Patent Publication Number: US-7225438-B2

Title: Lazy compilation of template-generated classes in dynamic compilation execution environments

Description:
RELATED APPLICATION 
   This application is a continuation of U.S. application Ser. No. 09/666,708, filed Sep. 21, 2000 now U.S. Pat. No. 6,760,905. The entire teachings of the above application is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   Compilers typically convert source code into object code or some other intermediate code representation, such as byte codes. Source code is a set of instructions written in a programming language by a software developer. Object code, in general, is machine code or other low level code, translated from source code toward execution by a system&#39;s central processing unit (CPU). Compilation of source code can occur either statically or dynamically. 
   Statically compiled programs are compiled once and the resulting binary object code is used repeatedly. Typically, static compilation occurs prior to shipping the program to the end user. Therefore, the time required to compile a program in static compilation environments does not effect the performance of the program when executed by the end user. 
   Dynamic compilation execution environments, such as web browsers, involve the compilation of source code simultaneously or time-sharing with the execution of a program. Dynamic compilation is advantageous because code is shipped in a platform independent form and the compilation can use information about the target machine and security environment. However, the time required to dynamically compile the source code directly impacts the speed of the program execution. 
   Programming and scripting languages provide the software developer with a set of features and tools for organizing source code within a program. Class templates and classes generated from class templates (hereinafter template-generated classes) are examples of tools provided by some programming and scripting languages within static compilation environments. 
   In object-oriented programming, a class is a descriptive tool used in source code for characterizing an object as a set of data structures (i.e. fields) or a set of routines (i.e. methods). For example, an array of integers can be represented in a class, IntegerArray. Such a class may include a field for storing the size of the array and another field comprising a data structure for representing the integers stored in an IntegerArray. In addition, the IntegerArray class may include methods for accessing the contents of the array, such as “get” and “set” methods. 
   Once compiled, the class is represented as an object defining storage allocation for the class fields and specifying the types of data that can be stored within them. In addition, the class object provides access to executable object code for executing class methods. Instances of a class are generated from this class representation. 
   An instance is an object that exists during program execution that functions in accordance with the guidelines specified by the class representation. An instance stores its own values in the storage allocated for the class fields and executes methods compiled and stored by the class representation. An instance is created when a class is instantiated during program execution. Instantiation is the process of creating an instance of a class, typically, by calling a special operator or function (e.g., “new{ }”). 
   Class templates are useful programming tools for generating classes that provide the same basic functionality, but differ with respect to the types of data that are stored and serviced within the class.  FIG. 1  is a block diagram illustrating an object hierarchy involving class templates. A class template  100  generates classes  200  which are used to declare and instantiate instances of classes  300 . 
   A class template has source code forming the foundation for new template-generated classes. Different template-generated classes can be generated from the same class template definition by specifying different parameters. A parameter can be a primitive data type, class, value, or other data structure. Therefore, the use of class templates facilitates the reuse of source code. 
   The use of class templates also leads to more robust software, because such template-generated classes can be type-checked when compiled. In addition, the use of class templates in generating classes leads to faster software, because the compiler can generate efficient code with the knowledge of the parameters defining such classes. 
   SUMMARY OF THE INVENTION 
   Using static compilation techniques for dynamically compiling a software program would be slow and inefficient. For example, compiling an entire program may be wasteful of both time and memory when some portions of a program, such as an error handling routine, may not be used. 
   With respect to template-generated classes, the unnecessary object code results, in part, from compiling source code that defines the body of class methods that are never invoked during program execution. In addition, unnecessary object code results from the generation of method bindings for class methods that are never referenced in the program code. A method binding is an object that stores information about an individual class method and is used to compile instructions in the program code that reference that method. Program code is the source code for the program in general. 
   The excess object code results in the program requiring more space on disk and, possibly, more memory (i.e. RAM) during program execution. When a program is statically compiled, the extra compilation time is tolerated because performance, in general, is not effected during program execution. However, in dynamic compilation execution environments, the program code is compiled at execution time so that extra compilation time makes the program run more slowly. Therefore, static compilation techniques implemented to compile template-generated classes would be prohibitive in both memory use and program performance. 
   The Java programming language is used, in part, to develop applets (i.e. programs that only execute within dynamic compilation execution environments, such as a web browser), but does not support class templates in its current language specification. Instead a Java programmer can use the class, Object, to store different data types, values, or structures. However, a disadvantage of using the Object class is that a Java compiler can not perform type-checking on Object-based classes until the program is running. This makes the software less robust, because any type errors are detected only at runtime by a runtime type dispatch. 
   There is a current proposal to add class templates to the Java programming language. However, the current proposal restricts the usage of parameters within the source code definition of a class template, limiting some of its benefits. 
   Therefore, it would be useful to be able to compile template-generated classes obtaining the full benefits of class templates without the issues of excess code generation and compilation time in dynamic compilation execution environments. 
   The following description discloses embodiments of the invention providing lazy compilation of template-generated classes in dynamic compilation execution environments. One embodiment involves creating a representation of the class template, creating a representation of a class generated from the class template, and generating method bindings, stored in the class representation, for only those methods referenced in the program code. This process of generating method bindings for referenced methods decreases the amount of code generated in compiling such classes and decreases the amount of time required to perform the compilation. 
   In addition, lazy compilation may include delaying compilation of the body of a referenced method until the class method is invoked by the execution of a method call instruction. Embodiments of the invention which provide for the compilation of a method body include execution of stub code initially referenced through an indexed method table for the invoked method. This action occurs in response to the class method being invoked to an instance of a template-generated class. The stub code initiates the compilation of the body of the invoked method resulting in executable object code. The address of the executable code replaces the address of the stub code in the method table for future invocation of the method by other instances of the same class. 
   Furthermore, embodiments of the invention which decrease the time required to compile a method body include a process of code sharing. Code sharing includes sharing executable object code for compatible methods among different classes generated from the same class template. Executable object code is stored or referenced by a cache of the class template representation. A determination is made as to the compatibility of the method to be compiled and the cached object code. If compatible, the compilation of the method is avoided by simply replacing the address of the stub code with the address of the cached code. 
   In general, full compilation of methods for a particular class is performed for only those methods that are referenced in the program code, invoked during program execution, and are incompatible with previously compiled methods. 
   Thus, certain aspects of the invention involve compiling program code which includes a class template and a template-generated class in a dynamic compilation execution environment. Such compilation involves (1) creating a representation of the class template from its definition within the program code; (2) creating a representation of the template-generated class from its declaration within the program code, such that it points to the class template representation; and (3) generating method bindings stored in the class representation for methods referenced within the program code. The method bindings generated are substantially limited to those methods referenced in the program code. In addition, the compilation may involve (4) delaying compilation of method bodies for those methods referenced in the program code until they are invoked. 
   Each method within a class template is represented by a method descriptor storing or referencing the source code representations of a type signature and method body for that method. The source code representation of the type signature is used in the process of generating a method binding for the template-generated class representation. 
   The process for generating method bindings is triggered whenever a method call instruction, referencing a class method of a template-generated class, requires compilation. It involves (1) scanning the class representation for a previously generated method binding representing the referenced method; and if not found, (2) creating a method binding; (3) scanning the class template representation for the source code representation of the method&#39;s type signature; (4) compiling the source code representation of the type signature; (5) associating the resulting type signature object and method offset with the method binding; and (6) associating the method binding with the class representation. Once the method binding is found or generated, the method call instruction is compiled using the type signature object and method offset. 
   The compiling of method bodies involves a class template representation which includes a method table mapping the methods of the template to stub code. The method table is referenced by the class template representation through a method table array. A copy of the method table is referenced by the class representation initially providing access to stub code for each method invoked on an instance of a template-generated class. 
   When a class method is invoked on an instance of a class representation during program execution, the method table of the class representation is referenced for the address of the stub code associated with the invoked method. The stub code is executed initiating the compilation of the source code representation of a method body such that a runtime compiler is invoked. The runtime compiler is invoked with the address of the source code representation of the method body stored within the class template representation. The address of the resulting executable code subsequently replaces the address of the stub code in the method table of the class representation. 
   Alternatively, the compilation of method bodies includes (1) referencing a cache of the class template representation for cached executable code for the invoked method; (2) determining whether the cached executable code is compatible with the invoked method; and if compatible, (3) replacing the address of the stub code in the method table with the address of the cached executable code. 
   A cache may be stored within a method descriptor of the class template representation, such that the cache includes the address of cached executable code or the address of a table mapping rules to cached executable code. The rules of the table provide criteria for determining whether the cached executable code is compatible with the invoked method. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a block diagram illustrating an object hierarchy involving class templates. 
       FIG. 2  is a flow diagram illustrating a high level overview of a process for lazy compilation of template-generated classes in dynamic compilation execution environments according to an embodiment of the invention. 
       FIG. 3A  is a block diagram illustrating a representation of a class template representation according to one embodiment of the invention. 
       FIG. 3B  illustrates a representation of a method descriptor of a class template representation according to one embodiment of the invention. 
       FIG. 3C  illustrates a representation of a field descriptor of a class template representation according to one embodiment of the invention. 
       FIG. 4A  is a block diagram illustrating a representation of a template-generated class representation according to one embodiment of the invention. 
       FIG. 4B  illustrates a representation of a field binding of a template-generated class representation according to one embodiment of the invention. 
       FIG. 4C  illustrates a representation of a method binding of a template-generated class representation according to one embodiment of the invention. 
       FIG. 5  is a flow diagram illustrating a first phase of the process for lazy compilation of class methods according to one embodiment of the invention. 
       FIG. 6  is a block diagram illustrating a representation of a template-generated class representation after the first phase of lazy compilation of the class methods according to one embodiment of the invention. 
       FIG. 7  is a flow diagram illustrating a second phase of the process for lazy compilation of class methods according to one embodiment of the invention. 
       FIG. 8  is a block diagram illustrating a representation of an instance of a template-generated class representation during program execution according to one embodiment of the invention. 
       FIG. 9  is a block diagram illustrating a representation of the template-generated class representation after the source code representation constituting the method body of a class method is compiled according to one embodiment of the invention. 
       FIG. 10A  is a block diagram illustrating a representation of a method descriptor of a class template representation for code sharing according to one embodiment of the invention. 
       FIG. 10B  is a block diagram illustrating another representation of a method descriptor of a class template representation for code sharing according to one embodiment of the invention. 
       FIG. 11  is a flow diagram illustrating a process for code sharing according to one embodiment of the invention. 
       FIG. 12A  illustrates a personal computer on which the invention may be implemented. 
       FIG. 12B  shows the internal structure of the personal computer of  FIG. 12A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A description of preferred embodiments of the invention follows. 
     FIG. 2  is a flow diagram illustrating a high level overview of a process for lazy compilation of template-generated classes in dynamic compilation execution environments according to an embodiment of the invention. The advantages include shorter compilation times and decreased code generation. 
   In step  1000 , source code defining the class template is compiled into an object representation suitable for use as a resource when subsequently compiling classes based on the template. An exemplary class template representation is shown in  FIG. 3A . This class template representation provides a resource of source code for compiling different template-generated classes. In particular, the class template representation includes objects storing source code representations for each method defined within the template. These objects are referred to as method descriptors. 
   The source code representations stored in these method descriptors are not necessarily source code. These source code representations may be some type of intermediate code (i.e. code categorized between source code and object code) as well. In general, a source code representation is an abstract representation of the source code of the class template not yet specialized for any particular template-generated class. In one embodiment, the source code representations stored in these method descriptors represent a type signature and a method body. The type signature for a method represents the arguments that the method expects and the value that the method returns. The method body is the source code constituting the behavior or functionality for a method defined in the class template. 
   In addition, a method table is generated with entries for each defined method. Each entry stores an address of executable object code called stub code. Stub code is generated for each method and initiates the compilation of the corresponding method body when that method is invoked. 
   Referring back to  FIG. 2 , source code declaring a class based on the class template is compiled in step  1010  such that the class template representation is used to generate an object representation of the template-generated class. The resulting template-generated class representation includes a pointer to the class template representation and a copy of the method table containing the pointers to stub code. 
   Lazy compilation involves, in particular, two separate compilations of the source code representations stored in or referenced by the method descriptors (i.e. steps  1020  and  1030 ). The first compilation results in the type signature of a method being compiled into a method binding as in step  1020 . The second compilation results in a method body being compiled into executable object code as in  1030 . The address of the resulting executable code replaces the previous address of the stub code stored as an entry in the method table. 
   In step  1020 , a source code instruction (i.e. method call instruction) that refers to a method of a template-generated class causes the generation of a method binding. A method binding is an object that stores information about a class method. Method bindings are generated for only those class methods that are referenced in the program code. Once generated, a method binding is used to compile the method call instruction. 
   Most programs having template-generated classes do not reference every method of the class. Therefore, limiting the creation of method bindings for class methods that are referenced in the program code avoids unnecessary object code generation and decreases total compilation time. 
   In step  1030 , the source code representation that constitutes the body of a method is compiled when the program executes a compiled method call instruction invoking that method. The execution of the compiled method call instruction causes executable code referenced by an entry within the method table associated with the invoked method to be executed. When the class method is invoked for the first time, stub code for that method initiates the compilation of the method body stored in or referenced by a corresponding method descriptor. Once compiled, the program toggles back to execution mode and the resulting executable object code is executed. 
   Method bodies are compiled once they are invoked during program execution, because some methods are never invoked even though they are referenced in the program code. Therefore, delaying compilation of method bodies in this manner avoids further generation of unnecessary object code. 
   Programs are generally broken up into source code components, such as source files. In dynamic compilation execution environments, the state of the environment toggles between compilation mode and execution mode, such that source files constituting the program are compiled at different times. Therefore, although the following descriptions illustrate lazy compilation and code sharing as occurring in one continuous sequence, lazy compilation and code sharing can occur over a series of compilation and execution phases. 
   When a source code definition for a class template is encountered in the program code, the class template is compiled to create a class template representation. A ClassTemplate is an object representation of the class template in memory. 
     FIG. 3A  is a block diagram illustrating a representation of an exemplary class template representation (i.e. “ClassTemplate”)  100  according to one embodiment of the invention. In brief overview, a ClassTemplate  100  includes a data structure  101  containing references to field and method descriptors, a data structure  120  containing references to any inherited classes  122 A and  122 B via an array data structure  121 , and a data structure  130  containing a method table array  131  referencing a method table  132 . 
   Describing Class Template  100  in more detail, a descriptor is generated for each field and method defined within the class template. Field and method descriptors are data objects storing or referencing the source code representations of the fields and methods defined by the class template. These descriptors provide a resource of source code when compiling the fields and methods for different classes. In one embodiment, field descriptors ( 105 A and  105 B, generally referred to as  105 ) and method descriptors ( 110 A and  110 B, generally referred to as  110 ) are stored in the ClassTemplate  100  as elements of a linked list. 
     FIG. 3B  illustrates a representation of a method descriptor  110  of a ClassTemplate  100  according to one embodiment of the invention. A method descriptor  110  includes an identifier  111  (i.e. name), an access privilege flag  112  (e.g., flag indicating either public, private, or protected access from other code) an address of a source code representation for the type signature  113  and an address of a source code representation for the body of the method  114 . The method body is the source code representation constituting the behavior or functionality for a method defined in the class template. The type signature for a method represents the arguments that the method expects and the values that the method returns. Methods can return more than one value. 
     FIG. 3C  illustrates a representation of a field descriptor  105  of a ClassTemplate  100  according to one embodiment of the invention. A field descriptor  105  includes an identifier (i.e. name)  106 , an access privilege flag  107  (e.g., flag indicating either public, private, or protected access from other code), an address of a source code representation of its declared type  108 , such as an integer or character, and an initial value  109 . The information stored within or referenced by a field descriptor  105  is used when a field needs to be compiled for a template-generated class. 
   Referring back to  FIG. 3A , the ClassTemplate  100  references a method table  132  through a method table array  131 . The method table  132  is generated from the method descriptors  110  such that there is one entry for each class method defined by the class template. Therefore, in one embodiment the offset of a particular method within the method table  132  is relative to the position of its method descriptor within the list of method descriptors  110 . The method table  132  initially maps the methods of the ClassTemplate  100  to stub code ( 133 A,  133 B,  133 C, and  133 D, generally referred to as  133 ). Each unit of stub code  133  is generated specifically to reference a specific method descriptor  110 . 
   Stub code  133  is executable code that invokes a compiler with the address of a method descriptor  110 . When invoked, the source code representation of the method body  114  stored within or referenced by the method descriptor  110  is compiled, resulting in executable object code for that method. The process of compiling a method body is described later with reference to  FIG. 7 . 
   In one embodiment, the method table  132  is referenced from an entry of the method table array  131 . Where the class template inherits from another class or class template, the method table array  131  also contains entries with pointers to the method tables of the inherited classes or class templates (not shown in  FIG. 3 ). 
   With respect to inherited class information  120 , the ClassTemplate  100  stores references to object representations in memory of inherited classes and class templates ( 122 A and  122 B, generally referred to as  122 ) as an array of pointers  121 . 
   The ClassTemplate  100  serves as the basis for generating object representations of template-generated classes in memory. The ClassTemplate  100  provides source code representations for all the methods and fields defined in the class template ( 110  and  105  respectively), method tables  130 , and inherited class information  120  for generating these class object representations. One ClassTemplate  100  is generated for each class template defined in the program code regardless of the number of classes that are generated from it. 
   In general, template-generated classes used in program code are data types. Such classes are declared in the program code by a variable declaration statement in which the parameters of the class template are replaced with actual data types or values. Specifying different parameters for the class template allows the software programmer to reuse the source code for the methods and fields defined within the class template. 
   When the compiler encounters in the program code a variable declaration in which the declared type is a template-generated class, the class is compiled into a template-generated class representation, an object representation of the template-generated class in memory. 
   If a template-generated class representation has already been generated for that class, no additional class representation is created. A single template-generated class representation is used to process references and method calls for all declared instances of that class. 
     FIG. 4A  is a block diagram illustrating an exemplary template-generated class representation (i.e. ClassObject)  200  according to one embodiment of the invention. In brief overview, the ClassObject  200  includes a data structure  210  containing the address of the ClassTemplate  100 , a data structure  220  initially containing references to field bindings  221 A and  221 B (generally referred to as  221 ), and a data structure  230  containing a method table array  231  referencing a method table  232  copied from the ClassTemplate  100 . 
   In brief overview, a binding is an object that stores information about a class method (i.e. method binding) or a field pertaining to the class (i.e. field binding  221 ). A method binding is used to compile source code instructions that reference a corresponding class method (i.e. method call instruction), while a field binding is used to define a corresponding field of an instance  300  of the ClassObject  200 . 
   In one embodiment, bindings are stored in the ClassObject  200  as elements of a linked list. When the ClassObject  200  is created, field bindings  221  are created and added to a bindings list for each field defined by a field descriptor  105  in the ClassTemplate  100 . 
   Initially, no method bindings are created and added to the bindings list  220 . Most programs having template-generated classes do not reference every method defined for such a class. Therefore, limiting the creation of method bindings for class methods that are referenced in the program code avoids unnecessary object code generation and decreases total compilation time. 
     FIG. 4B  illustrates a representation of a field binding  221  of a ClassObject  200  according to one embodiment of the invention. In a ClassObject  200 , a field binding  221  includes an identifier (i.e. name)  222 , an access privilege flag  223 , the address of a type object representing the type declared for the field  224 , and an initial value for the field  225 . 
   The information stored in or referenced by a field binding is used to define the characteristics for a corresponding field of an instance  300  of the ClassObject  200 . For instance, the type object defines the field in terms of what values it can store and operations it can perform. The address of the type object  224  is obtained from the resulting object code generated from the compilation of the source code representation of the field type  108  stored in or referenced by the associated field descriptor  105  of the ClassTemplate  100 . Alternatively, the address of the type object  224  is the address of the object code for the same field type previously compiled. 
   Referring back to  FIG. 4A , the method table  232  is copied from the ClassTemplate  100  for the ClassObject  200 . The ClassObject  200  references the method table  232  through its method table array  231 . The method table  232  contains all the addresses to stub code  133  generated during the creation of the ClassTemplate  100 . 
   However, the stub pointers stored within the method table  232  are temporary. After stub code for a corresponding class method is executed, the entry storing the stub pointer is replaced with the address of executable object code corresponding to the class method. All instances of the same class are represented by the same ClassObject  200 . Therefore, all instances of the ClassObject  200  refer to the same method table  232 . When a stub pointer is substituted with the address of executable object code corresponding to a class method, all instances that subsequently attempt to execute that method invoke the executable code directly, avoiding recompilation. 
   In addition, where the class template inherits from another class or class template, copies of the method tables of the inherited class or class template are referenced by the ClassObject  200  through its method table array  231  (not shown in  FIG. 4A ). 
   The address  210  of the ClassTemplate  100  is used to reference the field and method descriptors  101  during the process of generating a ClassObject  200 . 
   The next step in the process is to create method bindings for the ClassObject.  FIG. 4C  illustrates a representation of a method binding  226  associated with a ClassObject  200  according to one embodiment of the invention. A method binding  226  includes the address for the object code representation of the type signature  227  and a method offset  228  into the method table  232 . 
   In brief overview, a method binding  226  is created for each class method that is referenced by a source code instruction (i.e. method call instruction) within the program code. Furthermore, the method call instruction is compiled using the method binding, but compilation of the body of the method is delayed until the method is actually invoked during execution of the program. 
     FIG. 5  is a flow diagram illustrating a first phase of the process for lazy compilation of class methods according to one embodiment of the invention starting at step  2000 . 
   In step  2020  the compiler reads in an instruction from the program code. 
   In step  2040 , if the instruction is a method call to an instance (i.e. the method call instruction), the compiler proceeds to step  2060 . If not, a standard compile of the instruction occurs in step  2045 . Subsequently, the compiler returns to step  2020  to read in the next instruction. 
   In step  2060  the bindings  220  of the ClassObject  200  associated with the template-generated class are scanned for a method binding  226  corresponding to the class method specified in the method call instruction. If the method binding  226  does not exist, the compiler proceeds to step  2080 . 
   Otherwise, if the method binding was generated previously, the compiler extracts the method&#39;s type signature  227  and method offset  228  into the method table  232  from the method binding  226  in step  2070 . With the type signature  227  and offset  228 , the method call instruction is compiled in step  2075 . Subsequently, the compiler returns to step  2020  to read in the next instruction. 
   In step  2080  the ClassObject  200  references the ClassTemplate  100  through its stored address  210  and scans the list of method descriptors  10  for a method descriptor containing the source code representation of the class method referenced in the method call instruction. If the none of the method descriptors  110  of the ClassTemplate  100  correspond to the referenced class method, the compiler notifies the software programmer or end user of the compilation error through a user interface in step  2085 . Otherwise, the compiler proceeds to step  2100  in which the compiler creates a method binding  226  for the ClassObject  200 . 
   In step  2120 , the compiler compiles the source code representation of the type signature  113  ( FIG. 3B ) from the method descriptor  110  of the ClassTemplate  100  resulting in a type signature object. However, the compilation of the source code representation of the method body  114  is delayed until the program attempts to execute the method call instruction actually invoking the class method. 
   In step  2140 , the compiler stores or references the resulting type signature object and method offset (i.e. index into the method table  132 ) in the method binding  226  for the class method. In one embodiment, the method offset is determined by the relative position of the method descriptor for the referenced class method within the list of method descriptors  110 . 
   In step  2160 , the compiler adds the method binding  226  to the bindings data structure  220  of the ClassObject  200 . 
   Returning to step  2070 , the compiler extracts the method&#39;s type signature object  227  and offset  228  from the newly created method binding  226 . With the type signature  227  and method offset  228 , the method call instruction is compiled as an indirect function call through the method table in step  2075 . Subsequently, the compiler returns to step  2020  to read in the next instruction. 
   This process for creating and adding method bindings to the ClassObject and delaying compilation of the methods until the program invokes them at runtime decreases the code generation and time to compile due to the inherent overhead involved. 
     FIG. 6  is a block diagram illustrating a representation of a template-generated class representation (i.e. ClassObject  200 ) after the first phase of lazy compilation of the class methods according to one embodiment of the invention. The difference in the ClassObject  200  before and after lazy compilation of the class methods is that the ClassObject  200  includes method bindings ( 226 A and  266 B, generally referred to as  226 ) along with its list of field bindings  221 . Method bindings  226  are generated only for those methods that are referenced within the program code. 
   In the initial phase of lazy compilation previously described, the program code is converted from the original program code to executable object code. In the second phase, the source code representation which constitutes the body of a class method is compiled into executable object code upon the class method being invoked at runtime. 
     FIG. 7  is a flow diagram illustrating a second phase of the process for lazy compilation of class methods according to one embodiment of the invention. This phase includes compiling the source code representation constituting the method body of a class method when the method is invoked by the program at runtime. 
   In step  3000 , the program executes a set of compiled instructions which create an instance  300  of the ClassObject  200  as shown in  FIG. 8 .  FIG. 8  is a block diagram illustrating a representation of an instance  300  of a ClassObject  200  during program execution according to one embodiment of the invention. An instance is an object that is generated during program execution that functions in accordance with the guidelines specified by ClassObject  200 . Every instance  300  of a ClassObject  200  includes an address  310  to the method table  232  of the ClassObject  200  and memory  311 A and  311 B allocated for storing field values. Where other classes or class templates are inherited, the instance additionally contains addresses  312  to the respective inherited method tables as well as memory  313  allocated for inherited fields. 
   In step  3020 , the program executes object code, corresponding to the method call instruction in the program code, which invokes a class method to an instance  300  of the ClassObject  200 . 
   In step  3040 , the method table  232  of the ClassObject  200  is referenced at the offset of the invoked method to obtain the address of the executable code mapped to this method. The executable code may be the original stub code  133  or object code resulting from a previous compilation of the method body. 
   Although there is no specific check, step  3050  illustrates the direction of the process depending on whether the executable code is stub code  133  or object code compiled for the class method. In step  3050 , if the address does not point to stub code, then the method table is addressing executable object code that was previously compiled for this method. The program continues in execution mode executing the executable code mapped to the invoked method in step  3140 . Otherwise, the address is pointing to stub code and the process proceeds to step  3060 . 
   In step  3060 , the stub code  133 , which is mapped to the invoked method, is executed at the address specified in the method table entry. The execution of the stub code  133  invokes the compiler with the address of the method descriptor  110  of the ClassTemplate  100  for the invoked method. This step toggles the environment into compilation mode. 
   In step  3080 , the source code representation  114  stored in or referenced by the method descriptor  110  for the body of the invoked method is compiled resulting in executable object code that implements the intended functionality or behavior when executed. 
   In step  3100 , the address of the resulting executable object code replaces the address of the stub code  133  in the method table  232  for future invocations of the method. In addition, since all instances  300  of the same ClassObject  200  share the same method table  232 , future execution of the method by any instance  300  occurs without the need for recompilation of the method body. 
   In step  3120 , the environment toggles back to execution mode. 
   In step  3140 , the resulting executable object code of the invoked method is executed producing the desired result. 
     FIG. 9  is a block diagram illustrating a representation of the ClassObject  200  after the source code representation constituting the method body of a class method is compiled according to one embodiment of the invention. In particular, the method table entries corresponding to the compiled class methods are replaced with addresses to executable object code ( 240 A and  240 B, generally referred to as  240 ). By replacing the stub pointers of the method table entries, instances of this ClassObject can invoke a method and directly execute the object code without having to recompile the method body for that method. 
   In an alternative embodiment of the invention, a system and method for code sharing is implemented. Code sharing works in conjunction with lazy compilation to further decrease compilation time and code generation. 
   Code sharing provides the ability to cache and share executable object code of compatible methods among the ClassObjects representing different classes. Therefore, instead of recompiling the same source code for each ClassObject, the address of the stub code in the ClassObject&#39;s method table is replaced with the address of the cached object code. This process decreases the compilation time since the compilation step is avoided. In addition, code generation is decreased because the method tables of each individual ClassObject reference the same cached object code. 
   In order to implement code sharing, the method descriptors of the ClassTemplate further include a cache  115  for storing the address of the cached executable object code  116  as shown in  FIG. 10A . Alternatively, as shown in  FIG. 10B , the cache  115  stores the address of a compiler-generated table  117  mapping a set of rules with an address of the associated cached code for each rule. Each rule is a test for determining whether the method for a particular ClassObject is compatible with the cached object code for another ClassObject. 
   Since a ClassObject  200  is defined by its parameters, the rules, in general, are based on attributes of the parameters. An example of such an attribute is the memory representation of the parameter. For example, if the method is concerned only about the bit size of the parameter type, the rule may allow the class method to be compiled having a parameter that is 32 bits in length to share the cached object code previously compiled for a class method taking a parameter was 32 bits in length for the same ClassObject. 
   In addition, a method is compatible among all ClassObjects where the method does not refer to the parameter of the ClassObject at all. For example, a method that prints error strings or debug messages and does not reference a parameter is compatible for all ClassObjects generated from the same ClassTemplate. Such a method is compatible because the executable object code, compiled from its method body, is the same regardless of the ClassObject for which it is compiled. 
   More generally, the compiler can see which properties of the ClassObject parameters, if any, are relevant to the compilation of each method. Two methods can be shared between different ClassObjects if the type parameters have the same relevant properties. The compiler can automatically determine which properties are relevant by recording the type properties it queries during compilation of the method. 
   Furthermore, it may be possible to have a rule that will allow parameter types that can be converted from one to another to use the same cached code, such as in the case where the cached code is compiled for a class method taking a parameter of type “long” and the class method to be compiled taking a parameter of type “integer.” In this case, the integer could be cast into a type “long.” 
   Code sharing occurs during the compilation of the body of a method.  FIG. 11 , with reference to  FIG. 7 , illustrates the process for code sharing. 
   After invoking the compiler with the address of the appropriate method descriptor in step  3060  of  FIG. 7 , the compiler references the cache  115  of the method descriptor  110  for cached object code  116  or, alternatively, the compiler-generated table  117  mapping rules to the cached object code in step  3062 . 
   If, in step  3064 , the cache  115  is empty, the compilation process proceeds to step  3080  of  FIG. 7 , for compiling the source code representation of the method body. Otherwise, if the cache  115  contains cached object code  116  or a rule-based table of cached object code  117 , the compiler proceeds to step  3066 . 
   In step  3066 , the compiler determines whether the methods are compatible in step  3066 . For example, if the method to be compiled matches a rule of the table  117  corresponding to cached object code, then a method is deemed compatible. If the methods are deemed incompatible, the compilation process proceeds back to step  3080 . Otherwise, the methods are deemed compatible and the compiler avoids the process of compiling the method body proceeding to step  3068 . 
   In step  3068 , the address of the stub code for that method in the method table is replaced with the address of the cached object code. 
   After updating the method table, the process proceeds to step  3120  of  FIG. 7  to begin execution of the invoked method. 
     FIG. 12A  shows an example of a computer system on which embodiments of the present invention may be implemented. As shown, Computer  1  includes a variety of peripherals, among them being: i) a display screen  5  for displaying images/video or other information to a user, ii) a keyboard  6  for inputting text and user commands. Computer  1  may be a personal computer (PC), workstation, embedded system component, handheld computer, telecommunications device or any device containing a memory and processor. 
     FIG. 12B  shows the internal structure of Computer  1 . As illustrated, Computer  1  includes mass storage  12 , which comprises a computer-readable medium such as a computer hard disk and/or RAID (“redundant array of inexpensive disks”). Mass storage  12  is adapted to store applications  14 , databases  15 , and operating systems  16 . In preferred embodiments of the invention, the operating system  16  is a windowing operating system, such as RedHat® Linux or Microsoft® Windows98, although the invention may be used with other operating systems as well. Among the applications stored in memory  12  is a programming environment  17  and source files. 
   Programming environment  17  compiles the source files written in a language that creates the output generated by embodiments of the present invention. In the preferred embodiment of the invention, this language is Curl™, developed by Curl Corporation of Cambridge, Mass. The programming language is based upon a language developed at Massachusetts Institute of Technology and presented in “Curl: A Gentle Slope Language for the Web,”  WorldWideWeb Journal , by M. Hostetter et al., Vol II. Issue 2, O&#39;Reilly &amp; Associates, Spring 1997. 
   Computer  1  also includes display interface  20 , keyboard interface  21 , computer bus  26 , RAM  27 , and processor  29 . Processor  29  preferably comprises a Pentium II® (Intel Corporation, Santa Clara, Calif.) microprocessor or the like for executing applications. Such applications, including the programming environment and/or embodiments of the present invention  17 , may be stored in memory  12  (as above). Processor  29  accesses applications (or other data) stored in memory  12  via bus  26 . 
   Application execution and other tasks of Computer  1  may be initiated using keyboard  6  commands from which are transmitted to processor  29  via keyboard interface  21 . Output results from applications running on Computer  1  may be processed by display interface  20  and then displayed to a user on display  5 . To this end, display interface  20  preferably comprises a display processor for forming images based on image data provided by processor  29  over computer bus  26 , and for outputting those images to display  5 . 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.