4.6 Explanation of source coverage

This section will explain why the source reports can sometimes look like it would be incorrect and why the reports might differ from reports created by other tools, such as gcov.

Simics code coverage will rely only on what the debug information provides and does not have any extra information about if a source line is considered executable, contains branches, etc. Only lines which have instructions associated with them are considered executable, this means for example that variable declarations will not be marked as executable and that only one line of a multi-line expression might be marked as executable.

Sometimes the source report might look strange, especially when optimization and inlining is involved. As an example, see figure 22, which is a simple example that shows how a uncovered line can occur between two covered lines, without any branches involved. Similar cases usually occur when running coverage on a Linux kernel.

One would likely expect that line 50 would be green if both line 49 and line 51 are green as there is no branch involved. But this does not have to be the case and the report is actually correct.

This function is defined in a header file and will be inlined into two other functions, named one and another. Line 48 will be optimized out when inlining into the another function, while line 50 will be optimized out for the one function. Line 49 and 51 will be included in both functions. In the example only the one function has been executed so lines included in that will be green, while lines only included in the another function will be red. If a shared line has been executed by any function it will be marked as green.

The output in figure 23 is the same collected coverage as above, but with the -show-line-functions flag passed to the <code_coverage>.html-report command. This will show which functions make use of each line. This reveals what was explained above, that line 50 is only included by the another function and therefore it is red as the another function has not executed. Line 48 is only included by the one function, but that function has executed so that line is green. Both lines 49 and 51 are used by both functions so they will be green as one of the functions has executed.