Patent Publication Number: US-6041180-A

Title: System and method for optimizing template object files

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of information handling systems, and, more particularly, to a system and method for optimizing template object files used by program code executing in the information handling system. 
     BACKGROUND OF THE INVENTION 
     In an object-oriented programming language, such as C++, a class template defines the layout and operations for unbounded sets of related types. Templates provide a strongly-typed &#34;macro&#34; facility, and promote source code re-use. For example, a single class template for a stack might provide a common definition for a stack of integers, a stack of floats, or a stack of pointers to char strings. A template may be used by an individual program to implement an object, such as a stack. Rather than writing code to implement the stack, the programmer can simply use a stack template. Thus, templates reduce the amount of source code that programmers need to write, and promote code reuse. 
     The problem with the implementation of templates is that while the object code for different template class declarations may be similar, or even identical, the object code is not reused, where possible, for each different template class declaration. This means that every time a programmer defines and uses a different stack class, the template code is inserted into the program. This results in duplicate code being included in the program. 
     Consequently, it would be desirable to have a system and method for reusing code, where possible, to reduce the size of the program. It would also be desirable to have a system and method for reusing code which would have a negligible impact on the efficiency of the code. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a system and method to reuse code, and thus save code space within a program. A compiler and a linker work together to decide which object code to reuse, for implementation of template classes, and other repetitive code segments. 
     The compiler notes in the object file which functions are generated from template code, or other repetitive types of code. An intelligent linker then attempts to match code, and in the cases where the code matches, simply eliminates multiple versions of the same code by aliasing the function names. The compiler can reduce the amount of linker searching by storing a cyclic redundancy check (CRC) code with each method (note that a method is a procedure or routine associated with a class). Testing will show whether the CRC is sufficient or if the linker needs to perform a more exact match. 
     One of the embodiments of the invention is as sets of instructions resident in an information handling system. 
     The system and method of the present invention can be used by any compiler and linker to optimize generated object code. One advantage of the invention is that code is optimized so as to save code space in a program. Another advantage of the present invention is that true code reuse is made possible without sacrificing program efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will become more apparent from the detailed description of the best mode for carrying out the invention as rendered below. In the description to follow, reference will be made to the accompanying drawings, where like reference numerals are used to identify like parts in the various views and in which: 
     FIG. 1 is a block diagram of an information handling system capable of executing the optimization method of the present invention; and 
     FIG. 2 is a flow chart depicting the steps of compiling and linking code. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     The invention may be implemented on a variety of hardware platforms, including personal computers, workstations, mini-computers, and mainframe computers. Many of the steps of the method of the present invention may be advantageously implemented on parallel processors of various types. Referring now to FIG. 1, a typical configuration of an information handling system that may be used to practice the novel method of the present invention will be described. The computer system of FIG. 1 has at least one processor 10. Processor 10 is interconnected via system bus 12 to random access memory (RAM) 16, read only memory (ROM) 14, and input/output (I/O) adapter 18 for connecting peripheral devices such as disk units 20, tape drives 40, and printers 42 to bus 12, user interface adapter 22 for connecting keyboard 24, mouse 26 having buttons 17a and 17b, speaker 28, microphone 32, and/or other user interface devices such as a touch screen device 29 to bus 12, communication adapter 34 for connecting the information handling system to a data processing network, and display adapter 36 for connecting bus 12 to display device 38. Communication adaptor 34 may link the system depicted in FIG. 1 with hundreds or even thousands of similar systems, or other devices, such as remote printers, remote servers, or remote storage units. 
     The invention will be described with reference to optimizing C++ template object files. However, the system and method of the present invention may be used with any compiler, and any type of repetitively generated code. 
     An example class template definition for a stack is illustrated by the following C++ code: 
     
         ______________________________________                                    
         #ifndef .sub.-- STACK.sub.-- TPL.sub.-- H                        
         #define .sub.-- STACK.sub.-- TPL.sub.-- H                        
         template&lt;class T&gt;                                                
         class stack                                                      
         {                                                                
         private:                                                         
           T*  v;                                                         
           T*  p;                                                         
           int sz;                                                        
         public:                                                          
           stack(int);                                                    
           .sup.- stack( );                                               
           void push(T);                                                  
           T pop( );                                                      
           int getSize( );                                                
         };                                                               
         #endif                                                           
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     Note that the identifier T represents the template type. The class implementation for the above stack is as follows: 
     
         ______________________________________                                    
template&lt;class T&gt;stack&lt;T&gt;::stack(int s)                                   
       v = p = new T[sz=s];                                               
}                                                                         
template&lt;class T&gt;stack&lt;T&gt;::.sup.- stack( )                                
{                                                                         
       delete [] v;                                                       
}                                                                         
template&lt;class T&gt; void stack&lt;T&gt;::push(T a)                                
{                                                                         
       *p++ = a ;                                                         
}                                                                         
template&lt;class T&gt; T stack&lt;T&gt;::pop ( )                                     
{                                                                         
       return *(--p);                                                     
}                                                                         
template&lt;class T&gt; int stack&lt;T&gt;::getSize( )                                
{                                                                         
       return sz;                                                         
}                                                                         
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     Different declarations (i.e. template classes) of this class template are defined as follows: 
     
         ______________________________________                                    
#include &#34;stk.h&#34;                                                          
typedef char         *pCHAR;                                              
typedef unsigned short                                                    
                     *pUSHORT;                                            
typedef int          *pINT;                                               
typedef unsigned int *pUINT;                                              
stack&lt;unsigned int&gt;  dummy0(10);                                          
stack&lt;int&gt;           dummy1(10);                                          
stack&lt;unsigned short&gt;                                                     
                     dummy2(10);                                          
stack&lt;pCHAR&gt;         dummy3(10);                                          
stack&lt;PUSHORT&gt;       dummy4(10);                                          
stack&lt;pINT&gt;          dummy5(10);                                          
stack&lt;pUINT&gt;         dummy6(10);                                          
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     The stack class for integers can be used as follows: 
     
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#include &lt;iostream.h&gt;                                                     
#include &#34;stk.h&#34;                                                          
int main(void)                                                            
stack&lt;int&gt; s(10);                                                         
cout &lt;&lt; &#34;Pushing the sequence of numbers: 2 4 1 3\n&#34;;           
s.push(2);                                                                
s.push(4);                                                                
s.push(1);                                                                
s.push(3);                                                                
cout &lt;&lt; &#34;Popping the numbers:expecting 3 1 4 2\n&#34;;              
cout &lt;&lt; &#34;Sequence from Pop operations: &#34;;                                 
cout &lt;&lt; s.pop( ) &lt;&lt; &#39; &#39; ;                                                 
cout &lt;&lt; s.pop( ) &lt;&lt; &#39; &#39; ;                                                 
cout &lt;&lt; s.pop( ) &lt;&lt; &#39; &#39; ;                                                 
cout &lt;&lt; s.pop( ) &lt;&lt; &#39;\n&#39;;                                       
return 0;                                                                 
}                                                                         
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     When code is compiled, C++ compilers typically repeat the code for each method (e.g. push, pop, etc.) even if it is independent of the type (i.e. int, char, etc.) used for the template. In some cases, the code does need to be different because of differences in the type used in the template class declaration. However, in many cases the code will be identical, or nearly identical. 
     For example, the generated assembly code for the pop() method for each of the different types is shown below: 
     
         ______________________________________                                    
         unsigned int:                                                    
         pop.sub.-- stackXTUi.sub.-- Fv proc                              
         mov    ecx,eax                                                   
         mov    eax,[ecx+04h]                                             
         lea    edx,[eax-04h]                                             
         mov    [ecx+04h],edx                                             
         mov    eax,[eax-04h]                                             
         ret                                                              
         pop.sub.-- stackXTUi.sub.-- Fv endp                              
         int:                                                             
         pop.sub.-- stackXTi.sub.-- Fv proc                               
         mov    ecx,eax                                                   
         mov    eax,[ecx+04h]                                             
         lea    edx,[eax-04h]                                             
         mov    [ecx+04h],edx                                             
         mov    eax,[eax-04h]                                             
         ret                                                              
         pop.sub.-- stackXTi.sub.-- Fv endp                               
         unsigned short:                                                  
         pop.sub.-- stackXTUs.sub.-- Fv proc                              
         mov    edx,eax                                                   
         push   ebx                                                       
         mov    ecx,[edx+04h]                                             
         xor    eax,eax                                                   
         lea    ebx,[ecx-02h]                                             
         mov    [edx+04h],ebx                                             
         pop    ebx                                                       
         mov    ax,[ecx-02h]                                              
         ret                                                              
         pop.sub.-- stackXTUs.sub.-- Fv endp                              
         char:                                                            
         pop.sub.-- stackXTc.sub.-- Fv proc                               
         mov    edx,eax                                                   
         push   ebx                                                       
         mov    cx,[edx+04h]                                              
         xor    eax,eax                                                   
         lea    bx,[ecx-01h]                                              
         mov    [edx+04h],ebx                                             
         pop    ebx                                                       
         mov    a1,[ecx-01h]                                              
         ret                                                              
         pop.sub.-- stackXTc.sub.-- Fv endp                               
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     Note that the generated code for unsigned int and int is exactly the same. This is not unexpected as the size of the type is the same (i.e. 4 bytes each). The code for unsigned short and char is different, as expected, because an unsigned int is only two bytes in size and a char is only one byte in size. 
     As shown below, the generated code for all the pointer types are the same, as is the code for unsigned int and int: 
     
         ______________________________________                                    
        pointer to unsigned int:                                          
        pop.sub.-- stackXTPUi.sub.-- Fv proc                              
        mov    ecx,eax                                                    
        mov    eax,[ecx+04h]                                              
        lea    edx,[eax-04h]                                              
        mov    [ecx+04h],edx                                              
        mov    eax,[eax-04h]                                              
        ret                                                               
        pop.sub.-- stackXTPUi.sub.-- Fv endp                              
        pointer to int:                                                   
        pop.sub.-- stackXTPi.sub.-- Fv proc                               
        mov    ecx,eax                                                    
        mov    eax,[ecx+04h]                                              
        lea    edx,[eax-04h]                                              
        mov    [ecx+04h],edx                                              
        mov    eax,[eax-04h]                                              
        ret                                                               
        pop.sub.-- stackXTPi.sub.-- Fv endp                               
        pointer to unsigned short:                                        
        pop.sub.-- stackXTPUs.sub.-- Fv proc                              
        mov    ecx,eax                                                    
        mov    eax,[ecx+04h]                                              
        lea    edx,[eax-04h]                                              
        mov    [ecx+04h],edx                                              
        mov    eax,[eax-04h]                                              
        ret                                                               
        pop.sub.-- stackXTPUs.sub.-- Fv endp                              
        pointer to char:                                                  
        pop.sub.-- stackXTPc.sub.-- Fv proc                               
        mov    ecx,eax                                                    
        mov    eax,[ecx+04h]                                              
        lea    edx,[eax-04h]                                              
        1.     mov [ecx+04h],edx                                          
        mov    eax,[eax-04h]                                              
        ret                                                               
        pop.sub.-- stackXTPc.sub.-- Fv endp                               
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     The generated code for the getSize() method is also the same regardless of which template class is declared. In addition, the generated code for the constructor and destructor methods are the same regardless of which template class is declared. 
     Two principles arise from these examples: 
     1. The generated code is the same if the template type is not used in the method. 
     2. The generated code is usually the same if the size of the template types is the same. 
     Based on the above principles, the present invention provides a system and method which can be used by compilers and linkers to reuse object code, where possible, for implementation of template classes. The compiler notes in the object file which functions are generated from template code. The intelligent linker then attempts to match code, and in the cases where the code matches, simply eliminates multiple versions of the same code by aliasing the function names. The compiler can reduce the amount of linker searching by storing a cyclic redundancy check (CRC) with each method. Testing with various test cases will show whether the CRC is sufficient, or if the linker needs to perform a more exact match. The actual implementation will of course be compiler specific, however, an example is illustrated below with reference to FIG. 2. 
     The present invention is especially beneficial when optimizing class templates in which the methods perform data manipulation not directly tied to the template type. A common example is collection classes in which most methods are concerned with walking the collection, and only a few methods deal with adding, deleting or finding elements in the collection. 
     In addition, implementations of class libraries may be modified to derive the most benefit from the described optimization. For example, parameters that are template types can be converted to pointers before calling a relatively large implementation function. This results in only the wrapper function being different for each template type. 
     Referring now to FIG. 2, a method of compiling and linking code according to the present invention will be described. During the compile phase 102, a source file 104 is processed through a preprocessor 106, a syntax checker and parser 108, and a code generator 110. The preprocessor 106, syntax checker and parser 108, and code generator 110 produce a symbol table 112. The generated code is then optimized 114 and an object file 116 is produced. 
     During the linking phase 118, one or more object files 116 are linked together by being processed through a preprocessor 120 and an executable generator 122. Symbol table 124 is used for resolving linkages (i.e. collecting global names and resolving them). In addition, symbol table 124 is also used to collect and store all comment records found. The comment records are used to identify template object code, and are described more fully below. Finally, an executable code module 126 is created. 
     For the purposes of this example, assume the stack template implementation is in a file referred to as STK.CPP, and the following code snippet is from a source file 104, referred to as TEST.CPP: 
     
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#include &lt;iostream.h&gt;                                                     
#include &#34;stk.h&#34;                                                          
stack&lt;int&gt; i(10);                                                         
stack&lt;unsigned short *&gt; s(10);                                            
i.push(1);                                                                
i.push(2);                                                                
unsigned short j = 1;                                                     
s.push (&amp;j);                                                              
j = 2;                                                                    
s.push (&amp;j);                                                              
cout &lt;&lt; i.getSize( ) &lt;&lt; &#34;elements in integer stack\n&#34;;          
cout &lt;&lt; s.getSize ( ) &lt;&lt;                                                  
&#34;elements in unsigned short pointer stack\n&#34;;                   
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     When TEST.CPP is compiled, three object files 116 are produced, one for TEST.CPP and one for each of the different invocations of the stack template class. Assume that the template file objects for each of the two separate invocations are named STK1.OBJ and STK2.OBJ. 
     During the compiler phase 102, the compiler adds a comment record to each object file for each template function. The exact point at which the compiler adds the comment records will of course be compiler-dependent. The comment record contains the template filename, line number, function name and CRC (or checksum) for the object code. For example, STK1.OBJ contains: 
     
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filename  line number CRC      function                                   
______________________________________                                    
STK1.OBJ  1           8D23     .sub.-- C.sub.-- stack.sub.-- Fv           
STK1.OBJ  6           CAA1     .sub.-- D.sub.-- stack.sub.-- Fv           
STK1.OBJ  10          3A1F     push.sub.-- stack.sub.-- Fi                
STK1.OBJ  15          4A2C     pop.sub.-- stack.sub.-- XTi.sub.-- Fv      
STK1.OBJ  20          1CAD     getSize.sub.-- stackXTi.sub.-- Fv          
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     and STK2.OBJ contains: 
     
         ______________________________________                                    
filename                                                                  
        line number CRC      function                                     
______________________________________                                    
STK2.OBJ                                                                  
        1           8D23     .sub.-- C.sub.-- stack.sub.-- Fv             
STK2.OBJ                                                                  
        6           CAA1     .sub.-- D.sub.-- stack.sub.-- Fv             
STK2.OBJ                                                                  
        10          3A1F     push.sub.-- stack.sub.-- FPUs                
STK2.OBJ                                                                  
        15          4A2C     pop.sub.-- stackXTPUs.sub.-- Fv              
STK2.OBJ                                                                  
        20          1CAD     getSize.sub.-- stackXTPUs.sub.-- Fv          
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     During the preprocessor step 120 of the linker phase 118, the linker stores these comment records in symbol table 124. During the executable generation step 122 of the linker phase 118, the comment records in the object files are searched for common template code. The exact point in step 122 where this searching takes place will be linker-specific. The speed of this search is improved by using the CRC. If two CRC codes are equal, it is very likely that the bytes of code used to generate the CRC codes are also the same. Thus, if the linker finds matching CRC codes, it will then compare the actual instructions to determine if the underlying code is the same. 
     If a match is found, the linker creates a duplicate set of entry points in the executable code that aliases the found function and adjusts the addresses to the new function name. In the described example, the mapping would now be: 
     
         ______________________________________                                    
filename                                                                  
        line number CRC      function                                     
______________________________________                                    
STK1.OBJ                                                                  
        1           8D23     .sub.-- C.sub.-- stack.sub.-- Fv             
STK1.OBJ                                                                  
        6           CAA1     .sub.-- D.sub.-- stack.sub.-- Fv             
STK1.OBJ                                                                  
        10          3A1F     push.sub.-- stack.sub.-- Fi                  
STK1.OBJ                                                                  
        15          4A2C     pop.sub.-- stack.sub.-- XTi.sub.-- Fv        
STK1.OBJ                                                                  
        20          1CAD     GetSize.sub.-- stackXTi.sub.-- Fv            
STK2.OBJ                                                                  
        1           8D23     .sub.-- C.sub.-- stack.sub.-- Fv             
STK2.OBJ                                                                  
        6           CAA1     .sub.-- D.sub.-- stack.sub.-- Fv             
STK2.OBJ                                                                  
        10          3A1F     push.sub.-- stack.sub.-- FPUs                
STK2.OBJ                                                                  
        15          4A2C     pop.sub.-- stackXTPUs.sub.-- Fv              
STK2.OBJ                                                                  
        20          1CAD     getSize.sub.-- stackXTUs.sub.-- Fv           
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     The duplicate comment records for STK2.OBJ are also invalidated, as object STK1.OBJ has been used for resolving all symbol names and references. 
     
         ______________________________________                                    
filename  line number    CRC    function                                  
______________________________________                                    
STK2.OBJ  --             --     --                                        
STK2.OBJ  --             --     --                                        
STK2.OBJ  --             --     --                                        
STK2.OBJ  --             --     --                                        
STK2.OBJ  --             --     --                                        
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     In the example shown, 50% of the code has been eliminated, thus saving significant code space. For each of the stack functions (i.e. constructor, destructor, push, pop, and getSize), there is only one entry point in the executable code. For example, a call to push an integer onto the integer stack (i.e. i.push(1) and i.push(2) in the original source code) will call the same executable code as will a call to push a pointer to an unsigned short onto the pointer to unsigned short stack (i.e. s.push(&amp;j) in the original source code). For every function where the sets of instructions are the same, only one set of instructions is present in the executable module. 
     Although the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. One of the embodiments of the invention can be implemented as sets of instructions resident in the random access memory 16 of one or more computer systems configured generally as described in FIG. 1. Until required by the computer system, the set of instructions may be stored in another computer readable memory, for example in a hard disk drive, or in a removable memory such as an optical disk for eventual use in a CD-ROM drive or a floppy disk for eventual use in a floppy disk drive. Further, the set of instructions can be stored in the memory of another computer and transmitted over a local area network or a wide area network, such as the Internet, when desired by the user. One skilled in the art would appreciate that the physical storage of the sets of instructions physically changes the medium upon which it is stored electrically, magnetically, or chemically so that the medium carries computer readable information. The invention is limited only by the following claims and their equivalents.