Compilers

Fifteen C++ performance patterns from three sources — Bilokon & Gunduz’s HFT paper at Imperial, Jonathan Müller’s Cache-Friendly C++ deck at CppCon 2025, and Okade & Baker’s C++ Performance Tips at CppCon 2025 — implemented as single-file nanobench programs, run under Docker on GCC 14 with -O2 -std=c++23, every one of them linked to a working Godbolt session. The surprising half: three of the ’textbook’ speedups do not reproduce on a modern CPU. Cache warming is 7x slower than cold. Bitmask branch reduction is slower than the cascade it replaces. This post is the full runbook, numbers included, with the nuance that explains when each pattern actually earns its keep.

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.

Working through six C++ problems modeled on Optiver’s Senior SWE assessment: supermarket checkout simulation, order book matching, dividend pricing, lattice path DP, Dijkstra with K free edges, and a price-alert router built twice — AoS first, then SoA. Every bug, wrong turn, and data structure tradeoff included.

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.