[DO NOT MERGE] Find Windows empty base class inefficiency

[droberts195]

Visual Studio doesn't implement the empty base class
optimisation by default in cases of multiple inheritance.
To preserve ABI backwards compatibility it only does this
when explicitly told.

The secret /d1reportClassLayoutChanges option will reveal
where we might benefit from using __declspec(empty_bases).

Relates #840

[droberts195]

consoleText.txt from the Windows PR CI build contains the special output.

Search for "Future Default Layout" to see the opportunities.

For example one is:

Effective Layout: (Default)

class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >    size(48):
    +---
 0  | +--- (base class ml::maths::CBasicStatistics::COrderStatisticsImpl<struct std::pair<double,class ml::maths::CDenseVector<double> >,class std::vector<struct std::pair<double,class ml::maths::CDenseVector<double> >,class std::allocator<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >)
 0  | | ?$less@U?$pair@NV?$CDenseVector@N@maths@ml@@@std@@ m_Less
    | | <alignment member> (size=7)
 8  | | ?$vector@U?$pair@NV?$CDenseVector@N@maths@ml@@@std@@V?$allocator@U?$pair@NV?$CDenseVector@N@maths@ml@@@std@@@2@ m_Statistics
32  | | m_UnusedCount
    | +---
41  | +--- (base class boost::operators_impl::addable<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > >,struct boost::operators_impl::operators_detail::false_t>)
41  | | +--- (base class boost::operators_impl::addable1<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > > >)
41  | | | +--- (base class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > >)
    | | | +---
    | | +---
    | +---
    | <alignment member> (size=7)
    +---

Future Default Layout: (Empty Base Class Optimization)

class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >    size(40):
    +---
 0  | +--- (base class ml::maths::CBasicStatistics::COrderStatisticsImpl<struct std::pair<double,class ml::maths::CDenseVector<double> >,class std::vector<struct std::pair<double,class ml::maths::CDenseVector<double> >,class std::allocator<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >)
 0  | | ?$less@U?$pair@NV?$CDenseVector@N@maths@ml@@@std@@ m_Less
    | | <alignment member> (size=7)
 8  | | ?$vector@U?$pair@NV?$CDenseVector@N@maths@ml@@@std@@V?$allocator@U?$pair@NV?$CThreadDataWriter.cc
CDenseVector@N@maths@ml@@@std@@@2@ m_Statistics
32  | | m_UnusedCount
    | +---
 0  | +--- (base class boost::operators_impl::addable<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > >,struct boost::operators_impl::operators_detail::false_t>)
 0  | | +--- (base class boost::operators_impl::addable1<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > >,class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > > >)
 0  | | | +--- (base class boost::operators_impl::operators_detail::empty_base<class ml::maths::CBasicStatistics::COrderStatisticsHeap<struct std::pair<double,class ml::maths::CDenseVector<double> >,struct std::less<struct std::pair<double,class ml::maths::CDenseVector<double> > > > >)
    | | | +---
    | | +---
    | +---
    +---

The second base class adds 1 byte to the object size, but then padding for alignment increases that to 8 bytes.

Many of the classes that would benefit from the optimisation appear to be in the standard library (which Microsoft will not change until the next ABI-breaking release of Visual C++) and Eigen (which we're not allowed to edit because it's under the MPL). Still, there may be a few opportunities for making our code more memory-efficient on Windows.

[droberts195]

This is a way to find our classes in the output:

bash-3.2$ grep '^class ml' consoleText.txt | sed 's/<.*size/ size/' | sort -u | tr '\t' ' '
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(104):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(40):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(48):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(96):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(32):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(40):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(48):
class ml::maths::CLogJointProbabilityOfLessLikelySamples size(48):
class ml::maths::CLogJointProbabilityOfLessLikelySamples size(56):
class ml::maths::CTimeSeriesDecomposition size(664):
class ml::maths::CTimeSeriesDecomposition size(672):
class ml::maths::`anonymous-namespace'::detail::CHashIterator size(16):
class ml::maths::`anonymous-namespace'::detail::CHashIterator size(8):
class ml::model::CHierarchicalResultsNormalizer size(200):

CHierarchicalResultsNormalizer only appears once in that list because although its layout changes its overall size doesn't - there's already padding and the padding just gets shuffled to different places.

So, the list of classes that can benefit from __declspec(empty_bases) is:

1. ml::maths::CBasicStatistics::COrderStatisticsHeap
2. ml::maths::CBasicStatistics::COrderStatisticsStack
3. ml::maths::CLogJointProbabilityOfLessLikelySamples
4. ml::maths::CTimeSeriesDecomposition
5. ml::maths::`anonymous-namespace'::detail::CHashIterator

[edsavage]

This is a way to find our classes in the output:

bash-3.2$ grep '^class ml' consoleText.txt | sed 's/<.*size/ size/' | sort -u | tr '\t' ' '
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(104):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(40):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(48):
class ml::maths::CBasicStatistics::COrderStatisticsHeap size(96):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(32):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(40):
class ml::maths::CBasicStatistics::COrderStatisticsStack size(48):
class ml::maths::CLogJointProbabilityOfLessLikelySamples size(48):
class ml::maths::CLogJointProbabilityOfLessLikelySamples size(56):
class ml::maths::CTimeSeriesDecomposition size(664):
class ml::maths::CTimeSeriesDecomposition size(672):
class ml::maths::`anonymous-namespace'::detail::CHashIterator size(16):
class ml::maths::`anonymous-namespace'::detail::CHashIterator size(8):
class ml::model::CHierarchicalResultsNormalizer size(200):

CHierarchicalResultsNormalizer only appears once in that list because although its layout changes its overall size doesn't - there's already padding and the padding just gets shuffled to different places.

So, the list of classes that can benefit from __declspec(empty_bases) is:

1. ml::maths::CBasicStatistics::COrderStatisticsHeap
2. ml::maths::CBasicStatistics::COrderStatisticsStack
3. ml::maths::CLogJointProbabilityOfLessLikelySamples
4. ml::maths::CTimeSeriesDecomposition
5. ml::maths::`anonymous-namespace'::detail::CHashIterator

Cool stuff!

[droberts195]

#844 has the fixes.