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POGeneric wrappers for templates: optimization opportunities
 

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    <p>I'm trying to marry templates to generics with the accent on making access to primitive/value-typed properties as fast as (reasonably) possible, preferrably keeping it in C++/CLR. Consider this simple template:</p> <pre><code>template&lt; typename type &gt; class value_wrapper { public: type value; }; </code></pre> <p>and its counterpart:</p> <pre><code>generic&lt; typename T &gt; public ref class ValueWrapper { ... public: property T Value { T get() { if( T::typeid == System::Int32::typeid ) return ( T )( ( value_wrapper&lt; int &gt; * )ptr )-&gt;value; ...branches for some other types... // if all else fails: return ( T )( Object ^ )( ( value_wrapper&lt; gcroot&lt; Object ^ &gt; &gt; * )ptr )-&gt;value; } ... } ... private: void *ptr; }; </code></pre> <p><strong>Question 1.</strong> When a generic's MSIL is turned into specializations for value types, is the code further optimized? Is it possible to check types here in such a way that, e.g., in <code>ValueWrapper&lt;int&gt;</code> branches for non-int types and type comparisons themselves will be optimized away?</p> <p>Now, listing all supported types in every single method is somewhat painful, so I have made a separate function for this:</p> <pre><code>template&lt; typename action, typename context_type &gt; __forceinline static bool apply( System::Type ^type, void *value, System::Object ^*box, context_type context ) { if( type == System::Int32::typeid ) return action().operator()&lt; int &gt;( ( int * )value, context ), true; ...branches for some other types... // if all else fails: return action().operator()&lt; gcroot&lt; System::Object ^ &gt; &gt;( box, context ), false; } struct get_value { template&lt; typename type, typename result_type &gt; void operator () ( result_type *result, void *ptr ) { *result = ( ( value_wrapper&lt; type &gt; * )ptr )-&gt;value; } }; generic&lt; typename T &gt; public ref class ValueWrapper { ... property T Value { T get() { T result; System::Object ^box; return apply&lt; get_value &gt;( T::typeid, &amp;result, &amp;box, ptr ) ? result : ( T )box; } ... } ... }; </code></pre> <p>Turns out this is about 3x slower than the original code.</p> <p><strong>Question 2.</strong> What can be changed here to allow the optimizer to make the second implementation closer in speed to the first one (ideally, with speed difference within 10%-20%)?</p> <p>P.S. This is mostly about VC 2010. But if VC 2012 is somehow different in this regard, that would be good to know, too.</p>
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    1. COYou might gain some speed by using a `case`. Second implementation is slower, and can likely not be made as fast as the first one, because you first construct a `T` and then assign to it. Moreover you put a `System::Object^` on the stack even though it's not needed in all cases, and there might be some overhead of the function calls.
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    2. CO@stijn Do you mean `switch( T::typeid )`? I don't think it's possible. As for the second question, I'm OK with, say, 10%-20% speed difference, just not 3x. Editing for clarity. (Also: in theory, inlining could help with function call overhead, AND detecting the unnecessity of `box` & removing it, AND getting rid of extra operations on `result`. It's all about the optimizer's abilities.)
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    3. COoops, sorry indeed switching on typeid does not work
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