Abstract:
The invention relates to tracing the execution path of a computer program comprising at least one module including a plurality of instructions. At least one of these instructions is a branch instruction. Each branch instruction is identified and evaluated to be one of true and false. An evaluation of true results in a unique identifier being pushed into a predefined area of storage. This unique identifier is associated with the instructions executed as a result of an evaluation of true.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to tracing, and more particularly to tracing the execution path of a computer program. 
     2. Background of the Invention 
     Computers have proliferated into all aspects of society and increasingly form the backbone of large organisations. When a computer system fails to respond as intended, businesses are invariably unable to provide even the most basic of services. Money, reputation or even lives may be lost, dependant upon the criticality of the service, the outage time etc. 
     In today&#39;s increasingly competitive marketplace, quality is of the utmost concern. Users&#39; expectations are high and thus the reliability of hardware/software is especially important. Customers do not tolerate mistakes and the later a defect is discovered, the more costly it can prove to the manufacturer. 
     Typically therefore, software developers implement a rigorous testing regime in order to ensure that a product or system is fully operational and consistently performs according to its functional specification, before it is made available to the public. 
     Software development is however an increasingly complex process. Even using the most rigorous techniques, software developed by people will suffer failures. Most software developers understand this and therefore accept the need to be able to effectively diagnose the cause of such a software failure. 
     A most effective technique is known as “tracing”. In this technique a program records (“traces”) important information that can be used for further analysis after the occurrence of a software failure.  FIG. 1  is a component diagram of a computer system performing tracing according to the prior art. A computer system  10 , runs an operating system (o/s)  20 , which could for example be Windows NT available from Microsoft Corporation. A program  30  executes on top of operating system  20  and may consist of a number of separate modules (not shown), each defining separate functionality. As program  30  executes, it calls various trace functions  40  which are used to record certain information in a trace log  50 . Note, although the log is shown as present on the same computer system, computer system  10  may in fact be connected to another machine across a network (neither shown) and the trace log may instead reside on that machine. 
     Tracing typically occurs at three stages in the processing or executing of a program. These stages are known as the “entry”, “exit” and “exception” points of tracing. 
     The “entry” trace information is gathered when a module obtains control from another module. The information recorded is normally the input values provided to the module. 
     The “exit” trace information is gathered when a module returns control to a module from which it has previously received control. The information recorded is normally the output values returned to the module from which it obtained control in the first place. 
     The “exception” trace information is gathered whenever an extraordinary or interesting event occurs within a module after it has been entered but before it has returned control to the module from whom it originally received control. The information gathered is normally information that relates to the exceptional condition that has occurred. 
     As previously mentioned the entry, exit and exception information is outputted to a trace log. Upon the occurrence of a software failure, a software developer can analyse the trace information in this log. This information can then be used to determine which modules were called and what information was passed between them, including any interesting events that have occurred. This hopefully allows the developer to understand and locate the source of the error and fix it. 
       FIG. 2  shows entry, exit and exception tracing according to the prior art. A program  100  consists of two modules, “Main”  105  and “Function”  110 . The program runs on an operating system  120  which initially passes control to Main. Main receives two parameters, p 1  and p 2  as input. These may be specified by the user from the command-line or be the result of output from another program/module (not shown). Main calls a trace_in function to record entry trace to trace log  130 , and a trace_out function to record exit trace to trace log  130 . 
     Trace_in is passed three parameters, “Main”, p 1 , and p 2 . “Main” is used as an identifier in the trace log such that the values of p 1  and p 2  can be connected with module Main. It can be seen from the log that parameters p 1  and p 2  have values of  1  and  3  respectively upon entry to Main. Control is then passed to module FUNCTION  110 , which passes parameters p 3  and p 4  as parameters. FUNCTION performs a trace_in on both p 3  and p 4 , which are identified by the heading “FUNCTION” in trace log  130 . It can be seen that p 3  has a value of  2  and p 4  has a value of 4 upon entry to the module FUNCTION. 
     A trace_data function writes extraordinary events/events of interest to the trace log. It passes an identifier “FUNCTION” and the data to be read out DATA 1 , or a number of parameters. This is exception trace. 
     Module FUNCTION ends and a trace_out function is called passing it the “FUNCTION” identifier, along with parameters p 3  and p 4 . Note, the dotted lines connecting the various trace functions represent calculations and processing performed by the program but not shown (i.e. additional program instructions). It can be seen from trace log  130  that such program instructions have manipulated p 3  and p 4  such that their values have changed from those initially recorded at trace_in. P 3  now equals 5 and p 4  equals 2. 
     As previously mentioned, exit information (via the trace_out function) is recorded prior to a module relinquishing control to the module which originally called it. In this example control is returned to Main  105 , additional processing performed (dotted lines) and the trace_out function called. It is passed the “Main” identifier, along with p 1  and p 2 . The trace log once again shows that additional program instructions have manipulated p 1  and p 2  such that their values are now 4 and  1  respectively. Control is then returned to operating system  120 . 
     Tracing, typically occurring at entry, exit and exception, has a very coarse granularity. It provides very little information as to the path followed during program execution. It is possible to obtain a reasonable level of detail by calling the trace_data function on a regular basis (or whenever anything unusual happens (e.g. the value of a particular parameter is not as expected)), but this is expensive in terms of CPU time. Even on the OS/390 systems (available from IBM Corporation), in which trace information is dumped to fast volatile memory, performance is degraded by approximately 50%. For this reason customers often cannot afford to leave trace running continuously. Thus when a software failure occurs, the operating system often provides call hierarchy (i.e. which modules were called during program execution) but does not provide any more information than that. Time is lost because, trace then has to be turned on and the failure simulated before diagnostics can begin. Customers do not expect these sort of delays. In their eyes, software that they&#39;ve paid good money for is not supposed to fail. 
     Not only is tracing expensive in terms of CPU performance, but hand-instrumenting a program with trace instructions is a time consuming job. Knowing the most appropriate points at which to place trace_data statements also requires experience. Trace_in and trace_out statements, on the other hand, always go at the beginning and end of a module respectively. 
     Even when all the appropriate trace information is captured in a trace log, this information can be difficult to extract. When a module completes, it is only possible to determine which processing occurred within the module by searching for the previous entry trace for that module and examining all the trace entries that this module may have called. This is exceptionally tedious, especially if the flow of control through modules is complex. The situation may be further complicated in a multi-user system where trace records for multiple activities are traced at the same time. 
       FIG. 3  shows tracing in a multi-user system according to the prior art. A computer system (not shown) runs a Process  1 . This includes three program modules A, B and C, each with trace_in and trace_out function calls. Trace Log  150  shows the trace information outputted by process  1 . It can be seen that trace_in (ti) is called for all modules. Trace_out (to) is then called for module C. Note, although not shown, the log will in reality contain parameter values for each trace function call (see  FIG. 2 ). 
     Before trace_out can be called for modules B and A (in that order), process  2  is started. Process  2  also includes three modules, A, B and C, each calling the trace_in and trace_out functions. Thus process  2  dumps the trace_in parameter values for module A out to log  150 . This entry fills up the log. New information is thus written to the beginning of the log (shown by the circular arrows). Trace Log  150 ′ shows what happens next. Process  2  runs faster than process  1  and thus it continues to write to the trace log  150 ′ from the top. Process  2  thus outputs trace_in information for its module B, and C. It also dumps trace_out information for its modules C and B. Obviously both processes have not finished outputting information to the trace log  150 ′, but it can be seen that at this point in time all the trace information above the dotted line in trace log  150  for process  1 , has been overwritten by process  2 . Thus in the event of a software failure, log  150 ′ provides very little help in diagnosing the execution flow taken by process  1 . Note, Processes  1  and  2  may be on the same machine or on separate machines. 
     SUMMARY OF INVENTION 
     Disclosure of the Invention 
     Accordingly, the invention provides a method for tracing the execution path of a computer program comprising at least one module including a plurality of instructions, at least one of said instructions being a branch instruction, the method comprising the steps of: identifying each branch instruction; evaluating each branch instruction to be one of true and false; and responsive to an evaluation of true, pushing a unique identifier into a predefined area of storage, wherein said unique identifier is associated with the instructions executed as a result of said evaluation of true. 
     Preferably, the storage area is in volatile memory and the contents of the storage area is output to non-volatile memory (e.g. a file or trace log) at a predetermined point in time. Alternatively, the storage area itself is non-volatile memory. 
     In the preferred embodiment trace information is output to the trace log upon exit from the at least one module and the contents of the storage area is output at the same time. Thus the log contains a record of the values of any specified parameters upon exit from the at least one module, as well as a record of the execution path taken through that module. 
     Since the exit record is the most recent trace record to be written, it is preferably possible to determine the flow through the module even if this is the only trace record available (i.e. the only record that has not been overwritten). Thus the problem addressed with reference to  FIG. 3  is addressed. Furthermore, because the contents of the storage area is preferably outputted as one record, it is easy to extract the information from the trace log. The trace log may consist of multiple trace entries interwoven with trace entries from other systems. According to the preferred embodiment, one can easily extract the relevant entry as all the information is held on the one line. 
     Alternatively, the contents of the storage area is output to the log when the storage area is determined to be full. As yet another alternative, the contents of the storage area is output to the log upon the occurrence of a failure within the program. 
     In all cases customer satisfaction is increased because the information contained within the storage area can be used to diagnose problems more quickly. The information provides a much greater level of detail than previously possible. It indicates the execution path or flow followed through a program and is especially useful when the program being traced is complex. It provides a valuable insight into the behavior of the program at run-time. 
     According to the preferred embodiment, the cost of pushing a unique identifier into the storage area upon execution of a particular set of program instructions is trivial. Thus customers can afford to run with trace on continuously so that important problem diagnostics information can be collected in real-time. 
     Preferably, when the last available space in the storage area has been written, old identifiers are overwritten from the beginning of the area with new values. Thus the storage area can be of a limited size and does not consume an unnecessary amount of resources. A sensible size for the storage area depends upon the complexity of the program being traced. 
     According to the preferred embodiment, the storage area stores the position at which the last unique identifier was written. Thus it is possible to determine the number of unique identifiers that have been overwritten in the circular storage area, before being outputted to the trace log. If this number is excessively large, the size of the storage area is preferably increased. 
     In a further aspect, the invention provides an apparatus for tracing the execution path of a computer program comprising at least one module including a plurality of instructions, at least one of said instructions being a branch instruction, said apparatus comprising: means for identifying each branch instruction; means for evaluating each branch instruction to be one of true and false; and means, responsive to an evaluation of true, for pushing a unique identifier into a predefined area of storage, wherein said unique identifier is associated with the instructions executed as a result of said evaluation of true. 
     In a yet still further aspect, the invention provides a method for instrumenting a computer program comprising at least one module including a plurality of instructions, at least one of said instructions being a branch instruction, each branch instruction being evaluated to be one of true and false at run-time, with at least one signature instruction for indicating the execution path of said program at run-time, the method comprising the steps of: identifying each branch instruction; identifying the instructions associated with an evaluation of true at run-time; instrumenting said instructions associated with an evaluation of true with a signature instruction, wherein said signature instruction causes a unique identifier to be pushed into a predefined area of storage upon execution of said true instructions at run-time. 
     According to the preferred embodiment, little experience is required to hand-instrument the code, since such signature instructions are always associated with a branch instruction. This is unlike the placing of trace_data statements, which do require more experience. Preferably, it is only as a result of a branch instruction that the execution flow changes. Otherwise, each instruction is executed on a line by line basis. 
     According to another aspect, the invention provides a compiler for instrumenting a computer program comprising at least one module including a plurality of instructions, at least one of said instructions being a branch instruction, each branch instruction being evaluated to be one of true and false at run-time, with at least one signature instruction for indicating the execution path of said program at run-time, said compiler comprising: means for identifying each branch instruction; means for identifying the instructions associated with an evaluation of true at run-time; means for instrumenting said instructions associated with an evaluation of true with a signature instruction, wherein said signature instruction causes a unique identifier to be pushed into a predefined area of storage upon execution of said true instructions at run-time. 
     According to the preferred embodiment, automatically instrumenting a program with signature instructions greatly eases the job of the software developer. Detailed program execution flow can be obtained, without the time consuming task of modifying the code by hand. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A preferred embodiment of the present invention will now be described, by way of example only, and with reference to the following drawings: 
         FIG. 1  is a component diagram of a computer system performing tracing according to the prior art; 
         FIG. 2  shows entry, exit and exception tracing according to the prior art; 
         FIG. 3  shows tracing in a multi-user system according to the prior art; 
         FIGS. 4 and 5  illustrate tracing according to a preferred embodiment of the present invention. 
         FIG. 6  shows the process of outputting signature information at the first point of failure according to a preferred embodiment of the present invention; and 
         FIG. 7  shows the operation of a compiler in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Description of the Preferred Embodiments 
       FIGS. 4 and 5  should be read in conjunction with one another.  FIG. 5  shows a sample program, “test program”, from which execution information is gathered at run-time according to a preferred embodiment of the present invention.  FIG. 4  is a flow chart illustrating how the sample program executes in accordance with the preferred embodiment. 
     Upon execution of the program, a small, fixed size area called a signature area  300  is defined (step  200 ). In the example, the signature area may contain up to eight signature points  310  (define sig_area size(8)). 
     Each signature point comprises a unique 4 bit identifier (number). This identifier is, according to the preferred embodiment, used to indicate the execution path or flow followed through the program. 
     Typically a program executes each instruction line by line, until a decision point or branch instruction is reached. It is at such a point that the program may jump to a specified piece of code (set of instructions) or else ignore that code and continue with subsequent instructions. One example of a branch instruction is an “if statement” (found in most programming languages):            
     It can be seen from the above that either instructions  1  and  2  are executed, or instruction  3  alone, dependant upon whether “condition”=TRUE or FALSE. There are many other examples of branch instructions (e.g. case tests/statements found in the C programming language) and it is at such points that it is helpful to capture execution flow information. The signature area  300  holds this information. 
     At step  210  of  FIG. 4 , the signature area is initialised to 0 (set sig_area=0) (not shown in  FIG. 5 ). In this example, the trace_in function is called as before (step  220 ), although this does not have to be so. Note, trace_in is passed one or more parameters as previously explained (not shown in  FIG. 5 ). 
     In the example, the program then loops until a condition evaluates to TRUE (step  230 ), at which point the program calls the trace_out function (step  280 ). This function also accepts parameters as previously discussed (not shown). Whilst however the condition evaluates to FALSE, the logic between “repeat until . . . ” and “end repeat” is performed. In the example, this logic consists of a number of case statements (branch instructions). A case statement is a Boolean test. In the example, the first case statement evaluates to TRUE when a parameter (not shown)=1, the second case statement evaluates to TRUE when another parameter (not shown)=2, and so on. An evaluation of TRUE results in the code forming part of the current case statement being executed, otherwise the next case statement is evaluated. Thus it will be appreciated that the execution flow through the program will vary dependant upon the evaluation of each case statement. 
     Thus in accordance with  FIG. 4 , a case statement is evaluated (step  230 ). If it evaluates to FALSE, then step  270  checks for another case statement. If there aren&#39;t any, then step  230  evaluates the original condition. The program then either loops round again or outputs trace at step  280 . 
     If on the other hand, a case statement evaluates to TRUE at step  240 , then an identifier is pushed into the signature area at step  250  (e.g. push 1 into sig_area). Although in the example, this identifier is hard-coded, this need not be the case. The hard-coded numeral may, for example, be replaced with a variable, initially set to 0 and then incremented each time a push into the signature area is made. A pointer  320  keeps note of the current position in the signature area. Having pushed a unique identifier into this signature area, the pointer is incremented at step  260 . 
     According to the preferred embodiment, signature points are added into the signature area from left to right (Note, this does not have to be the case and other variations are considered within the spirit of the invention). In the example, the seven most recent signature points are held in the signature area. These are used to track the execution flow through the program. The purpose of the last element  330  will be explained later. 
     In the example, case statement  1  is evaluated to TRUE first such that 1 is pushed into the signature area. Execution then jumps to case statement 4 (i.e. case statements  2  and  3  evaluate to false)(arrow  1 ); case statement  2  (arrow  2 ); case statement  3  (arrow  3 ); case statement  1  (arrow  4 ); and finally case statement  2  (arrow  5 ). It can be seen that the corresponding identifiers have been pushed into the signature area ( 1 ;  4 ;  2 ;  3 ;  1 ;  2 ). These numbers indicate which set of instructions have been executed at run-time and in what order. 
     According to the preferred embodiment, when the program or module finishes, trace is output to trace log  340  as before (step  280 ). Additionally the information contained within the signature area is also output to the trace log (step  290 ). The signature information provides valuable insight into the behavior of the program. Should the program fail or behave erroneously, then the signature points can be used in subsequent problem diagnostics. 
     According to the preferred embodiment, once the signature area  300  is full, old signature points are overwritten. Note, according to the preferred embodiment signature information is only written out to the trace log along with a trace exit record (i.e. at the end of the module/program). If a module/program is particularly complex, then the signature area may not be large enough. Thus signature points may be overwritten before they are recorded in the trace log. The last element  330  holds the number of the last signature point that is written (6). It is possible to determine from this number how much signature information is missing as a result of being overwritten. For example, if a signature point is written into element  7 , this fills the signature area. A subsequent signature point is thus written into the first element of the signature area. Last element  330  is then updated to read 1. It is thus possible to determine that the first element in the signature area has been overwritten before being outputted to the trace log (i.e. it is possible to determine that this information is missing). If a large number of signature points are continually overwritten before being recorded in the trace log, then a larger signature area can be defined. In an alternative embodiment, the signature information is written straight out to the trace log every time the signature area is filled. 
     For many modules it is sufficient to have a small number (e.g. 15) of signature points and a signature area limited to 32 bits (i.e. 4 bits per signature point, making 8 signature points). The push operation usually becomes two low-level instructions (a SHIFT followed by an OR). Therefore, for all practical purposes, the implementation cost is negligible. Even for larger signature areas and more signature points, a trivial marginal processing overhead is required. Thus a simple and efficient determination of module processing is provided. As a result, customers can afford to run with a more detailed trace (i.e. than that provided by the trace_in and trace_out functions) on continuously. 
     As previously mentioned, according to the preferred embodiment, the signature information is always written out with the exit trace record (i.e. the trace_out data). Since the exit trace record is the most recent trace record to be written, it is possible to determine the flow through the module even if this is the only trace record available (i.e. the only record that has not been overwritten). Thus the problem discussed with reference to  FIG. 3  is addressed. 
     In an alternative embodiment, the signature information is outputted to the trace log at the point of failure (see  FIG. 6 ). This is useful in the situation where a failure causes the program/module to loop indefinitely and thus never output that extremely helpful exit record. Furthermore, customer satisfaction is increased since problems can be diagnosed much more quickly. There is no need to instrument the program with the more detailed trace —data  statements, which takes time and experience. Instead data is captured upon a first failure and technicians can set about solving the problem immediately. Of course outputting the signature information also increases customer satisfaction because it too decreases the time taken to solve a software failure. 
     In one embodiment a program&#39;s code is hand-instrumented with instructions that enable a signature point to be pushed into the signature area. Note, as previously mentioned, such signature points are preferably only required at points where the code branches. Thus less experience is needed to determine where to place the signature points. This is unlike the placing of the trace_data statements, which do require more experience. According to an alternative embodiment, the code is automatically instrumented with such instructions at compile-time. 
       FIG. 7  shows the operation of a compiler in accordance with a preferred embodiment of the present invention. A compiler  410  is provided with a program&#39;s source code  400  as input. The compiler processes this source code to produce object code  420 . A listing of the source code  415  can also be produced at this stage. The object code is then passed to a linker  430  which uses this code  420  to produce an executable  440 . 
     Typically, there are three stages to the compilation process: lexical analysis; syntax analysis; and code generation. During the lexical analysis, symbols (e.g. alphabetic characters) are grouped together to form tokens. For example the characters P R I N T are grouped to form the command (token) PRINT. In some systems, certain key words are replaced by shorter, more efficient tokens. This part of the compilation process also verifies that the tokens are valid. Next, the syntax analyser checks whether each string of tokens forms a valid sentence. Finally, the code generation stage produces the appropriate object code. 
     The syntax analyser is able to identify branch instructions in a program&#39;s source code. According to the alternative embodiment, the compiler is modified to instrument the code with instructions for pushing a signature point into a defined signature area after each occurrence of a branch instruction. By allowing the compiler to automatically instrument the code at compile-time, the job of the software developer is greatly eased. Detailed execution flow information can be obtained, without the time consuming task of modifying the code by hand. Preferably, the compiler also instruments the code with an instruction to define the size of the signature area and an instruction to write the signature information out to a trace log at the appropriate point (any one of those discussed above). 
     According to the alternative embodiment, source code is compiled with one or both of two compiler switches set. The first switch determines whether the code is automatically instrumented with signature points or not, and the second switch allows a user to modify the size of the signature area. If the program being traced is particularly complex, then the size of the signature area may need to be increased.