As we head into the holidays, the C++ community gets a nice surprise: Matt Godbolt, long-time CppCon speaker and creator of Compiler Explorer, has announced “Advent of Compiler Optimisations 2025.” From December 1 to 25, he’ll post a new example each day showing how modern C and C++ compilers transform code.

This series is useful because it makes compiler behavior easier to see. Matt shows real code and the resulting assembly, which helps you understand what the compiler improves and when those improvements don’t apply. It’s also a good follow-up to his 2025 keynote “C++: Some Assembly Required”. And, honestly, it’s just fun to have a daily technical treat during December.

As we wrap up 2025, it’s easy to lose sight of the “invisible helpers” that make our code fast, safe, and efficient. Matt’s Advent calendar is a lovely reminder — and a gift — that compilers are not magic, but they are powerful tools when we understand them. We hope you’ll join us each day in December, pull back the curtain, and learn something new about what’s really happening under the hood.

Happy holidays, and happy optimizing!