New submission from STINNER Victor <[email protected]>:
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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