Abstract:
Interface optimization is provided using a closed system in which all the individual software components in the system are known to the compiler at a single point in time. This knowledge enables significant opportunities to optimize the implementation of interfaces on a set of implemented objects. When code is compiled, because the compiler knows the full list of interfaces and the objects which implement the interfaces, it can improve execution and working set (i.e., recently referenced pages in a program&#39;s virtual address space) when implementing the interfaces on objects. This improvement may be realized by reducing the size of interface lookup tables which map each interface to the object types which implement that particular interface.

Description:
BACKGROUND 
       [0001]    A traditional “open system”, such as the .NET runtime environment supported by Microsoft Corporation, describes a situation where individual components such as DLLs (dynamically linked libraries) may be compiled at any time, and loaded into a common process at any time. For example, an “interface” declared in “A.dll” may be implemented or consumed by another component in “B.dll”, that was unknown at the time that “A.dll” was compiled. Accommodating this situation typically requires that any compile-time implementation or runtime data-structures be flexible enough to allow for an interface to be implemented or consumed by anyone in the future, even if the situation actually never occurs in a given program when executed. Accordingly, providing for such flexibility can often incur a high performance cost when implementing and then calling interfaces in a program. 
         [0002]    This Background is provided to introduce a brief context for the Summary and Detailed Description that follow. This Background is not intended to be an aid in determining the scope of the claimed subject matter nor be viewed as limiting the claimed subject matter to implementations that solve any or all of the disadvantages or problems presented above. 
       SUMMARY 
       [0003]    Interface optimization is provided using a closed system in which all the individual software components in the system are known to the compiler at a single point in time. This knowledge enables significant opportunities to optimize the implementation of interfaces on a set of implemented objects. When code is compiled, because the compiler knows the full list of interfaces and the objects which implement the interfaces, it can improve execution and working set (i.e., recently referenced pages in a program&#39;s virtual address space) when implementing the interfaces on objects. This improvement may be realized by reducing the size of interface lookup tables which map each interface to the object types which implement the particular interface. 
         [0004]    In various illustrative examples, a set of five optimizations are applied to the interface lookup table including i) grouping object types that actually implement interfaces at the beginning of the interface lookup table; ii) progressively collapsing singularly-implemented interfaces; iii) applying statistical compression of the interface lookup table based on usage patterns of the object types throughout a program; iv) constant-folding for interface-to-instance lookup; and v) progressively collapsing never-called members and interfaces to remove unused members of the interface. The optimizations may be repeatedly applied until no further transformations of the interface lookup table occur. 
         [0005]    Advantageously, the present interface optimizations enable software developers to use interface constructs without incurring the usual overhead that is associated with a managed development environment such as .NET or Java. The optimized program code will typically execute faster and consume significantly fewer resources at runtime. For example, these optimizations can reduce the overhead of interface lookup tables from 2500 KB to 45 KB for a typical program. 
         [0006]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows an illustrative set of object-oriented programming constructs including objects, classes, and interfaces; 
           [0008]      FIG. 2  shows an illustrative closed runtime environment in which a compiler has awareness of all interfaces and objects that implement interfaces at a single point in time; 
           [0009]      FIG. 3  shows an illustrative interface lookup table that maps each interface to the object type that implements that interface; 
           [0010]      FIG. 4  shows an illustrative compressed interface lookup table; and 
           [0011]      FIG. 5  shows a flowchart for an illustrative method for performing interface optimization in a closed system. 
       
    
    
       [0012]    Like reference numerals indicate like elements in the drawings. 
       DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows an illustrative set  100  of object-oriented programming constructs including objects, classes, and interfaces that define the fundamental terminology that is used through the description of the present interface optimization in a closed system. Set  100  includes a plurality of objects  105   1, 2 . . . N . The objects  105  have some state and expose various methods or functions as may be required to implement some desired programming goal or functionality. The objects  105  will typically be instances of one or more classes  109   1, 2 . . . N  which provide a blueprint from which the individual objects  105  are created. 
         [0014]    Interaction with the methods exposed by the objects  105  is implemented using one or more interfaces (representatively illustrated by interface  112 ). The interface  112  essentially is composed of members which can include any of the following: i) methods; ii) properties; iii) constructors; iv) events; and, v) fields. Accordingly, the interface  112  forms a contract for supporting some functionality to the “outside world” (as indicated by reference numeral  116 ) which is enforced at build time by a compiler. Typically, if a class  109  implements an interface, then it must provide the implementation details for all members defined by the interface. In a traditional open system, an interface may be implemented or consumed by a component at any time, for example, even after the compilation of the component that declared the interface originally. 
         [0015]      FIG. 2  shows an illustrative closed runtime environment  200 . In comparison to the traditional open system, a compiler  206  here has awareness of all interfaces (collectively identified by reference numeral  212 ) and classes (collectively identified by reference numeral  209 ) that implement interfaces at a single point in time. 
         [0016]    In the closed runtime environment  200 , consider a set of declarations for Classes (A . . . F) that implement interfaces (I 1  . . . I 5 ): 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 class A : I1, I2 
               
               
                   
                 class B : I3, I4 
               
               
                   
                 class C : I1, I2 
               
               
                   
                 class D : I2, I3 
               
               
                   
                 class E : I1, I4 
               
               
                   
                 class F : I5 
               
               
                   
                   
               
             
          
         
       
     
         [0017]    A set of auxiliary lookup tables (termed “interface lookup tables” here) can be built that maps each interface to the object types that implement that particular interface. In the interface lookup tables, each class  209  is assigned a unique index represented by a single row in the interface lookup table. Each interface  212  is assigned a unique index which is represented by a single column in the built up table. 
         [0018]    An illustrative interface lookup table  300  is shown in  FIG. 3 . Each element in the table  300  refers to the function&#39;s v-table (i.e., the “virtual table” that acts as an array of pointers to functions) for the given interface. For example, ‘a2’ is class A&#39;s v-table for interface I 2 , ‘b3’ is class B&#39;s v-table for interface I 3 , and so on. 
         [0019]    A typical application could have 4,000 non-interface object types across 150 interfaces. For a naive implementation, this would result in an interface lookup table that would be approximately 2400 KB in size on a 32 bit system (i.e., 4,000 types*150 interfaces*4 bytes). 
         [0020]    When code invokes this interface, for example 
         [0000]                                            I1 p1 = ...           p1→Foo( )                        
then the v-table would naively be retrieved as
       Table(Idx — 1, p1-&gt;GetIface( ))
 
where the GetIface ( ) function is a function to return the object&#39;s row index into the interface lookup table.
         
         [0022]    Because the runtime environment is closed, as described in the text accompanying  FIG. 2 , and all the classes and interfaces are known at compile time, five different optimizations by the compiler may be applied to the interface lookup table  300 . Each optimization may be applied repeatedly until no further transformations to the table  300  occur. 
         [0023]    The optimizations make use of the observation that many interfaces in managed environments are generic and are intended to be utilized by a wide variety of different applications. Most programs only use a subset of the members of an interface, and rarely are all of the members utilized. Since programs tend to use different members, a given interface may have a lot of members. But while interfaces are broadly defined to provide general applicability, by making use of the closed system, not all the members in the interface need to be retained when a specific program is compiled. After the optimizations are performed, the optimized code produced by the compiler will typically enable the program to be smaller and run in the most efficient implementation for a given device, while consuming measurably fewer resources (e.g., memory and computational cycles) given that interface calls tend to be expensive. Such performance enhancements can be particularly beneficial on devices that are resource constrained like handheld electronic devices or other “thin” clients. 
         [0024]    The first optimization comprises grouping object types that implement interfaces at the beginning of the interface lookup table. Here, classes such as “System.Object”, the base class for all objects in the .NET framework, would be assigned a “type index” at the beginning of the interface lookup table  300 . Instead, it can be assigned an index following all classes across all modules that implement interfaces. In an open system such optimization would normally not be possible since modules loaded later in the compilation process would have their type indices appended at the end of all lists. 
         [0025]    The second optimization comprises progressively collapsing singularly-implemented interfaces. In this case, the compiler  206  will search for interfaces that only implement one class. In the interface lookup table  300 , for example, interface I 5  only implements class F. Accordingly, all instances of interface I 5  must actually be an instance of the class F, and thus the corresponding row and column can be completely removed from the interface lookup table  300 . This has the effect of shrinking the table size. 
         [0026]    Calls to the interface can be translated, for example, from 
         [0000]                                            // Make an interface call           I5 p5 = . . . ;           p5-&gt;Foo ( );                        
to
 
         [0000]                                            // Now a non-interface call to the de-referenced method           F p5 = (F) . . . ;           p5→I5_Foo ( );                        
as interface calls are normally expensive.
 
         [0027]    The third optimization comprises the application of statistical compression of the interface lookup table  300  based on usage patterns through a program. In this case, the interface lookup table  300  can be compressed so long as it is guaranteed that reverse mapping can be performed from interface type and object type to the object v-table for a given interface. An illustrative compressed interface lookup table  400  is shown in  FIG. 4 . 
         [0028]    The compiler  206  can perform compression on the original interface lookup table  300  at compile time because the table inputs are known in the closed system. The compiler  206  can apply an arbitrarily aggressive packing algorithm at compile time to get the desired compaction efficiency. 
         [0029]    In this example, the original interface lookup table  300  was compressed from a 5×6 table (30 cells) to 4×3 (12 cells) which represents a 60% reduction in size. 
         [0030]    The fourth optimization comprises constant-folding for interface-to-instance lookup. The implementations of the Table( ) and GetIface( ) methods may be utilized here to replace variable lookups with compile-time constants. This optimization enables faster dereferencing of an interface on a given object instance. 
         [0031]    The fifth optimization comprises progressively collapsing never-called members and interfaces. In this optimization, dead code across interfaces can be detected by the compiler  206  and removed at compile time. Interface members that are never called can also be removed. In addition, interfaces that become empty or are never called can be completely removed. This result becomes very valuable when implementing large “Base Class Libraries” in managed runtime environments where an interface may expose a significant amount of unneeded functionality. For example, if “ICollection.SyncRoot” is never called by the application, this member may be completely removed from the interface. An interface may be removed from a single type if it is provable that the code never accesses that interface on that type. And, an interface may be removed completely if it is never used in the program. 
         [0032]      FIG. 5  shows a flowchart for an illustrative method  500  for performing interface optimization in a closed system. Program code is received and compiled by the compiler  206  ( 505 ). A set of auxiliary lookup tables (i.e., interface lookup tables) are built that map each interface used in the program to the object types that implement the interfaces ( 510 ). One or more of the five optimizations discussed above can then be applied to reduce the size of the table ( 515 ). The optimizations may be repeatedly applied until no more transformations of the interface lookup table occur ( 520 ). 
         [0033]    Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.