Abstract:
Computer method and apparatus allows instrumentation of program modules while maintaining exception-handling unwinding context. In the case of instrumenting procedure prologues, the invention preserves the calling context. A sanitized copy of the prologue and rewind instructions to reverse the effects of duplicate prologue instructions are employed.

Description:
RELATED APPLICATION  
       [0001]    This application is a continuation of U.S. application Ser. No. 09/410,388, filed Sep. 30, 1999, the entire teachings of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A computer application (i.e., program) is formed of a series of instructions referred to as “lines of code”. Initially, these instructions are written in a human readable programming language, e.g., Fortran, C++, etc. This is referred to as the program source code. A compiler transforms the source code into machine readable code. It is the machine readable code, or a binary image of the same, that is the executable form (by a digital processor) of the application program.  
           [0003]    To assist in debugging or analyzing a program, certain additional machine readable lines of code called instrumentation points are desired to be inserted into the executable image. At these specified points, new procedure invocations are introduced into the program. Instrumented programs are executed in the same manner as the original program, with the new procedures being invoked at the instrumented points in the program. These new procedures, or analysis routines, provide, for example, the values of parameters at the instrumentation point in the program, the number of times a procedure in the program is called, and other information about certain parts of the program. To that end, the new procedures/analysis routines of the instrumented program provide the capability for debugging and other types of analysis of the program. When executed, the instrumented application produces the desired type of analysis action as it runs.  
           [0004]    Instrumentation points are allowed at procedure boundaries, basic block boundaries and/or instruction boundaries. Introducing binary instrumentation into the prologue code (initialization portion) of a procedure is a difficult problem. The primary issue is that if prologue code is used as an exception-handling mechanism for unwinding the stack frame, the introduction of other instructions into the prologue may compromise or break exception handling.  
           [0005]    In the Atom and NT-Atom (both by Digital Equipment Corporation of Maynard, Massachusetts) instrumentation frameworks, the ability to properly instrument the prologue is a fundamental requirement for doing simulated performance studies. The current method that Atom utilizes on Tru64 Unix is to insert instrumentation code directly in the prologue potentially breaking exception handling of instrumented Tru64 applications. The current released version of NT-Atom does not instrument the prologue of any routine, thus guaranteeing that exception handling will operate properly.  
         SUMMARY OF THE INVENTION  
         [0006]    Thus there is need for a technique which allows binary instrumentation of prologue code in program procedures while preserving the exception-handling&#39;s mechanism for unwinding the procedures stack frame. Such is the object of the present invention.  
           [0007]    The present invention provides a method and apparatus to safely insert instrumentation code into the prologue of a procedure. The invention works with programming models that respond to exceptions or unwinding mechanisms by reverse executing the prologue code. Reverse execution of the prologue returns the state of the program to a condition that it was prior to the execution of the procedure.  
           [0008]    The invention method introduces binary instrumentation into a procedure&#39;s prologue by duplicating components of the prologue and then instrumenting the original version of the prologue. The non-standard prologue instructions are preserved in the original sequence of instrumented prologue instructions. The net effect is that exception handling is preserved and the full prologue with non-standard instructions can be instrumented and executed without altering the program&#39;s intended behavior.  
           [0009]    In the preferred embodiment, the method includes the steps of: (a) identifying a portion of the subject program into which insertion of instrumentation code is desired; (b) duplicating the identified portion such that a working copy of the identified portion is formed, said duplicating including placing the working copy before the identified portion; (c) removing instructions from the working copy such that only instructions necessary to restore calling context remains; (d) instrumenting the identified portion; and (e) enabling the working copy for use during reverse execution of the subject program and otherwise effecting use of the instrumented identified portion in the subject program.  
           [0010]    Further, rewind instructions are inserted between the working copy and the original identified portions to reverse the effect of duplicated instructions in the identified portion. Preferably, working constructs (i.e., pointers) are set such that the working copy serves as an entry point and the original identified portion is effectively part of the main body of a program module.  
           [0011]    Apparatus of the present invention includes an instrumentation assembly coupled to an instrumentation coder. The instrumentation assembly effects steps (a), (b), (c) and (e) of the foregoing method. The instrumentation coder effects step (d). Preferably, the instrumentation coder follows techniques of U.S. Pat. Nos. 5,539,907 and 5,732,273. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    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.  
         [0013]    [0013]FIG. 1 is a block diagram of a computer processor assembly embodying the present invention.  
         [0014]    FIGS.  2 A- 2 B are schematic views illustrating instrumentation of a procedure prologue.  
         [0015]    [0015]FIG. 3 illustrates a prologue implemented according to the invention.  
         [0016]    [0016]FIG. 4 is a block diagram outlining the steps of the invention method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Illustrated in FIG. 1 is a digital processor  11  holding the binary code of an application program  13  in working memory  15 . Typically, the binary code  13  comprises a plurality of modules. Each module is formed of several to thousands of lines of instructions. Shown is a procedure module  14  and a main body module  16  in which there is a call  19  to the procedure module  14 . During execution, one instruction after another in the main body  16  of the subject program  13  is processed. When the call instruction  19  is reached, the procedure module (indicated in the call instruction) is invoked.  
         [0018]    Invocation of the procedure module  14  involves initializing a stack frame  17  to store the calling context. That is, memory address of the calling instruction  19  is saved/ recorded on the stack frame  17 , as is the return address (i.e., memory location to which to continue processing after the called procedure module) and other information required to restore the procedure calling context. Lastly, initial values of the procedure module variables are assigned using parameters passed into the called procedure module  14  via the call instruction  19 . This sets the initial state of the procedure module  14 .  
         [0019]    The digital processor  11  then processes one instruction after the other in the procedure module  14 .  
         [0020]    In the case of an exception that is not processed in the immediate context of the procedure, the digital processor  11  reverse executes a current subject module one instruction at a time. This is referred to as “unwinding”. The stack frame is used in the exception handling of a procedure  14  to unwind to the point/state where the procedure module  14  was invoked and originally called. That is, the state data stored in the stack frame  17  is used to return variables to the state at which the procedure module  14  was invoked/called. The calling address is used to backtrack to the call instruction  19  in the main body  16  of the subject program  13  that called the procedure module  14 . Further unwinding of the procedure prologue associated with program body  16  may then be enabled.  
         [0021]    Being supported by stack frames  17 , pointers and similar stored memory address information, the backtracking or unwinding is fairly certain and stable. It is the altering of the program code  13  with additional adjustments to the stack frame pointer and other memory address changing instructions during instrumentation of the prior art that causes unwinding to fail. Thus, as previously mentioned, exception handling becomes problematic in instrumented programs of the prior art.  
         [0022]    Referring back to FIG. 1, an instrumentation assembly  31  of the present invention is also stored in processor memory. Instrumentation assembly  31  provides instrumentation of program  13  before and/or after (e.g., outside of) execution runs in working memory  15  as detailed later in FIG. 4.  
         [0023]    More specifically, the procedure module  14  is formed of a prologue portion  21  and main procedure body portion  23 , as illustrated in FIG. 2A. The prologue portion  21  contains the instructions for initializing the stack frame with calling context information and state data. As shown in FIG. 2B, during instrumentation (outside of execution), it is desirable to insert into the procedure prologue  21  a series of instructions that adjust the stack frame, save information onto the stack, branch to an analysis routine and then restore the stack frame pointer to its former position. Such a branch instruction and associated analysis routine enable monitoring of state values, counting of the number of times the procedure module has been invoked, and other measuring of program operation.  
         [0024]    When prior art instrumentation has inserted a branch (or other memory address changing) instruction  25  in the prologue  21 , the exception-handling unwinding process breaks down as follows. Invocation and initialization of the procedure  14 /prologue is as before. Processor  11  processes one instruction after another sequencing through prologue  21 . As shown in FIG. 2B, an “lda” instruction is processed, followed by a “stq” instruction, etc. If an exception is invoked in the body of the procedure that requires unwinding the procedure, then the processor  11  proceeds to unwind reverse execute and effectively undo each reversible instruction in the prologue  21 . That is, processor  11  reverse executes each of the store instructions  4  and  3 . Then processor  11  attempts to adjust the stack pointer at instruction i 4 . This and subsequent adjustments to the stack pointer (i 1  and regular prologue instruction  1 ) result in an over adjustment of the stack that breaks the unwinding mechanism.  
         [0025]    In the present invention, instrumentation of the prologue  21  is performed in a manner that preserves, in proper sequence, the instructions in the prologue needed for unwinding. This is accomplished as follows and illustrated in FIGS. 3 and 4. Referring to FIGS. 1 and 4, an API (application program interface)  27  provides an interactive user interface for instrumenting program  13 . In response to user request to insert instrumentation code into the prologue  21  of procedure module  14 , instrumentation assembly  31  locates the binary code (step  41  in FIG. 4) for the designated prologue  21  and makes a copy (step  43 ) into scratch memory  33 .  
         [0026]    With the scratch memory copy, the instrumentation assembly  31  assesses each instruction for unwinding purposes. That is, if a subject instruction in the scratch memory copy is not necessary for the unwinding process, then at step  45  the instrumentation assembly  31  deletes that instruction from the scratch memory  33  copy of the prologue. As such, the instrumentation assembly  31  sanitizes the scratch memory copy of prologue  21  and only the instructions necessary for restoring the calling context during unwinding handling remain in the prologue copy at scratch memory  33 .  
         [0027]    Next, for each instruction remaining in the resulting prologue copy (in scratch memory  33 ), the instrumentation assembly  31  generates a corresponding rewind or reverse effect of that instruction (step  47 ), if required. The generated rewind instructions are appended to the prologue copy in scratch memory  33 . The final contents of the scratch memory  33  are inserted before the original prologue  21  in the subject program binary code  13  in working memory  15  (FIG. 1).  
         [0028]    At step  49  the instrumentation assembly  31  redefines beginning and ending boundaries of the original prologue  21  to be set to (i.e., point to) the copy of the prologue that came from scratch memory  33 . The new rewind instructions from step  47  are seen as part of the main body  23  of the procedure  14  along with the original prologue instructions.  
         [0029]    Instrumentation of the original prologue  21  (step  51 ) is then enabled and effected in accordance with U.S. Pat. No&#39;s. 5,539,907 and 5,732,273, or the like.  
         [0030]    [0030]FIG. 3 illustrates prologue  21  instrumented according to the present invention. The duplicate and sanitized version of the prologue is at  37 , i.e., instructions at DPINS ( 1 ) . . . DPINS (n). The beginning point or entry point to the now instrumented procedure module  14   a  is at  29  and an ending point prologue and procedure main body is set at  39 . This effectively poses the DPINS( 1 ) . . . (n) instructions at  37  as the prologue of instrumented procedure  14   a . The rewind instructions from step  47  (FIG. 4) are at  35  and indicated as RWINS ( 1 ) . . . RWINS (n). The original prologue instructions (OPINS ( 1 ) . . . OPINS (n)) with desired instrumentation instructions (IINS( 1 ) . . . IINS(n)) inserted are at  21   a  in FIG. 3. The original procedure  14  is at  23 . The combination of code at  21   a ,  35  and  23  is as the body of main procedure body for instrumented procedure  14   a.    
         [0031]    That is, during execution, the processor  11  effectively executes the prologue instructions twice, the first time at  37  without instrumentation and the second time at  21   a  with instrumentation. The duplicate prologue instructions  37  are seen as the official prologue of the instrumented procedure  14   a  from the viewpoint of the procedure descriptor and the unwinding mechanism. The original prologue instructions at  21   a  are considered part of the procedure body along with the original procedure body  23 .  
         [0032]    With regard to execution and exception handling, the processor  11  processes DPINS instructions  37  one at a time as the prologue of the invoked procedure  14   a . Next, the processor  11  processes the main body of the procedure  14   a  starting with the RWINS instructions  35 . If an exception is invoked, the exception-handling rewinding begins at the set ending boundary  39  of what is seen as the official prologue  37  but is the duplicate or prologue copy from scratch memory  33  (FIG. 1). Where this version of the prologue  37  does not contain any instrumentation code and fully preserves the calling context, the unwinding mechanism succeeds in reverse executing the then apparent prologue  37 .  
         [0033]    To that end, the present invention enables the instrumentation of prologue code in a manner such that in cases where an exception occurs which causes the unwinding of the calling context to occur, the call chain is maintained.  
         [0034]    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.  
         [0035]    For example, instead of copying the prologue (or program module to be instrumented) into scratch memory  33 , prologue instructions may be copied directly into the subject binary program. Similarly, the rewind instructions may be inserted directly into the binary program instead of appended in scratch memory.  
         [0036]    Further, the foregoing has discussed instrumentation of a procedure prologue. Other program modules may be similarly instrumented using the above-described techniques/steps of the present invention.