CS 110: Code Analysis

1. CS 110: Code Analysis C. M. Overstreet Old Dominion University Fall 2007

2. Overview • Why code analysis? • Examples: • some easy • some hard • some impossible • Will give a few examples of where used

3. Example 1: elimination of infinite loops • Infinite loops are usually undesirable. • But not always. Why? • Can you write a program that checks another program for infinite loops? • That works some of the time? • That finds all infinite loops • Might be a useful compiler feature • Initial idea: just consider while loops: • Check loop body to see if any action modifies some loop control variables. • Does this work? • If no change, is loop infinite? • Can one always do this? How? What about input data?

4. Examples while ( x < Max ) { if ( x > 10 ) x--; y = Max + x; } • Is this an infinite loop? • How do you know? while ( x < MaxVal ) { ftn1(); y = anotherFunction( a, b, c ); j = x + 1; } • Is this an infinite loop? • How do you know?

5. Example 1 (cont.) • Point: often partial solutions are useful even if a general solution does not exist. • Question: What is the Halting Problem and why is it important?

6. Static & Dynamic Analysis • From their beginning, compilers have tried to optimize code. • Done primarily by static analysis • Figure things out by analyzing source code • Some useful information about code can only be obtained by running to code and monitoring its behaviors. • This is dynamic analysis

7. Possible Common Compiler Optimizations • Flag unused variables • Very common • Make loops more efficient by reordering code • Very common • Eliminating code that cannot affect program output • Generally not done; too hard! • Worked on a project for Navy a few years ago in which program manager suspected ~80% of code was useless • About 2,000,000 lines of source code

8. Examples of analysis tools • gcov: Tool to identify which statements have been executed while testing a program • gprof: Tool to tell a programmer which parts of a program use the most execution time. Useful when program performance is crucial. • What are memory leaks?

9. Example 3: uninitialized variables • Uninitialized variables are usually undesirable. • Should compilers make them illegal? • Can compilers find all of them? • Can you write a program which detects uninitialized variables? • Always? Sometimes? • Using static analysis? • i.e., uninitialized vars detected at compile time • Using dynamic analysis? • i.e. uninitialized vars detected at run time

10. Another example: side effects of function calls • Consider the statement: x = f(&a,b) + a + g(&a,c); • Can be a dangerous statement • OK if you always run the code using the same compiler • And the compiler never gets updated • Careful programmers avoid this type of coding. • Why? • Fix?

11. Another example • Consider the program #include <iostream> using namespace std; int main() { int a[5], i; a[0] = 2; a[1] = 4; a[2] = 8; a[3] = 32; a[4] = 64; i = 2; i = a[i++]; cout << i << endl; return 0; } • What’s the difference in “++i” and “i++”? • What’s the value of i? • Run on three compilers (Microsoft, gnu, Sun) • Got three different answers

12. 1 void main() 2 { int x, y, total, sum, z; 3 cin >> x >> y; 4 total = 0.0; 5 sum = 0.0; 6 if ( x < 1 ) 7 sum = y; 8 else { 9 cin >> z; 10 total = x*y; 11 } 12 cout << total << sum << endl; 13 } slice criterion: <13, {z}> 1, 2, 3, 6, 8, 9, 11, 13 slice criterion: <10, {x}> 1, 2, 3, 13 slice criterion: <13, {total}> 1, 2, 3, 4, 6, 8, 10, 11, 13 Weiser: Program Slices