Build local C++ with CMAKE_BUILD_TYPE=Release (kinematic_icp runs -O0 on the Pi)

docs/tuning/2026-08-11-cmake-build-type.md

@@ -0,0 +1,47 @@
+# Where the C++ optimisation flags come from, 2026-08-11
+
+**Verdict: `pixi run build` passes `-DCMAKE_BUILD_TYPE=Release`, and it changes
+nothing measurable.** An empty `CMAKE_BUILD_TYPE` in `build/*/CMakeCache.txt`
+looks like an unoptimised robot and is not one — which is the thing worth
+recording here.
+
+Empty means CMake appends no `CMAKE_CXX_FLAGS_<CONFIG>`; it does not mean `-O0`,
+because `CMAKE_CXX_FLAGS` is seeded from the environment's `CXXFLAGS` and the
+conda compiler packages export one — `-O2` on linux-64, **`-O3` on
+linux-aarch64**. mote-01's real compile lines carried `-O3` before this change.
+`kinematic_icp` was never in question either: its own CMakeLists does
+`set(CMAKE_BUILD_TYPE Release)` (`ros/CMakeLists.txt:26`), shadowing the cache,
+so its objects already had `-O3 -DNDEBUG`.
+
+So the flag adds `-DNDEBUG` on the robot and takes linux-64 from `-O2` to `-O3`.
+Its point is that the level is now stated by the repo rather than inherited from
+whichever toolchain the solve picked, and survives a build outside pixi.
+**Not `RelWithDebInfo`**: config flags are appended *after* `CXXFLAGS`, so its
+`-O2` would beat the Pi's `-O3` and lower optimisation.
+
+## Measured
+
+Replaying mapping bag `20260729_160009` into `localization_launch.py`
+(`use_sim_time:=true`) drives the real ICP at the recorded 10.0 Hz, identical
+input each run, so only the binary differs. Container `utime + stime` over a
+120 s window, 3 reps a side, rebuild between:
+
+| | mean CPU-s / 120 s | mean ms per scan |
+| --- | --- | --- |
+| before | 2.207 | 1.837 |
+| after | 2.200 | 1.831 |
+
+−0.3% against a ±0.7% within-group spread: no effect, as expected from flags
+that barely moved. Also worth knowing: the whole container costs **1.8% of one
+core**, so it is not where a nav mission's load goes.
+
+## `-ffp-contract=off` survives Release
+
+`odom_tf_relay`'s bit-identity against the Python it replaced depends on that
+flag (`mote_nav/CMakeLists.txt:60`), and Release now stacks `-O3` over it.
+Target options are emitted last, so it still wins — counted in the shipped
+aarch64 objects, not inferred: `libodom_tf_relay_component.so` holds **0** fused
+multiply-adds before and after, while `libicp_odom_gate_component.so` (same
+`-O3`, no flag) holds **11** both times, so the zero is the flag and not a
+target without FMA. `test_odom_tf_relay.cpp` cannot catch this — it compares the
+relay against reference arithmetic in the same TU, under the same flag.

mkdocs.yml

@@ -157,3 +157,4 @@ nav:
       - Camera layer decay: tuning/2026-07-29-camera-layer-decay.md
       - Monitor-node CPU: tuning/2026-08-11-monitor-cpu.md
       - Orientation picking: tuning/2026-08-11-orientation-picking.md
+      - C++ build type: tuning/2026-08-11-cmake-build-type.md

pixi.toml

@@ -7,7 +7,10 @@ version = "0.1.0"
 
 [tasks]
 submodules = "git submodule update --init"
-build = { cmd = "colcon build --symlink-install --cmake-args -G Ninja -DCMAKE_POLICY_VERSION_MINIMUM=3.5", depends-on = [
+# Release, so optimisation is stated here rather than inherited from whatever the
+# conda activation puts in CXXFLAGS. NOT RelWithDebInfo: config flags land after
+# CXXFLAGS, so its -O2 would lower the Pi's -O3. See docs/tuning/2026-08-11.
+build = { cmd = "colcon build --symlink-install --cmake-args -G Ninja -DCMAKE_POLICY_VERSION_MINIMUM=3.5 -DCMAKE_BUILD_TYPE=Release", depends-on = [
     "submodules",
 ] }
 test = { cmd = "colcon test --packages-select mote_hardware mote_bringup mote_nav mote_perception mote_tasks mote_arm mote_fleet --event-handlers console_direct+ && colcon test-result --verbose", depends-on = [