Patent Publication Number: US-6986126-B2

Title: Method and apparatus for detecting violations of type rules in a computer program

Description:
BACKGROUND 
   1. Related Application 
   The subject matter of this application is related to the subject matter in a co-pending non-provisional application by inventors Nicolai Kosche, Milton E. Barber, Peter C. Damron, Douglas Walls and Sidney J. Hummert filed on Apr. 15, 2000 entitled, “Disambiguating Memory References Based Upon User-Specified Programming Constraints,” having Ser. No. 09/549,806. This related application is hereby incorporated by reference in order to support the instant application. 
   2. Field of the Invention 
   The present invention relates to the process of developing and debugging software for computer systems. More specifically, the present invention relates to a method and an apparatus for detecting violations of type rules in a computer program. 
   3. Related Art 
   Compilers perform many optimizations during the process of translating computer programs from human-readable source code form into machine-readable executable code form. Some of these optimizations improve the performance of a computer program by reorganizing instructions within the computer program so that the instructions execute more efficiently. For example, it is often advantageous to initiate a read operation in advance of where the data returned by the read operation is used in the program so that other instructions can be executed while the read operation is taking place. 
   Unfortunately, the problem of “aliasing” greatly restricts the freedom of a compiler to reorganize instructions to improve the performance of a computer program. The problem of aliasing arises when two memory references can potentially access the same location in memory. If this is the case, one of the memory references must be completed before the other memory reference takes place in order to ensure that the program executes correctly. For example, an instruction that writes a new value into a memory location cannot be moved so that it occurs before a preceding instruction that reads from the memory location without changing the value that is read from the memory location. 
   The problem of aliasing is particularly acute for programs that make extensive use of memory references through pointers, because pointers can be easily modified during program execution to point to other memory locations. Hence, an optimizer must typically assume that a pointer can reference any memory location. This assumption greatly limits the performance improvements that can the achieved by a code optimizer. 
   One solution to this problem is to use a strongly typed computer programming language, such as Pascal, that restricts the way in which pointers can be manipulated. For example, in a strongly typed language, a pointer to a floating point number cannot be modified to point to an integer. Hence, an optimizer is able to assume that pointers to floating pointer numbers cannot be modified to point to integers, and vice versa. The drawback of using strongly typed languages is that strong type restrictions can greatly reduce the freedom of the programmer. 
   An alternative solution is to construct a code optimizer that detects all of the aliasing conditions that can arise during program execution. Unfortunately, the task of detecting all of the aliasing conditions that can potentially arise is computationally intractable and/or undecidable for all but the most trivial computer programs. 
   Another solution is to use programming standards. The C programming language standard imposes type-based restrictions on the way pointers may be used in standard-conforming programs. Unfortunately, these programming standards are flagrantly ignored in programs of enormous economic importance, such as major database applications. Consequently, compilers do not use the restrictions imposed by programming standards to achieve better performance. 
   The process of determining whether two memory references alias is known as alias “disambiguation.” Note that alias disambiguation is typically performed through inter-procedural pointer analysis, which is intractable in both space and time for large commercial applications. 
   What is needed is a method and an apparatus that makes validation of type-based restrictions tractable for large commercial applications. 
   SUMMARY 
   One embodiment of the present invention provides a system that detects violations of type rules in a computer program. The system operates by locating a type casting operation within the computer program, wherein the type casting operation involves a first pointer and a second pointer. The system then checks the type casting operation for a violation of a type rule. If a violation is detected, the system indicates the violation. 
   In one embodiment of the present invention, if the first pointer is defined to be a structure pointer and the second pointer is not defined to be a structure pointer, the system indicates a violation of a type rule. 
   In one embodiment of the present invention, if the first pointer is a structure pointer and the second pointer is a void or char pointer, the system indicates the violation of the type rule by generating a warning to warn a programmer of a potential type violation. On the other hand, if the second pointer is a pointer to a scalar, the system generates an error to indicate a type violation to the programmer. 
   In one embodiment of the present invention, if the first pointer is defined to point to a first structure type and the second pointer is defined to point to a second structure type, the system determines whether the first structure type and the second structure type belong to the same alias group. If not, the system generates an error to indicate a type violation. In a variation on this embodiment, if the system is operating at a strict alias level or higher, and the first and second pointers are not explicitly aliased, the system generates an error to indicate a type violation. 
   In a variation in this embodiment, the system determines whether the first structure type and the second structure type belong to the same alias group by keeping track of special program statements that link structure types into alias groups. The system then determines that the first structure type and the second structure type belong to the same alias group if the first structure type and the second structure type are the same structure type, or if one or more special procedures (such as program instructions or compilation command line options) link the first structure type and the second structure type into the same alias group. In a variation in this variation, the system additionally determines that the first structure type and the second structure type belong to the same alias group if the first structure type and the second structure type have all the same basic types in the same order. 
   In one embodiment of the present invention, the computer program is received in source code form, and the system parses the computer program into an intermediate form prior to locating the type casting operation. 
   In one embodiment of the present invention, the system is configured to receive an identifier for a set of constraints on memory references that a programmer has adhered to in writing the computer program. The system uses the identifier to select a type casting rule from a set of type casting rules, wherein the selected type casting rule is associated with the set of constraints, and wherein each type casting rule is associated with a different set of constraints on memory references. 
   In one embodiment of the present invention, the system is part of a compiler. 
   In one embodiment of the present invention, the system is part of an error checking application, which is not part of a compiler. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates a computer system in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates how a filter program is used in accordance with an embodiment of the present invention. 
       FIG. 3  illustrates the internal structure of a filter program in accordance with an embodiment of the present invention. 
       FIG. 4  illustrates how constraints are used to select a type casting rule in accordance with an embodiment of the present invention. 
       FIG. 5  illustrates how special aliasing statements are identified and processed in accordance with an embodiment of the present invention. 
       FIG. 6  is a flow chart illustrating the process of validating type casting operations in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet. 
   Computer System 
     FIG. 1  illustrates a computer system  100  in accordance with an embodiment of the present invention. Computer system  100  includes central processing unit (CPU)  102 , bridge  104 , memory  106 , disk controller  112  and disk  114 . CPU  102  can include any type of computational circuitry, including, but not limited to, a microprocessor, a mainframe computer, a digital signal processor, a personal organizer, a device controller and a computational device within an appliance. 
   CPU  102  is coupled to memory  106  through bridge  104 . Bridge  104  can include any type of circuitry for coupling CPU  102  with other components in computer system  100 . Memory  106  can include any type of random access memory that can be used to store code and data for CPU  102 . 
   CPU  102  is coupled to disk  114  through disk controller  112 , bridge  104  and I/O bus  110 . I/O bus  110  can include any type of communication channel for coupling I/O devices with computer system  100 . Disk controller  112  can include any type of circuitry for controlling the actions of storage devices, such as disk  114 . Disk  114  can include any type of non-volatile storage for computer system  100 . This includes, but is not limited to, magnetic storage, flash memory, ROM, EPROM, EEPROM, and battery-backed-up RAM. 
   Memory  106  contains a filter program  108 , such as the “lint(1)” UNIX operating system utility, that has been augmented to check type cast operations in accordance with an embodiment of the present invention. Note that filter program  108  is generally used to detect bugs and irregularities in a program. 
   Also note that the present invention can generally be used within any type of computing system, and is not limited to the computing system illustrated in  FIG. 1 . 
   Filter Program 
     FIG. 2  illustrates how a filter program  108  is used in accordance with an embodiment of the present invention. Filter program  108  analyzes source code  202  to produce warnings and/or errors  206 , which indicate potential bugs and irregularities in source code  202 . This allows a programmer to correct the potential bugs and irregularities. After these corrections are made, the programmer processes source code through compiler  208 , which converts source code  202  into machine-readable object code  210  for execution on CPU  102 . Note that filter program  108  has been augmented to additionally validate type casting operations, to ensure that type casting operations within source code  202  conform to one or more rules specifying legal type casting operations. 
   In another embodiment of the present invention, the functions of filter program  108  are embedded within compiler  208 , instead of residing in a separate filter program  108 . 
     FIG. 3  illustrates the internal structure of filter program  108  in accordance with an embodiment of the present invention. Source code  202  is first processed through a parser  302  to produce intermediate form  304 . This intermediate form  304  is processed through a first pass  306 . First pass  306  generally checks assignment operations, arguments and expressions as in a normal lint program. First pass  306  has been additionally augmented to check type casting operations against a set of rules for type casting operation in accordance with an embodiment of the present invention. The output of first pass  306  is processed through a second pass  308 , which performs global analysis on the program. 
   Note that first pass  306  and second pass  308  can generate errors and warnings  310  if any potential bugs and irregularities are detected. This allows the programmer to make corrections to source code  202 . 
   Selection of Type Casting Rules 
     FIG. 4  illustrates how constraints are used to select a type casting rule in accordance with an embodiment of the present invention. The system first receives an identifier for a set of constraints on memory references that the programmer has adhered to (step  402 ). This identifier can be received as a command line argument during the compilation process, or can be received through explicit commands (or pragmas) within the code. 
   This identifier is used to select a type casting rule (or set of type casting rules) to apply (step  404 ), and this type casting rule is subsequently used to detect problematic type casting operations. 
   Locating Aliasing Statements 
     FIG. 5  illustrates how special aliasing statements are identified and processed in accordance with an embodiment of the present invention. The system first locates a special program statement that expressly aliases two structures (step  502 ). For example, the statement, “#pragma alias(struct foo, struct bar)” indicates that the structure “foo” should alias with the structure “bar.” Next, the system adds the located alias to a linked list containing pragmas that apply to structures (step  504 ). This enables type casting operations to be checked against the aliases in the linked list. 
   Note that the process illustrated in  FIG. 5  takes place during first pass  306 , at the same time that the type cast operations are being validated. Hence, the linked list will only contain aliases that have been encountered so far during the first pass. Therefore, subsequent aliases will not apply to preceding type casting operations. It is consequently advantageous to define aliases in a global header file to ensure that they apply to all type cast statements in the code. 
   Validating Type Casting Operations 
     FIG. 6  is a flow chart illustrating the process of validating type casting operations in accordance with an embodiment of the present invention. Note that this flow chart covers type cast checking for both the “weak” and the “strict” cases that are described in more detail in a related patent application by inventors Nicolai Kosche, Milton F. Barber, Peter C. Damron, Douglas Walls and Sidney J. Hummert filed on Apr. 15, 2000 entitled, “Disambiguating Memory References Based Upon User-Specified Programming Constraints,” having Ser. No. 09/549,806. This related application is hereby incorporated by reference in order to provide additional details of the “weak” and “strict” cases. 
   The system first receives the program in parsed form (step  602 ). Next, the system locates type casting operations within the program that involve pointers (step  604 ). Note that these type casting operations can occur at a number of locations, such as within assignment operations, within function calls and within expressions. For example, if there are two structures “foo” and “bar” defined, with associated pointers “fp” and “bp,” a cast can be made between pointers and the structures as follows. 
   
     
       
         
             
             
           
             
                 
                 
             
           
          
             
                 
               struct foo { 
             
          
         
         
             
             
          
             
                 
               int f1; 
             
             
                 
               int f2; 
             
          
         
         
             
             
          
             
                 
               } *fp; 
             
             
                 
               struct bar { 
             
          
         
         
             
             
          
             
                 
               int b1; 
             
             
                 
               short b2; 
             
             
                 
               short b3; 
             
          
         
         
             
             
          
             
                 
               } *bp; 
             
             
                 
               fp = (struct foo*) bp; 
             
             
                 
                 
             
          
         
       
     
   
   Next, the system determines if both pointers, fp and bp, involved in the type casting operation are structure pointers (step  606 ). If so, and if the type casting rule is associated a rule that is less than strict, both pointers are assumed to alias, and the system returns to step  604  (through the dashed line) to validate the next type casting operation. 
   If both pointers involved in the type casting operation are structure pointers, and if the type casting rule is associated with the strict type rule or higher, the system determines whether they belong to the same alias group (step  608 ). If not, the system generates an error (step  610 ) and returns to step  604 . Note that under a strict alias level, a cast of a struct pointer to a struct pointer requires explicit aliasing. 
   Otherwise, if they belong to the same alias group, the system takes no action and returns to step  604  to validate the next type casting operation. Note that the two pointers belong to the same alias group if, (1) both pointers point to the same type of structure, (2) both structure types have all the same basic types in the same order, or (3) if one or more special program instructions link both structure types into the same alias group. Note that this is not the only way an alias group can be defined. In general, many other definitions of alias groups can be used. 
   If both pointers involved in the type casting operation are not structure pointers, the system determines if the “to” pointer is a struct pointer and the “from” pointer is a scalar pointer (step  612 ). If not, the system returns to step  604  to validate the next type casting operation. 
   Otherwise, the system determines if there is a char exception (step  614 ). If not, the system generates an error to alert the programmer that there could be a cast of a scalar pointer to a struct pointer (step  618 ) before returning to step  604  to process the next type casting operation. 
   If there is a char exception, the system determines if the from pointer is a void pointer (step  620 ). If not, the system next returns to step  604  to process the next type casting operation. If so, the system generates a warning indicating that there is a cast of a void pointer to a struct pointer (step  610 ) before returning to step  604  to process the next type casting operation. Otherwise, the system returns to step  604  directly. 
   Note that casting is not in general a transitive operation. For example, casting from any structure to a void is typically allowed, whereas casting from a void to other structures may create problems. Hence, type checking may have to be performed. 
   The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.