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### 11.1 Introduction to `gcov-tool`

`gcov-tool` is an offline tool to process gcc’s gcda profile files.

Current gcov-tool supports the following functionalities:

- merge two sets of profiles with weights.
- read one set of profile and rewrite profile contents. One can scale or normalize the count values.

Examples of the use cases for this tool are:

- Collect the profiles for different set of inputs, and use this tool to merge them. One can specify the weight to factor in the relative importance of each input.
- Rewrite the profile after removing a subset of the gcda files, while maintaining the consistency of the summary and the histogram.
- It can also be used to debug or libgcov code as the tools shares the majority code as the runtime library.

Note that for the merging operation, this profile generated offline may contain slight different values from the online merged profile. Here are a list of typical differences:

- histogram difference: This offline tool recomputes the histogram after merging the counters. The resulting histogram, therefore, is precise. The online merging does not have this capability – the histogram is merged from two histograms and the result is an approximation.
- summary checksum difference: Summary checksum uses a CRC32 operation. The value depends on the link list order of gcov-info objects. This order is different in gcov-tool from that in the online merge. It’s expected to have different summary checksums. It does not really matter as the compiler does not use this checksum anywhere.
- value profile counter values difference: Some counter values for value profile are runtime dependent, like heap addresses. It’s normal to see some difference in these kind of counters.

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