Force-cover

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Getting accurate test coverage information about C++ code containing templates is challenging; uninstantiated templates don’t make it into the compiled binary, so compilers don’t instrument them for coverage tracking (i.e. if you never use a template the compiler thinks it isn’t runnable code and doesn’t count it as lines that should be covered). Since templates with no test coverage are likely to never get instantiated this results in overly accurate test coverage metrics.

Force-cover is a set of tools for dealing with this problem. It consists of two parts:

Requirements

Theoretically force-cover should work on any operating system, but it’s currently only been tested on Ubuntu and Linux Mint.

Installation

You can install the requirements on Ubuntu-flavored Linux with:

sudo apt install -y clang llvm-dev libclang-dev

You can build force-cover by cloning this repo and running Make inside it:

git clone https://github.com/emilydolson/force-cover.git
cd force-cover
make

This will create the force_cover executable. No additional work is needed to set up the Python script.

Troubleshooting

If you have multiple versions of clang or llvm on your computer, the Make command may fail. You may be able to fix this by changing the default version as described at the bottom of this page. Alternatively, you can modify the Makefile to include absolute paths to the installation location. Set LLVM_SRC_PATH equal to the path to your llvm installation location (e.g. /usr/lib/llvm-11). Uncomment the LLVM_CONFIG := $(LLVM_BIN_PATH)/llvm-config line and comment out the line above it.

Alternately, save yourself a trip through install hell by using a containerized environment a la Singularity! Build from our handy-dandy Singularity recipe (sudo singularity build force-cover.simg Singularity) or grab a pre-built container from SingularityHub (singularity pull --name "force-cover.simg" shub://emilydolson/force-cover). Then, hop on to an interactive shell by singularity shell force-cover.simg. Cowabunga!

Quick-start guide

Here is the basic sequence of commands you need to execute to use force-cover with LLVM Source-Based coverage (the recommended approach):

./force_cover [C++ code file to be evaluated] -- [any flags you would pass to the compiler when compiling this program] > [name of file to store modified code in]
clang++ -fprofile-instr-generate -fcoverage-mapping -O0 -fno-inline -fno-elide-constructors [.cpp file] -o [executable name]
[run executable]
llvm-profdata merge default.profraw -o default.profdata
llvm-cov show [executable name] -instr-profile=default.profdata > coverage.txt
python fix_coverage.py coverage.txt

Example (using included example.cc file):

./force_cover examples/example.cc -- --language c++ -std=c++11 > examples/example_with_template_coverage_info.cc
clang++ -fprofile-instr-generate -fcoverage-mapping -O0 -fno-inline -fno-elide-constructors examples/example_with_template_coverage_info.cc -o example
./example
llvm-profdata merge default.profraw -o default.profdata
llvm-cov show ./example -instr-profile=default.profdata > coverage.txt
python fix_coverage.py coverage.txt

Using force-cover (in detail)

The workflow for using force-cover is as follows:

In theory, this should be possible with a variety of compilers and code coverage programs. Thus far, I have only tested it with LLVM Source Based coverage. If you have tested it and found that it worked with a different toolchain, let me know so I can add it to this documentation!

Step 1: Run force_cover on your code

The syntax for running the force_cover C++ program is:

./force_cover [C++ code file to be evaluated] -- [any flags you would pass to the compiler when compiling this program]

For instance, to run it on the example you could use:

./force_cover examples/example.cc -- --language c++ -std=c++11

By default, it prints the modified version of the code to stdout. In order to compile programs using the modified code, you’ll need to pipe this new code to a file. For instance:

./force_cover examples/example.cc -- --language c++ -std=c++11 > examples/example_with_template_coverage_info.cc

For larger code-bases, one option is to make a copy of your code, rewrite all of the files in the copy, and use those files to compile your tests. This can be achieved with a few lines of bash code. For instance, let’s say you’re writing a header-only library and all of the headers live in a directory called source. You could run the following code:

cp -r source coverage_source
for filename in `find ../coverage_source -name "*.h"`
do
    ./force_cover $filename -- -I../coverage_source --language c++ -std=c++14 | xargs -0 echo > $filename.temp
    mv $filename.temp $filename
done

Then when you go to compile your tests for coverage, instead of including source you would include coverage_source (i.e. replace -Isource with -Icoverage_source).

If you are running tests on a continuous integration platform you may choose to skip the step of copying the code to a different directory. Just be aware that this is dangerous because it will overwrite your code.

Step 2: Compile your program

In order to get coverage information, you need to compile your program with coverage instrumentation turned on. This can be achieved by passing a few flags to the compiler. In LLVM, there are a number of different systems of coverage instrumentation. The one I have had by far the most luck with is Source Based coverage, which can be enabled with the -fprofile-instr-generate and -fcoverage-mapping flags. The other version, which mirrors GCC’s gcov system, sometimes optimizes unused class methods out of the binary, preventing them from getting appropriately flagged as not covered.

Some other useful flags to prevent the compiler from making optimizations that hide uncovered code are: -O0 -fno-inline -fno-elide-constructors.

So your compilation step will probably look something like:

clang++ -fprofile-instr-generate -fcoverage-mapping -O0 -fno-inline -fno-elide-constructors examples/example_with_template_coverage_info.cc -o example

Note that Source Based coverage is only available in clang. Theoretically, the tools in this repo should work on code instrumented in other ways but, as mentioned before, it hasn’t been tested on them.

Step 3: Run your program

The most straightforward step! Run your program so that the coverage instrumentation can record which lines were executed.

For instance:

./example

Step 4: Extract coverage information

Now that you’ve run your program, coverage data exists but it’s probably not in an easy-to-interpret form. You’ll have to run a program to extract it. For LLVM Source Based coverage, that will look like:

llvm-profdata merge default.profraw -o default.profdata
llvm-cov show ./example -instr-profile=default.profdata > coverage.txt

This processes the raw coverage data and then compares that information to the executable to generate a report indicating the number of time each line was executed. Specifically, the format should look like this:

[line_number] |     [times_line_executed]|  [code from source file]

Whatever compiler and tools you used, you need to end up with data in this format for step 5 to work. Fortunately, it seems to be a relatively common format (Note: if anyone knows the actual name of this format, send me a PR! I wrote this tool because I needed it and thought others might too, not because I’m some kind of code coverage expert).

Step 5: Run fix_coverage.py

For the final step, run fix_coverage.py on your output file from the previous step. Note that this will overwrite your output file.

python fix_coverage.py coverage.txt

This script will go through and find all of the regions that are erroneously being excluded from coverage analysis and modify the coverage file to indicate that they should be covered but are not.

Step 6: Profit!

Ta-da! You have code coverage data that includes uninstantiated templates! You can look at the file directly, or pass it along to a service like codecov that will give you a more user-friendly way to examine your coverage (codecov’s documentation on using llvm-cov isn’t super clear, but it will accept files in this format with names matching the pattern coverage*.txt).

Caveats

Code coverage is a flawed metric. Just because a line of code is executed doesn’t mean it’s being rigorously tested. This is especially true for templates, since different instantiations of the same template could be wildly different from each other. That’s the whole reason uninstantiated templates don’t get included in the binary in the first place: template definitions only have a meaning with an appropriate set of arguments. Force-cover can increase the accuracy of your code coverage and alert you to uninstantiated templates, but it can’t guarantee that your tests are actually good.

Bugs? Contributions?

Open an issue or send me a PR! I’m not an expert on this stuff, so I’m sure there are myriad ways force-cover could be better. I welcome all contributions. The code is pretty succinct, so hopefully it’s not too overwhelming to wade into.

In particular I would love to receive: