[Patches] [ python-Patches-1590352 ] The "lazy strings" patch

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Patches item #1590352, was opened at 2006-11-04 06:30
Message generated for change (Comment added) made by paulhankin
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Category: Core (C code)
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Larry Hastings (lhastings)
Assigned to: Nobody/Anonymous (nobody)
Summary: The "lazy strings" patch

Initial Comment:
This patch consists of three changes to CPython:
 * changing PyStringObject.ob_sval,
 * "lazy concatenations", and
 * "lazy slices".
None of these changes adds new functionality to CPython;
they are all speed or memory optimizations.


In detail:

PyStringObject.ob_sval was changed from a char[] array
to a char *.  This is not in and of itself particularly
desirable.  It was necessary in order to implement the
other two changes.

"lazy concatenations" change string concatenation ("a" + "b") so that,
instead of directly calculating the resulting string, it returns a
placeholder object representing the result.  As a result, string
concatenation in CPython is now more than 150% faster on average (as
reported by pystone 2.0), and is approximately as fast as the standard
string concatenation idiom ("".join([a + b + c])).

"lazy slices" changes string slicing ("abc"[1], "a".strip()) so
that, instead of directly calculating the resulting string, it
returns a placeholder object representing the result.  As a result,
string slicing in CPython is now more than 60% faster on average
(as reported by pystone 2.0).

When considering this patch, please keep in mind that the "lazy" changes
are distinct, and could be incorporated independently.  In particular
I'm guessing that "lazy concatenations" have a lot higher chance of
being accepted than "lazy slices".


These changes were implemented almost entirely in
Include/stringobject.h and Objects/stringobject.c.

With this patch applied, trunk builds and passes all expected tests
on Win32 and Linux.


For a more thorough discussion of this patch, please see the attached
text file(s).

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Comment By: Paul Hankin (paulhankin)
Date: 2007-03-11 17:27

Message:
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I really like the idea of the lazy cats, and can believe that it's a
really good optimisation, but before I review this code properly I'd like
to see:
a. convincing that it doesn't break strict aliasing (a casual reading
suggests it does)
b. lazy slices removed into their own patch (or just removed) - I don't
want to recommend a patch containing them
c. adherence to coding standard
d. a little more explanation of how the cat objects work: it's important
because they're a future minefield of bugs.



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