This post describes a proposal and is a call for clients, if you will. I have an idea for an optimization in the GCC MC/DC instrumentation (-fcondition-coverage) that I estimate would reduce the overhead in both compile time, object size, and run time, roughly 2-3 times, and I am looking for clients to fund this work.

GCC has supported MC/DC since version 14 (released May 2024) and works quite well. I recently fixed a performance bug that made a massive difference for (very) large expressions, which went unnoticed for a while because such large expressions are quite uncommon and the performance has been adequate. I do believe we can improve both the compile time and the quality of instrumentation further.

Graph improvements

I wanted zcov to quickly and naturally show the coverage status of a piece of code. Most of the time we’re interested in missing coverage as that is where we need to do work, either adding a test, improving the spec, or figuring out if there is a mismatch between the code as-is and then goals we’re trying to achieve. zcov can now colour code the blocks to show this, using a common red/yellow/green system. This works quite well; the red blocks immediately stand out. In fact, differently coloured blocks stand out, so if a function is mostly covered, missing coverage stands out, and vice versa.

GCC 16 will probably release in a couple of months, and comes with a couple of my patches. There’s nothing too big this time, but a couple of bug fixes and some quality-of-life changes.

You no longer need to explicitly pass -ftest-coverage for -fcondition-coverage and -fpath-coverage to be useful, it is now implied. The -ftest-coverage flag controls if GCC creates the .gcno files gcov needs to create the report. The coverage support in GCC is built on top of arc profiling which underpins profile guided optimization (PGO), and the PGO doesn’t need the .gcno, only the .gcda (counters). Coverage was a sort of side effect, and MC/DC and prime path coverage was built on that framework. Unlike arc profiling, it doesn’t make much sense to ask for MC/DC and prime path coverage without also wanting to read the reports, so this makes GCC a bit easier to use.