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Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]10 <title>JaCoCo - Control Flow Analysis</title>
11</head>
12<body onload="prettyPrint()">
13
14<div class="breadcrumb">
15 <a href="../index.html" class="el_report">JaCoCo</a> &gt;
16 <a href="index.html" class="el_group">Documentation</a> &gt;
17 <span class="el_source">Control Flow Analysis</span>
18</div>
19<div id="content">
20
21<h1>Control Flow Analysis for Java Methods</h1>
22
23<p class="hint">
24 Implementing a coverage tool that supports statement (C0) as well as branch
25 coverage coverage (C1) requires detailed analysis of the internal control flow
26 of Java methods. Due to the architecture of JaCoCo this analysis happens on
27 the bytecode of compiled class files. This document describes JaCoCo's
28 strategies for inserting probes into the control flow at runtime and analyzing
29 the actual code coverage. Marc R. Hoffmann, November 2011
30</p>
31
32<h2>Control Flow Graphs for Java Bytecode</h2>
33
34<p>
35 As an starting point we take the following example method that contains a
36 single branching point:
37</p>
38
39<pre class="source lang-java linenums">
40public static void example() {
41 a();
42 if (cond()) {
43 b();
44 } else {
45 c();
46 }
47 d();
48}
49</pre>
50
51<p>
52 A Java compiler will create the following bytecode from this example method.
53 Java bytecode is a linear sequence of instructions. Control flow is
54 implemented with <i>jump</i> instructions like the conditional
55 <code>IFEQ</code> or the unconditional <code>GOTO</code> opcode. The jump
56 targets are technically relative offsets to the target instruction. For better
57 readability we use symbolic labels (<code>L1</code>, <code>L2</code>) instead
58 (also the ASM API uses such symbolic labels):
59</p>
60
61<pre class="source linenums">
62public static example()V
63 INVOKESTATIC a()V
64 INVOKESTATIC cond()Z
65 IFEQ L1
66 INVOKESTATIC b()V
67 GOTO L2
68 L1: INVOKESTATIC c()V
69 L2: INVOKESTATIC d()V
70 RETURN
71</pre>
72
73<p>
74 The possible control flow in the bytecode above can be represented by a graph.
75 The nodes are byte code instruction, the edged of the graph represent the
76 possible control flow between the instructions. The control flow of the
77 example is shown in the left box of this diagram:
78</p>
79
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame]80<img src="resources/flow-example.png" alt="Bytecode Control Flow"/>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]81
82
83<h3>Flow Edges</h3>
84
85<p>
86 The control flow graph of a Java method defined by Java byte code may have
87 the following Edges. Each edge connects a source instruction with a target
88 instruction. In some cases the source instruction or the target instruction
89 does not exist (virtual edges for method entry and exit) or cannot be
90 exactly specified (exception handlers).
91</p>
92
93<table class="coverage">
94 <thead>
95 <tr>
96 <td>Type</td>
97 <td>Source</td>
98 <td>Target</td>
99 <td>Remarks</td>
100 </tr>
101 </thead>
102 <tbody>
103 <tr>
104 <td>ENTRY</td>
105 <td>-</td>
106 <td>First instruction in method</td>
107 <td></td>
108 </tr>
109 <tr>
110 <td>SEQUENCE</td>
111 <td>Instruction, except <code>GOTO</code>, <code>xRETURN</code>,
112 <code>THROW</code>, <code>TABLESWITCH</code> and <code>LOOKUPSWITCH</code></td>
113 <td>Subsequent instruction</td>
114 <td></td>
115 </tr>
116 <tr>
117 <td>JUMP</td>
118 <td><code>GOTO</code>, <code>IFx</code>, <code>TABLESWITCH</code> or
119 <code>LOOKUPSWITCH</code> instruction</td>
120 <td>Target instruction</td>
121 <td><code>TABLESWITCH</code> and <code>LOOKUPSWITCH</code> will define
122 multiple edges.</td>
123 </tr>
124 <tr>
125 <td>EXHANDLER</td>
126 <td>Any instruction in handler scope</td>
127 <td>Target instruction</td>
128 <td></td>
129 </tr>
130 <tr>
131 <td>EXIT</td>
132 <td><code>xRETURN</code> or <code>THROW</code> instruction</td>
133 <td>-</td>
134 <td></td>
135 </tr>
136 <tr>
137 <td>EXEXIT</td>
138 <td>Any instruction</td>
139 <td>-</td>
140 <td>Unhandled exception.</td>
141 </tr>
142 </tbody>
143</table>
144
145<p>
146 The current JaCoCo implementation ignores edges caused by implicit exceptions
147 and the the method entry. This means we consider SEQUENCE, JUMP, EXIT.
148</p>
149
150
151<h2>Probe Insertion Strategy</h2>
152
153<p>
154 Probes are additional instructions that can be inserted between existing
155 instructions. They do not change the behavior of the method but record the
156 fact that they have been executed. One can think probes are placed on edges of
157 the control flow graph. Theoretically we could insert a probe at every edge of
158 the control flow graph. As a probe implementation itself requires multiple
159 bytecode instructions this would increase the size of the class files several
160 times and significantly slow down execution speed of the instrumented classes.
161 Fortunately this is not required, in fact we only need a few probes per method
162 depending on the control flow of the method. For example a method without any
163 branches requires a single probe only. The reason for this is that starting
164 from a certain probe we can back-trace the execution path and typically get
165 coverage information for multiple instructions.
166</p>
167
168<p>
169 If a probe has been executed we know that the corresponding edge has been
170 visited. From this edge we can conclude to other preceding nodes and edges:
171</p>
172
173<ul>
174 <li>If a edge has been visited, we know that the source node of the this edge
175 has been executed.</li>
176 <li>If a node has been executed and the node is the target of only one edge
177 we know that this edge has been visited.</li>
178</ul>
179
180<p>
181 Recursively applying these rules allows to determine the execution status of
182 all instructions of a method &ndash; given that we have probes at the right
183 positions. Therefore JaCoCo inserts probes
184</p>
185
186<ul>
187 <li>at every method exit (return or throws) and</li>
188 <li>at every edge where the target instruction is the target of more than one
189 edge.</li>
190</ul>
191
192<p>
193 We recall that a probe is simply a small sequence of additional instructions
194 that needs to be inserted at a control flow edge. The following table
195 illustrates how this extra instructions are added in case of different edge
196 types.
197</p>
198
199<table class="coverage">
200 <thead>
201 <tr>
202 <td>Type</td>
203 <td>Before</td>
204 <td>After</td>
205 <td>Remarks</td>
206 </tr>
207 </thead>
208 <tbody>
209 <tr>
210 <td>SEQUENCE</td>
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame]211 <td><img src="resources/flow-sequence.png" alt="Sequence"/></td>
212 <td><img src="resources/flow-sequence-probe.png" alt="Sequence with Probe"/></td>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]213 <td>
214 In case of a simple sequence the probe is simply inserted between the
215 two instructions.
216 </td>
217 </tr>
218 <tr>
219 <td>JUMP (unconditional)</td>
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame]220 <td><img src="resources/flow-goto.png" alt="Unconditional Jump"/></td>
221 <td><img src="resources/flow-goto-probe.png" alt="Unconditional Jump with Probe"/></td>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]222 <td>
223 As an unconditional jump is executed in any case, we can also insert the
224 probe just before the GOTO instruction.
225 </td>
226 </tr>
227 <tr>
228 <td>JUMP (conditional)</td>
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame]229 <td><img src="resources/flow-cond.png" alt="Conditional Jump"/></td>
230 <td><img src="resources/flow-cond-probe.png" alt="Conditional Jump with Probe"/></td>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]231 <td>
Marc R. Hoffmann99f74092012-06-21 18:46:52 +0000[diff] [blame]232 Adding a probe to an conditional jump is little bit more tricky. We
233 invert the semantic of the opcode and add the probe right after the
234 conditional jump. With a subsequent <code>GOTO</code> instruction we
235 jump to the original target. Note that this approach will not introduce
236 a backward jump, which would cause trouble with the Java verifier if we
237 have an uninitialized object on the stack.
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]238 </td>
239 </tr>
240 <tr>
241 <td>EXIT</td>
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame]242 <td><img src="resources/flow-exit.png" alt="Exit"/></td>
243 <td><img src="resources/flow-exit-probe.png" alt="Exit with Probe"/></td>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]244 <td>
245 As is is the nature of RETURN and THROW statements to actually leave the
246 method we add the probe right before these statements.
247 </td>
248 </tr>
249 </tbody>
250</table>
251
252<p>
253 Now let's see how this rules apply to the example snippet above. We see that
254 <code>INVOKE d()</code> instruction is the only node with more than one
255 incoming edge. So we need to place probes on those edges and another probe on
256 the only exit node. The result is shown the the right box of the diagram
257 above.
258</p>
259
Marc R. Hoffmann87311452015-02-16 09:09:25 +0100[diff] [blame]260<h2>Additional Probes Between Lines</h2>
261
262<p>
263 The probe insertion strategy described so far does not consider implicit
264 exceptions thrown for example from invoked methods. If the control flow
265 between two probes is interrupted by a exception not explicitly created with
266 a <code>throw</code> statement all instruction in between are considered as
267 not covered. This leads to unexpected results especially when the the block of
268 instructions spans multiple lines of source code.
269</p>
270
271<p>
272 Therefore JaCoCo adds an additional probe between the instructions of two
273 lines whenever the subsequent line contains at least one method invocation.
274 This limits the effect of implicit exceptions from method invocations to
275 single lines of source. The approach only works for class files compiled with
276 debug information (line numbers) and does not consider implicit exceptions
277 from other instructions than method invocations (e.g.
278 <code>NullPointerException</code> or <code>ArrayIndexOutOfBoundsException</code>).
279</p>
280
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]281<h2>Probe Implementation</h2>
282
283<p>
284 Code coverage analysis is a runtime metric that provides execution details
285 of the software under test. This requires detailed recording about the
286 instructions (instruction coverage) that have been executed. For branch
287 coverage also the outcome of decisions has to be recorded. In any case
288 execution data is collected by so called probes:
289</p>
290
291<p class="hint">
292 A <b>probe</b> is a sequence of bytecode instructions that can be inserted
293 into a Java method. When the probe is executed, this fact is recorded and can
294 be reported by the coverage runtime. The probe must not change the behavior
295 of the original code.
296</p>
297
298<p>
299 The only purpose of the probe is to record that it has been executed at least
300 once. The probe does not record the number of times it has been called or
301 collect any timing information. The latter is out of scope for code coverage
302 analysis and more in the objective of a performance analysis tool. Typically
303 multiple probes needs to be inserted into each method, therefore probes needs
304 to be identified. Also the probe implementation and the storage mechanism it
305 depends on needs to be thread safe as multi-threaded execution is a common
306 scenario for java applications (albeit not for plain unit tests). Probes must
307 not have any side effects on the original code of the method. Also they should
308 add minimal overhead.
309</p>
310
311<p>
312 So to summarize the requirements for execution probes:
313</p>
314
315<ul>
316 <li>Record execution</li>
317 <li>Identification for different probes</li>
318 <li>Thread safe</li>
319 <li>No side effects on application code</li>
320 <li>Minimal runtime overhead</li>
321</ul>
322
323<p>
324 JaCoCo implements probes with a <code>boolean[]</code> array instance per
325 class. Each probe corresponds to a entry in this array. Whenever the probe is
326 executed the entry is set to <code>true</code> with the following four
327 bytecode instructions:
328</p>
329
330<pre class="source">
331ALOAD probearray
332xPUSH probeid
333ICONST_1
334BASTORE
335</pre>
336
337<p>
338 Note that this probe code is thread safe, does not modify the operand stack
339 or modify local variables and is also thread safe. It does also not leave the
340 method though an external call. The only prerequisite is that the probe array
341 is available as a local variable. For this at the beginning of each method
342 additional instrumentation code needs to be added to obtain the array instance
343 associated with the belonging class. To avoid code duplication the
344 initialization is delegated to a static private method
345 <code>$jacocoinit()</code> which is added to every non-interface class.
346</p>
347
348<p>
349 The size of the probe code above depends on the position of the probe array
350 variable and the value of the probe identifier as different opcodes can be
351 used. As calculated in the table below the overhead per probe ranges between 4
352 and 7 bytes of additional bytecode:
353</p>
354
355<table class="coverage">
356 <thead>
357 <tr>
358 <td>Possible Opcodes</td>
359 <td>Min. Size [bytes]</td>
360 <td>Max. Size [bytes]</td>
361 </tr>
362 </thead>
363 <tfoot>
364 <tr>
365 <td>Total:</td>
366 <td>4</td>
367 <td>7</td>
368 </tr>
369 </tfoot>
370 <tbody>
371 <tr>
372 <td><code>ALOAD_x</code>, <code>ALOAD</code> <sup>1</sup></td>
373 <td>1</td>
374 <td>2</td>
375 </tr>
376 <tr>
377 <td><code>ICONST_x</code>, <code>BIPUSH</code>, <code>SIPUSH</code>, <code>LDC</code>, <code>LDC_W</code> <sup>2</sup></td>
378 <td>1</td>
379 <td>3</td>
380 </tr>
381 <tr>
382 <td><code>ICONST_1</code></td>
383 <td>1</td>
384 <td>1</td>
385 </tr>
386 <tr>
387 <td><code>BASTORE</code></td>
388 <td>1</td>
389 <td>1</td>
390 </tr>
391 </tbody>
392</table>
393
394<p>
395 <sup>1</sup> The probe array is the first variable after the arguments.
396 If the method arguments do not consume more that 3 slots the 1-byte opcode can
397 be used.<br/>
398 <sup>2</sup> 1-byte opcodes for ids 0 to 5, 2-byte opcode for ids up to 127,
399 3-byte opcode for ids up to 32767. Ids values of 32768 or more require an
400 additional constant pool entry. For normal class files it is very unlikely to
401 require more than 32,000 probes.
402</p>
403
404<h2>Performance</h2>
405
406<p>
407 The control flow analysis and probe insertion strategy described in this
408 document allows to efficiently record instruction and branch coverage. In
409 total classes instrumented with JaCoCo increase their size by about 30%. Due
410 to the fact that probe execution does not require any method calls, only local
411 instructions, the observed execution time overhead for instrumented
412 applications typically is less than 10%.
413</p>
414
415<h2>References</h2>
416
417<ul>
418 <li><a href="http://asm.objectweb.org/">ASM byte code library</a> by Eric Bruneton at al.</li>
419 <li><a href="http://andrei.gmxhome.de/bytecode/index.html">Bytecode Outline Plug-In</a> by Andrei Loskutov</li>
420 <li><a href="http://en.wikipedia.org/wiki/Glossary_of_graph_theory">Wikipedia: Glossary of Graph Theory</a></li>
421</ul>
422
423</div>
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