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Thursday, April 16, 2026

No imageProposal: faster and smaller MC/DCThis 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. I did some more experiments and compared the compile times for a couple of real-world programs. The baseline is built with no flags, coverage with -ftest-coverage -fprofile-arcs , and MC/DC with -ftest-coverage -fcondition-coverage on GCC 14.2. This table shows that compile times are very sensitive to the program structure and that MC/DC slows down compiles significantly, sometimes up to 3 times. All compile times are in seconds. Baseline Coverage MC/DC SQLite 2.3 3.4 5.2 TagLib 2.2 3.2 2.9 FreeType 1.3 1.7 2.5 JUCE 1.4 1.9 1.9 Object sizes too are greatly affected by instrumentation, albeit slightly less consistent, but does support that the generated instrumentation (and not the analysis) is the main driver for compile times. The object sizes are in megabytes (MB). Baseline Coverage MC/DC SQLite 1.4 2.5 3.9 TagLib 3.0 6.5 4.5 FreeType 1.0 2.3 3.8 JUCE 1.5 3.4 2.8 Faster compiles are important and goes beyond simple ergonomics. Increased latency in the edit-compile-test cycle is very detrimental to both efficiency and effectiveness; reduced latency empowers engineers to solve harder problems and, in my experience, in a better way. The faster compile does not just save time (which is already precious and expensive), but reduces context switches and makes for a stronger feedback loop. The importance of fast and solid feedback loops is well understood, and has been covered by countless books, articles, blog posts, and talks. I would argue it is vital to optimize the edit-compile-test cycle when working with MC/DC since reaching (and understanding) coverage often is the most expensive and time consuming phase of system development. There’s efficiency to be gained, too, just by reducing the time wasted waiting for the compiler to finish. When developing the test suite for MC/DC the program will be recompiled many times which will effectively be idle, so just a few seconds here and there add up fast. The improvements in size- and runtime overhead means even larger programs can fit on small embedded system, which is particularly relevant the context of MC/DC as safety critical systems often run on small computers, ECUs, microcontrollers. This goes beyond just edit-compile-test friction; if instrumented programs cannot run on the device it is not just a mere inconvenience, it is a barrier to progress. The compile time and size tests show that the overhead of coverage instrumentation greatly depends on the structure of the program. Note that compilation is a complicated process and the 2-3x reduction in overhead is an estimate. To make this improvement in GCC happen, send an email to j@patch.no and I will send you a formal proposal and discuss the terms. After we sign the contract I will implement these optimizations and take care of upstreaming and integrating the changes into GCC.📝patch – Blog

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Wednesday, April 15, 2026

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Saturday, April 11, 2026

Preventing Integer Overflow in Physical ComputationsPreventing Integer Overflow in Physical Computations Integers overflow. That is not a controversial statement. What is surprising is how easily overflow can hide behind the abstraction of a units library. Most developers immediately think of explicit or implicit scaling operations — calling .in(unit) to convert a quantity, constructing a quantity from a different unit, or assigning between quantities with different units. These are indeed places where overflow can occur, and the library cannot prevent it at compile time when the values are only known at runtime. But at least these operations are visible in your code : you wrote the conversion, you asked for the scaling, and you can reason about whether the multiplication or division might overflow your integer type. The far more insidious problem is what happens when you don't ask for a conversion. When you write 1 * m + 1 * ft , the library must automatically convert both operands to a common unit before performing the addition. That conversion — which you never explicitly requested — involves multiplication or division by scaling factors. With integer representations, those scaling operations can overflow silently, producing garbage results that propagate through your calculations undetected. No compile-time programming can prevent this. The values are only known at runtime. But very few libraries provide proper tools to detect it. This article explains why that limitation is real, how other libraries have tried to work around it, and what mp-units provides to close the gap as tightly as the language allows.📝mp-units

Friday, April 10, 2026

Building C/C++ libraries for HarmonyOS with vcpkgWe're currently working on porting Qt to HarmonyOS . For our CI and developer machines, we need a number of third-party libraries built for HarmonyOS. Cross-compiling open-source C and C++ libraries for this platform has been a manual, error-prone process. Each library has its own build system, whether CMake, Autotools, or Meson. Each needs individual attention to produce correct binaries for the OHOS target. We have been maintaining a hand-written shell script that builds libraries one by one, with per-library workarounds for cross-compilation quirks. With our vcpkg fork, that script is now a single command.📝Qt Blog