Abstract:
A method for analyzing program execution characteristics such as the minimum and maximum execution time, and maximum stack depth of a processor. The process examines every possible path through a program accumulating required information. The results guarantee best or worst case results since every path is examined. The time to produce these results is greatly reduced over other methods.

Description:
This application is a continuation of application Ser. No. 07/935,594, filed Aug. 26, 1992, now abandoned. 
    
    
     BACKGROUND 
     1. Field of Inventions 
     This invention relates to the analysis of execution time and stack depth for computers. 
     2. Description of Prior Art 
     It is beneficial in real-time programs to know execution characteristics such as the maximum execution time, minimum execution time, and worst case stack depth. This is one way to verify that a real-time program will finish all tasks within its time and memory constraints. 
     The flow of a simple program diverts from a linear flow of execution by the use of a conditional branch. A conditional branch has two paths of execution that it may take based on some logical test. The first direction is the path taken when the conditional branch falls through to the next instruction. The second direction is when the conditional branch takes the branch to another instruction that does not follow it. 
     A common method of measuring execution time and stack depth is to use a logic analyzer. This works well only if the program executions in a simple linear fashion. It becomes more complex if the timing depends upon calculations or inputs. The problem is to catch the program in its worst case path. There is no way of knowing what that path is. If the path is known then it is usually very difficult to have the program execute it. 
     The problem with this method and all other methods that measure the performance of a processor is that they can not guarantee worst case results. The execution time and stack depth are discovered using a trial-and-error approach. They hope that the random set of events that causes the worst case execution time and stack depth to occur will happen while being examined. 
     A program that only simulates a processor has a similar disadvantage. It does not try every possible combination of execution paths because that is impractical. A simulator may try a large number of paths, especially at the extremes, but that does not prove the worst case results will be found. 
     The number of paths in a program that uses conditional branches is between N+1 and 2 N  where N is the number of conditional branches. A small program with 20 conditional branches would have between 21 and 1048576 possible separate paths through the program. It clearly becomes unrealistic to use conventional means to look at all of these paths to determine the worst case, especially in programs of significant size. 
     OBJECTS AND ADVANTAGES 
     Accordingly, several objects and advantages of my invention are: 
     (a) to provide a means of determining a computer program&#39;s slowest execution time; 
     (b) to provide a means of determining a computer program&#39;s fastest execution time; 
     (c) to provide a means of determining a computer program&#39;s largest stack depth usage; and 
     (d) to provide this information in a deterministic way. 
     Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description. 
    
    
     DRAWING FIGURES 
     In the drawings, closely related figures have the same number but different alphabetic suffixes. 
     FIG. 1A to 1B show a simple program and a corresponding flowchart. 
     FIG. 2A to 2F show different views of a branch tree formed by the program in FIG. 1A. 
     FIG. 3 shows a flow chart detailing the process of obtaining execution time and stack information from a computer program. 
     
         ______________________________________Reference Numerals In Drawings______________________________________101  first conditional statement                    102    first part of code103  first end of conditional statement                    104    second conditional                           statement105  second part of code 106    second end of                           conditional state-                           ment201  first conditional node                    202    first conditional                           fall path203  first conditional take path                    204    second conditional                           node205  second conditional fall path                    206    second conditional                           take path301  variable initialization                    302    test for condition-                           al branch303  nominal instruction information                    304    test if condition-                           al branch data                           complete305  use previous information                    306    analysis of take                           path307  analysis of fall path                    308    complete condi-                           tional branch data309  add information to running totals                    300    test if analysis                           should stop311  increment instruction pointer______________________________________ 
    
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the present invention is now explained in detail with reference to the drawings. 
     FIG. 1A shows a simple program as an example. It contains a first conditional statement 101 and a second conditional statement 104. If first conditional statement 101 is true then execution falls through and a first part of code 102 executes; otherwise execution goes to a first end of conditional statement 103. If second conditional statement 104 is true then execution falls through and a second part of code 105 executes; otherwise execution goes to a second end of conditional statement 106. 
     FIG. 1B shows a flowchart of the program in FIG. 1A. The arrangement of conditional branches in the program in FIG. 1A produces the greatest number of possible paths that a program may have. This is calculated as 2 N  where N is the number of conditional branches. The number of possible paths through the program in FIG. 1A is four. 
     FIG. 2A shows a tree structure diagram of all possible paths through the program in FIG. 1A. The first conditional branch 101 is represented by a node 201. A line segment 202 represents the information obtained when execution falls through the first conditional branch 101 to the first part of the code 102. The information obtained when the conditional branch 101 is taken to the conditional statement end 103 is represented by a line segment 
     The second conditional branch 104 is represented by a node 204. A line segment 205 represents the information obtained when execution falls through the second conditional branch 104 to the second part of the code 105. The information obtained when the conditional branch 104 is taken to the conditional statement end 106 is represented by a line segment 206. 
     FIG. 3 shows a detailed flowchart for obtaining an analysis of execution information. A variable initialization 301 is performed to set a program counter. This program counter will contain the address of the first instruction to analyze. 
     A test for conditional branch 302 is performed on the current instruction. If the instruction is not a conditional branch instruction a normal information gathering 303 is done. This information is used by an add information to running totals 309 procedure. 
     If the opcode is a conditional branch instruction then a test if conditional branch data complete 304 is made. This is important because it allows the total number of paths searched to be greatly reduced. If the conditional branch instruction is completed a use previous information 305 is available. This contains analysis information relating to the conditional branch and everything following it. This is used to add information to running totals 309. 
     If the conditional branch is not complete then both of its paths must be analyzed. A analysis of take path 306 is arbitrarily shown first. One way to do this is by the use of a recursive call. The new starting address is the address to which the conditional branch will go on the take path. A analysis of fall path 307 is also done. A recursive call would set the new starting address to the address following the conditional branch. 
     Once both conditional branch paths are analyzed a complete conditional branch data 308 is done. This includes setting a flag and saving the information obtained about both paths associated with the conditional branch. The flag is use by the test if conditional branch data complete 304. The information is available for use previous information 305. The desired results of the analysis are used to add information to running totals 309. 
     A test if analysis should stop 310 is done. The current program counter is compared with the desired stop address. If the analysis is not done then an increment instruction pointer 311 is carried out and the algorithm starts over with test for conditional branch 302. 
     This process carried out on the program in FIG. 2A starts with FIG. 2B. The first conditional take path 203 is arbitrarily analyzed first. The second conditional take path 206 is also arbitrarily analyzed first. The analysis backs up in FIG. 2C and looks at the second conditional fall path 205. FIG. 2D shows that both paths of second conditional node 204 have been analyzed. The information associated with node 204 is saved and a flag set to indicate that the information is valid. 
     The analysis backs up in FIG. 2E and looks at the first conditional fall path 202. Node 204 is encountered again and the information for it is determined to be valid based on a flag associated with it. This information is used instead of repeating a complete analysis on both of its paths. FIG. 2F shows that both paths of first conditional node 201 have been analyzed. The information associated with node 201 is saved and a flag set to indicate that the information is valid. 
     SUMMARY, RAMIFICATIONS AND SCOPE 
     Thus the reader will see that the process of analyzing computer execution time and stack depth by this invention provides results in a deterministic and quick way. Furthermore, this method has the additional advantages in that 
     each and every path of the program is analyzed; 
     it finishes much faster than processing paths that have already been processed once; 
     the maximum and minimum execution time may be calculated; and 
     the maximum stack depth may be calculated. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, other program execution instruction characteristics such as maximum interrupt suppression, time between instructions, possible stack imbalances, and current stack depth may be determined; wait states, pre-fetches, and other variables may be accounted for; loops may be handled by following the same path numerous times; branches may be forced in one direction by following only one of the two paths, etc. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.