wiki:Developer/Coverage/Theory

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Added example explanations

Coverage Analysis Theory

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The subject of Code Coverage Analysis is broad and has been written about many times over. This background material is not intented to summerize or rehash what can be read elsewhere. Instead, the focus here will be on the aspects of Code Coverage Analysis as they pertain to the RTEMS Coverage Analysis effort.

The ultimate goal of Code Coverage Analysis is to ensure that a test suite adequately tests a particular body of code. In order to achieve this goal, several different coverage criteria may have to be examined. Let's consider the following criteria:

  • Statement Coverage - Has each line of the source code been executed?
  • Decision Coverage (also known as Branch coverage) - Has each control structure (such as an if statement) evaluated both to true and false?
  • Condition Coverage - Has each boolean sub-expression evaluated both to true and false (this does not necessarily imply decision coverage)?
  • Modified Condition/Decision? Coverage (MC/DC) - Has every condition in a decision taken on all possible outcomes at least once? Has each condition been shown to affect that decision outcome independently?
  • Object Coverage - Has each line of generated assembly been executed?

In order to illustrate what is covered by each of the different criteria, consider the following example showing the source code for a simple if statement along with its generated pseudo-code instructions.

{| border="1" style="margin: 1em auto 1em auto;text-align: left;" |+

|Block | A
Source Code Block Object Pseudo-code
if (x OR y) A1 cmp x, 0

branch if FALSE to do something

|
A2 cmp y, 0

branch if TRUE around do something

| B
do something B do something instructions

|}

Statement Coverage

Statement Coverage requires that each line of source code must be executed. In our example, a single test case that allows the if statement to evaluate to TRUE will achieve 100% Statement Coverage.

Decision Coverage

Decision Coverage requires that each control structure evaluate to both TRUE and FALSE. In our example, two test cases are required to achieve 100% Decision Coverage. One case must allow the if statement to evaluate to TRUE and the other case must allow the if statement to evaluate to FALSE.

MC/DC

Modified Condition/Decision Coverage requires that every condition in a decision taken on all possible outcomes at least once? Has each condition been shown to affect that decision outcome independently?

Object Coverage

Object Coverage requires that each line of generated assembly been executed. In our example, only a test case where x is FALSE and y is TRUE will achieve 100% Object Coverage.

Criteria Relationships

Imported from old wiki.]]

Each of these criteria can be used independently to analyze the code in question. Application of any one criteria will likely improve the test suite to some degree albeit at some cost. Examination of the criteria collectively, shows that there are clear relationships between the different criteria as shown in the picture. The completness of the test suite increases as it satifies first Statement Coverage and then Decision Coverage and finally MC/DC. If the test suite satisfies Statement Coverage, it will partially satisfy Decision Coverage and MC/DC. If the test suite satisfies Decision Coverage, it will completely satisfy Statement Coverage and partially satisfy MC/DC. There is also a complexity relationship where Statement Coverage is the least complex to satisfy and MC/DC is the most complex to satisfy.

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