<http://bugs.python.org/issue14422>
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