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From Stefan Teleman <stefan.tele...@gmail.com>
Subject STDCXX-1056 : numpunct fix
Date Wed, 19 Sep 2012 17:56:54 GMT
This is a proposed fix for the numpunct facet for stdcxx-1056:

0.  Number of reported race conditions is now 0 (zero).

1.  No memory leaks in stdcxx (there are memory leaks reported in either
    libc or glibc, but there's nothing we can do about these anyway).

2.  This fix preserves perfect forwarding in the _numpunct.h header file.

3.  This fix eliminates code from facet.cpp and locale_body.cpp which
    was creating unnecessary overhead, with the potential of causing
    memory corruption, while providing no discernable benefit.

    More specifically:

    It is not true that there was no eviction policy of cached locales or
    facets in stdcxx. Not only cache eviction code existed, and still exists
    today, but cache cleanups and resizing were performed periodically,
    either when an object's reference count dropped to 0 (zero), or whenever
    the number of cached objects fell below sizeof(cache) / 2.

    In the latter case, both the facet cache and the locale cache performed
    a new allocation of the cache array, followed by a memcopy and a delete[]
    of the old cache array.

    First, the default size of the facets and locales caches was too small:
    it was set to 8. I raised this to 32. A direct consequence of this
    insufficient default size of 8 was that the cache had to resize itself
    very soon after program startup. This cache resize operation consists of:
    allocate memory for a new cache, copy the existing cached objects
    from the old cache to the new cache, and then delete[] the old cache.

    This is a first unnecessary overhead.

    Second, and as I mentioned above, whenever the number of cached objects
    fell below sizeof(cache) / 2, the cache resized itself, by performing
    the same sequence of operations as described above.

    This is a second unnecessary overhead.

    Third, cached objects were automatically evicted whenever their reference
    count dropped to 0 (zero). There are two consequences to this eviction
    policy: if the program needs to re-use an object (facet or locale) which
    has been evicted and subsequently destroyed, this object needs then to be
    constructed again later on, and subsequently re-inserted into the cache.
    This, in turn, would trigger a cache resize, followed by copying and
    delete[] of the old cache buffer.

    Object eviction followed by destruction followed by reconstruction is
    a third unnecessary overhead. Re-inserting a re-constructed object into,
    the cache, followed by a potential cache resize involving allocation of
    a new buffer, copying pointers from the old cache to the new cache,
    followed by delete[] of the old cache is a fourth unnecessary overhead.

    Real-life programs tend to reuse locales and/or facets they have created.
    There is no point in destroying and evicting these objects simply because
    there may be periods when the object isn't referenced at time. The object
    is likely to be needed again, later on.

    The fix proposed here eliminates the cache eviction and object destruction
    policy completely. Once created, objects remain in the cache, even though
    they may reside in the cache with no references. This eliminates the
    cache resize / copy / delete[] overhead. It also eliminates the overhead
    of re-constructing an evicted / destroyed object, if it is needed again
    later.

4.  Tests and Analysis Results:

    4.1. SunPro 12.3 / Solaris / SPARC / Race Conditions Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.solaris-sparc.datarace.er.html/index.html

    4.2. SunPro 12.3 / Solaris / SPARC / Heap and Memory Leaks Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.solaris-sparc.heapcheck.er.html/index.html

    4.3. SunPro 12.3 / Linux / Intel / Race Conditions Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.linux-intel.datarace.er.html/index.html

    4.4. SunPro 12.3 / Linux / Intel / Heap and Memory Leaks Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.sunpro.linux-intel.heapcheck.er.html/index.html

    4.5. Intel 2013 / Linux / Intel / Race Conditions Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.intel.linux.datarace.r007ti3.inspxez

    4.6. Intel 2013 / Linux / Intel / Heap and Memory Leaks Test:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/22.locale.numpunct.mt.intel.linux.heapcheck.r008mi1.inspxez

5.  Source code for this fix:

    http://s247136804.onlinehome.us/stdcxx-1056-20120919/_numpunct.h
    http://s247136804.onlinehome.us/stdcxx-1056-20120919/facet.cpp
    http://s247136804.onlinehome.us/stdcxx-1056-20120919/locale_body.cpp
    http://s247136804.onlinehome.us/stdcxx-1056-20120919/punct.cpp

These files are based on stdcxx 4.2.1.

-- 
Stefan Teleman
KDE e.V.
stefan.teleman@gmail.com

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