GCC

Follow-up to the 15-pattern HFT post. The first post asked ‘which patterns work?’ This post answers ‘how do you force your team to keep using them?’ Seven new nanobench programs, each with a working Godbolt link, covering __builtin_popcountll (24x faster than a bit-count loop), __builtin_unreachable in switch defaults (1.55x), __builtin_bswap64 (same speed as the portable idiom — GCC already folds it), [[likely]] and [[unlikely]] (no measurable effect on tight loops — an honest null result), and a flags matrix showing -ffast-math + -march=x86-64-v3 giving 6.9x over -O2. Then a .clang-tidy config that fails CI on every common perf regression, with the performance-for-range-copy warning demonstrated at 41x real runtime cost.

A day-long battle fixing a CI pipeline that builds Docker images on a self-hosted Jetson Xavier runner. From stale git submodules to GCC-15 assembler errors, expired tokens, DNS failures inside Docker, and deleted upstream branches — every layer of the stack had something broken.

A performance optimization in Eigen’s fill path assumed all scalar types are equal. GCC’s libstdc++ disagreed — and AutoDiffScalar paid the price.

How a GCC 13 false-positive -Warray-bounds warning in Eigen’s TensorContraction led to a lesson about if constexpr, C++14 portability, and the right way to suppress compiler warnings in a large codebase.