CS 110: Code Analysis

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CS 110 Code Analysis

• C. M. Overstreet
• Old Dominion University
• Fall 2005

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Overview

• Some motivation for code analysis
• Some examples some easy, some hard, some
  impossible
• Some examples of where used

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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 all of the time?
• 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?
• What about for loops?

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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?

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Static Dynamic Analysis

• From their beginning, compilers have tried to
  optimize code.
• Done by primarily by static analysis
• Figure things out by analyzing source code
• Many types of useful information about code
  itself can only be obtained by running to code
  and monitoring its behaviors.
• This is dynamic analysis

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Possible Common Compiler Optimizations

• Flag unused variables
• Very common
• Make loops more efficient by moving code around
• Very common
• Eliminating code that cannot affect program
  output
• 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 1,000,000 lines of source code

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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
  improving program speed.
• What are memory leaks?

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Example 3 uninitialized variables

• Uninitialized variables are usually undesirable.
• Should compilers make them illegal?
• Can compilers make them illegal?
• 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

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Another example side effects of function calls

• Consider the statement
• x f(a,b) a g(a,c)
• Considered to be a dangerous statement.
  "Careful" programmers avoid this type of coding.
• Why?

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Weiser Program Slices

• slice criterion lt13, zgt
• 1, 2, 3, 6, 8, 9, 11, 13
• slice criterion lt10, xgt
• 1, 2, 3, 13
• slice criterion lt13, totalgt
• 1, 2, 3, 4, 6, 8, 10, 11, 13

• 1 void main()
• 2 int x, y, total, sum, z
• 3 cin gtgt x gtgt y
• 4 total 0.0
• 5 sum 0.0
• 6 if ( x lt 1 )
• 7 sum y
• 8 else
• 9 cin gtgt z
• 10 total xy
• 11
• 12 cout ltlt total ltlt sum ltlt endl
• 13