Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 1 | <?xml version="1.0" encoding="ISO-8859-1" ?>
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Marc R. Hoffmann | d7d2f75 | 2010-05-06 21:12:31 +0000 | [diff] [blame] | 7 | <link rel="shortcut icon" href=".resources/report.gif" type="image/gif" />
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 8 | <title>JaCoCo - Control Flow Analysis</title>
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| 9 | </head>
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| 10 | <body>
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| 11 |
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| 12 | <div class="breadcrumb">
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Marc R. Hoffmann | d7d2f75 | 2010-05-06 21:12:31 +0000 | [diff] [blame] | 13 | <a href="../index.html" class="el_report">JaCoCo</a> >
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 14 | <a href="index.html" class="el_group">Documentation</a> >
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| 15 | <span class="el_source">Control Flow Analysis</span>
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| 16 | </div>
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| 17 | <div id="content">
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| 18 |
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| 19 | <h1>Control Flow Analysis for Java Methods</h1>
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| 20 |
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| 21 | <p style="font-weight:bold;">
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| 22 | DRAFT - This document does not reflect the current JaCoCo implementation.
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| 23 | </p>
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| 24 |
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| 25 | <p class="hint">
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| 26 | Implementing a coverage tool for branch coverage requires detailed analysis
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| 27 | of the internal control flow of Java methods. Due to the architecture of
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 28 | JaCoCo this analysis needs to happen on compiled class files (bytecode).
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| 29 | This document defines graph structures for control flow analysis of Java
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| 30 | bytecode and discusses strategies for probe insertion.
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| 31 | Marc R. Hoffmann, April 2010
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 32 | </p>
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| 33 |
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| 34 | <h2>Motivation and Requirements</h2>
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| 35 |
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| 36 | <ul>
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| 37 | <li>Path Coverage</li>
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| 38 | <li>Exception Detection</li>
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| 39 | </ul>
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| 40 |
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| 41 | <h2>The Control Flow Graph</h2>
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| 42 |
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| 43 | <ul>
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| 44 | <li>Virtual Entry and Exit Nodes</li>
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| 45 | <li>Subsequent Instructions</li>
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| 46 | <li>(Conditional) Jump</li>
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| 47 | <li>Table/Lookup Switch</li>
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| 48 | <li>Exception Handlers</li>
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| 49 | <li>Unhandled Exceptions</li>
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| 50 | </ul>
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| 51 |
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| 52 | <h2>Probe Insertion</h2>
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| 53 |
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| 54 | <p>
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| 55 | Code coverage analysis is a runtime metric that provides execution details
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| 56 | of the software under test. This requires detailed recording about the
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| 57 | instructions (instruction coverage) that have been executed. For path coverage
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| 58 | also the outcome of decisions has to be recorded. In any case execution data
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| 59 | is collected by so called probes:
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| 60 | </p>
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| 61 |
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| 62 | <p class="hint">
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 63 | A <b>probe</b> is a sequence of bytecode instructions that can be inserted
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 64 | into a Java method. When the probe is executed, this fact is recorded and can
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| 65 | be reported by the coverage runtime.
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| 66 | </p>
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| 67 |
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| 68 | <p>
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| 69 | The only purpose of the probe is to record that it has been executed at least
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| 70 | once. The probe does not record the number of times it has been called or
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| 71 | collect any timing information. The latter is out of scope for code coverage
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| 72 | analysis and more in the objective of a performance analysis tool. Typically
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| 73 | multiple probes needs to be inserted into each method, therefore probes needs
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| 74 | to be identified. Also the probe implementation and the storage mechanism it
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| 75 | depends on needs to be thread safe as multi-threaded execution is a common
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| 76 | scenario for java applications (albeit not for plain unit tests). Probes must
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| 77 | not have any side effects on the original code of the method. Also they should
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| 78 | add minimal overhead.
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| 79 | </p>
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| 80 |
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| 81 | <p>
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| 82 | So to summarize the requirements for execution probes:
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| 83 | </p>
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| 84 |
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| 85 | <ul>
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| 86 | <li>Record execution</li>
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| 87 | <li>Identification for different probes</li>
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| 88 | <li>Thread safe</li>
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| 89 | <li>No side effects on application code</li>
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| 90 | <li>Minimal runtime overhead</li>
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| 91 | </ul>
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| 92 |
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| 93 | <p>
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| 94 | JaCoCo implements probes with a <code>boolean[]</code> array instance per
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| 95 | class. Each probe corresponds to a entry in this array. Whenever the probe is
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 96 | executed the entry is set to <code>true</code> with the following four
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| 97 | bytecode instructions:
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 98 | </p>
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| 99 |
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| 100 | <pre class="source">
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| 101 | ALOAD probearray
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| 102 | xPUSH probeid
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| 103 | ICONST_1
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| 104 | BASTORE
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| 105 | </pre>
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| 106 |
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| 107 | <p>
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| 108 | Note that this probe code is thread safe, does not modify the operand stack
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| 109 | or modify local variables and is also thread safe. It does also not leave the
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| 110 | method though an external call. The only prerequisite is that the probe array
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| 111 | is available as a local variable. For this at the beginning of each method
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| 112 | additional instrumentation code needs to be added to obtain the array instance
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Marc R. Hoffmann | 9772829 | 2010-04-27 20:29:23 +0000 | [diff] [blame] | 113 | associated with the belonging class. To avoid code duplication the
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| 114 | initialization is delegated to a static private method
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| 115 | <code>$jacocoinit()</code> which is added to every non-interface class.
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 116 | </p>
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| 117 |
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| 118 | <p>
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| 119 | The size of the probe code above depends on the position of the probe array
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| 120 | variable and the value of the probe identifier as different opcodes can be
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| 121 | used. As calculated in the table below the overhead per probe ranges between 4
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| 122 | and 7 bytes of additional bytecode:
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| 123 | </p>
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| 124 |
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| 125 | <table class="coverage">
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| 126 | <thead>
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| 127 | <tr>
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| 128 | <td>Possible Opcodes</td>
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| 129 | <td>Min. Size [bytes]</td>
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| 130 | <td>Max. Size [bytes]</td>
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 131 | </tr>
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| 132 | </thead>
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 133 | <tfoot>
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| 134 | <tr>
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| 135 | <td>Total:</td>
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| 136 | <td>4</td>
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| 137 | <td>7</td>
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 138 | </tr>
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| 139 | </tfoot>
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Marc R. Hoffmann | 9772829 | 2010-04-27 20:29:23 +0000 | [diff] [blame] | 140 | <tbody>
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| 141 | <tr>
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| 142 | <td><code>ALOAD_x</code>, <code>ALOAD</code> <sup>1</sup></td>
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| 143 | <td>1</td>
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| 144 | <td>2</td>
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| 145 | </tr>
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| 146 | <tr>
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| 147 | <td><code>ICONST_x</code>, <code>BIPUSH</code>, <code>SIPUSH</code>, <code>LDC</code>, <code>LDC_W</code> <sup>2</sup></td>
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| 148 | <td>1</td>
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| 149 | <td>3</td>
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| 150 | </tr>
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| 151 | <tr>
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| 152 | <td><code>ICONST_1</code></td>
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| 153 | <td>1</td>
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| 154 | <td>1</td>
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| 155 | </tr>
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| 156 | <tr>
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| 157 | <td><code>BASTORE</code></td>
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| 158 | <td>1</td>
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| 159 | <td>1</td>
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| 160 | </tr>
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| 161 | </tbody>
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Marc R. Hoffmann | 102e837 | 2010-04-27 15:46:23 +0000 | [diff] [blame] | 162 | </table>
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| 163 |
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| 164 | <p>
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Marc R. Hoffmann | 9772829 | 2010-04-27 20:29:23 +0000 | [diff] [blame] | 165 | <sup>1</sup> The probe array is the first variable after the arguments.
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| 166 | If the method arguments do not consume more that 3 slots the 1-byte opcode can
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| 167 | be used.<br/>
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| 168 | <sup>2</sup> 1-byte opcodes for ids 0 to 5, 2-byte opcode for ids up to 127,
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| 169 | 3-byte opcode for ids up to 32767. Ids values of 32768 or more require an
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| 170 | additional constant pool entry. For normal class files it is very unlikely to
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| 171 | require more than 32,000 probes.
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 172 | </p>
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| 173 |
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| 174 | <ul>
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Marc R. Hoffmann | 096bd1d | 2010-04-27 04:21:39 +0000 | [diff] [blame] | 175 | <li>Limitation: Only proves that the probe itself has been executed,
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| 176 | assumptions about the surrounding application code is interpolation</li>
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| 177 | <li>Probe in every edge of the control flow graph</li>
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| 178 | <li>Every exit path known (branch coverage)</li>
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| 179 | <li>Block entry known (exceptions within blocks)</li>
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| 180 | </ul>
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| 181 |
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| 182 | <h2>Different Types of Edges</h2>
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| 183 |
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| 184 | <ul>
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| 185 | <li>Probe insertion strategies</li>
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| 186 | </ul>
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| 187 |
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| 188 | <h2>Runtime Overhead</h2>
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| 189 |
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| 190 | <ul>
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| 191 | <li>Comparison to current basic block probes</li>
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| 192 | </ul>
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| 193 |
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| 194 | </div>
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| 195 | <div class="footer">
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| 196 | <div class="versioninfo"><a href="@jacoco.home.url@">JaCoCo</a> @qualified.bundle.version@</div>
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| 197 | <a href="license.html">Copyright</a> © @copyright.years@ Mountainminds GmbH & Co. KG and Contributors
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| 198 | </div>
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| 199 |
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