New submission from STINNER Victor:
Python 2 has fast path in ceval.c for operations (a+b, a-b, etc.) on small
integers ("int" type) if the operation does not overflow.
We loose these fast-path in Python 3 when we dropped the int type in favor of
the long type.
Antoine Pitrou proposed a fast-path, but only for int singletons (integers in
the range [-5; 255]): issue #10044. His patch was rejected because it
introduces undefined behaviour.
I propose to reimplemenet Python 2 optimization for long with a single digit,
which are the most common numbers.
Pseudo-code for BINARY_ADD:
---
if (PyLong_CheckExact(x) && Py_ABS(Py_SIZE(x)) == 1
&& PyLong_CheckExact(y) && Py_ABS(Py_SIZE(y)) == 1)
{
stwodigits a = ..., b = ...;
stwodigits c;
if (... a+b will not overflow ...) {
c = a + b;
return PyLong_FromLongLong(c);
}
}
/* fall back to PyNumber_Add() */
---
The code can be copied from longobject.c, there are already fast-path for
single digit numbers. See for example long_mul():
---
/* fast path for single-digit multiplication */
if (Py_ABS(Py_SIZE(a)) <= 1 && Py_ABS(Py_SIZE(b)) <= 1) {
....
}
---
As any other optimization, it should be proved to be faster with benchmarks.
----------
messages: 222731
nosy: haypo, mark.dickinson
priority: normal
severity: normal
status: open
title: ceval.c: implement fast path for integers with a single digit
type: performance
versions: Python 3.5
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue21955>
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