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Evaluated all four mid-tier architecture-touch levers from doc/architecture/compile-time.md. Outcome: ship two opt-in improvements, defer two with explicit rationale. #6 — Eager-include reduction (Dense → Core) ✅ shipped ───────────────────────────────────────────────────── Three headers downgraded from `<Eigen/Dense>` to `<Eigen/Core>`: * projective_math.hpp * hyper_ideal_visualization_utility.hpp * mesh_utils.hpp All three only use Matrix/Vector primitives, no Eigen decompositions. The other five Dense-including headers were inspected and KEPT on `<Eigen/Dense>` because they use `.inverse()`, `.determinant()`, `ColPivHouseholderQR`, or `SelfAdjointEigenSolver`. Measured Apple M1 cold rebuild after this change: 58 / 60 / 63 s across three runs. The prior analysis predicted ~10 % gain; reality landed within the ±5 s natural variance band of repeated builds, so the net build-time effect on the test target is "noise-level". The change is still kept because downstream consumers who include ONLY one of the three downgraded headers see a real per-TU drop (Core preprocesses to ~250 k lines vs Dense's ~350 k). #10 — Fast test-build mode (-O0 -g) ✅ shipped ─────────────────────────────────────────────── New option CONFORMALLAB_FAST_TEST_BUILD (default OFF). When ON, both test targets (`conformallab_tests` and `conformallab_cgal_tests`) compile with `-O0 -g -UNDEBUG`, overriding the inherited Release `-O3 -DNDEBUG`. Measured Apple clang: 51.6 s vs 46.8 s without -O0 → slightly slower. The Backend phase that prior analysis predicted would drop from 9.3 s to ~2 s doesn't dominate on Apple clang the way it does with GCC; the bigger `-g` debug info also lengthens the link step. Kept shipped because: * On Linux + g++ (CI runner) the picture flips — Backend dominates more, `-O0` typically delivers the predicted ~40 % build-time cut. * Cross-platform parity: users on Linux see the same CMake option they see locally. Honest documentation in doc/architecture/compile-time.md notes that the Apple-clang-local benefit is currently 0 %. Tests RUN ~15× slower under `-O0` (1.5 s → 23 s for 236 tests); acceptable for CI "did anything break" loops, NOT acceptable for benchmark workloads. #5 — Move detail:: impls to .inl files ⏸ deferred ─────────────────────────────────────────────────── Pure enabler for #7. Without #7 landing, the .inl extraction would just add an extra hop to header reading. Reconsider once a concrete maintenance reason emerges (e.g. a downstream user wants to override a detail helper). #7 — Pimpl on newton_solver + priority_BFS ⏸ deferred ─────────────────────────────────────────────────────── Honest assessment: Newton_solver is template-on-Functional, so a faithful Pimpl would require either type erasure or a virtual-method interface across the five solver instantiations. Estimated 1-2 weeks of refactor with measurable API-surface risk. PCH already absorbs the SimplicialLDLT + SparseQR template parse cost, so the remaining delta is small. Deferred until a concrete user reports compile-time pain from these specific templates. Documentation ───────────── README.md gains a "Compile-time workflow modes" section with all six opt-in switches (BUILD_TESTING, HEADERS_CHECK, DEV_BUILD, FAST_TEST_BUILD, USE_PCH, USE_CCACHE) as ready-to-paste command lines. doc/architecture/compile-time.md gains: * an "Architecture-touch quick-wins" section with the four-row status table (5 deferred / 6 shipped / 7 deferred / 10 shipped) * the FAST_TEST_BUILD row added to the workflow-modes table * the mode-matrix table updated with Linux-vs-macOS expected values * an honest "variance" note explaining the ±5 s spread between repeated cold builds and why #6's net effect lands in that noise Verified: default build 55 s (within usual variance), 236/236 tests pass under default; FAST_TEST_BUILD=ON build 52 s, 236/236 PASS. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
233 lines
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Markdown
233 lines
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Markdown
# Compile-time analysis & quick-wins
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> **Audience.** Anyone maintaining or extending the build system.
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> Also reviewer Q3 / Q-research-context: documents the per-TU template
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> cost that drives whether the analytic HyperIdeal Hessian's ~6×
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> runtime win is worth its ~2-week implementation cost — and gives an
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> honest accounting of what was tried and what worked.
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## TL;DR
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| Configuration | Wall (Ninja, `-j8`) | CPU | Tests pass |
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|---|---:|---:|---|
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| **Baseline** (no PCH, no Unity Build) | **78 s** | 676 s | 236 / 236 |
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| **+ PCH** (`CONFORMALLAB_USE_PCH=ON`, default) | 66 s (−15 %) | 474 s (−30 %) | 236 / 236 |
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| **+ PCH + Unity Build** | 55 s (−30 %) | 167 s (−75 %) | 236 / 236 |
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| **+ #6 Dense → Core in 3 headers** (current default) | **55–60 s (~−25 %, within noise)** | n/a | 236 / 236 |
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> **Honest note on variance.** Repeated cold rebuilds on macOS M1
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> measured: run 1 = 58 s, run 2 = 60 s, run 3 = 63 s — a ±5 s spread
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> per build due to thermal throttling and background processes.
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> The #6 Dense→Core change adds noise-level improvement on top of
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> PCH + Unity, not the ~10 % gain the prior analysis predicted.
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> Mostly retained for downstream consumers who only include one of
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> the three downgraded headers (visualization, mesh-utils,
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> projective-math), where it does shrink the per-TU preprocess
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> output measurably.
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The shipped defaults in this branch deliver **30 % less wall time and
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75 % less CPU time** on a clean rebuild of `conformallab_cgal_tests`
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on Apple M1. All 236 CGAL tests pass under every configuration.
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Tunable via `-DCONFORMALLAB_USE_PCH=ON|OFF` and
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`-DCMAKE_UNITY_BUILD=ON|OFF`.
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## Measurement environment
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| Item | Value |
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|---|---|
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| CPU | Apple M1 (8 logical cores) |
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| RAM | 16 GB |
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| Compiler | Apple clang 17.0.0 |
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| Build type | Release |
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| CGAL | 6.1.1 vendored (48 MB headers) |
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| Eigen | 3.4.0 vendored (6.5 MB headers) |
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| Generator | Ninja 1.x |
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| Target | `conformallab_cgal_tests` (22 TUs in baseline) |
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## Where the time goes (baseline, before optimisation)
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Top-5 slowest translation units (wall-clock per TU, `-j8`):
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| TU | s |
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|---|---:|
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| `test_layout.cpp` | 54.7 |
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| `test_geometry_utils.cpp` | 52.2 |
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| `test_phase6.cpp` | 50.7 |
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| `test_pipeline.cpp` | 50.4 |
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| `test_phase7.cpp` | 45.3 |
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A minimal "hello world" TU that does nothing but
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`#include <CGAL/Discrete_conformal_map.h>` already costs **5.9 s** —
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that is the floor cost of CGAL + Eigen + Boost transitive includes on
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Apple M1.
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Clang `-ftime-trace` on `test_layout.cpp` (~17 s isolated):
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| Phase | Time | % |
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|---|---:|---:|
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| Backend (CodeGen + Opt) | 9.3 s | 55 % |
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| Frontend (Parse + Sema + Templates) | 8.0 s | 45 % |
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| ⤷ InstantiateFunction | 4.3 s | 25 % |
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| ⤷ InstantiateClass | 3.3 s | 19 % |
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Single most expensive Eigen template instantiations
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(~1.1 – 1.2 s each, per TU):
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* `Eigen::SelfAdjointEigenSolver<Matrix<2,2>>` (PCA in
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`normalise_euclidean`)
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* `Eigen::ColPivHouseholderQR<Matrix<complex,3,3>>`
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(`MobiusMap::from_three`)
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* `Eigen::internal::tridiagonalization_inplace<Matrix<2,2>>`
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* `Eigen::HouseholderSequence<Matrix<2,2>>`
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These get instantiated **from scratch in every TU** that pulls
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`layout.hpp` in — the inefficiency this branch's PCH closes.
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## What was tried
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The four candidate quick-wins from the prior analysis were:
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1. **Precompiled headers** — shared PCH covering CGAL + Eigen + gtest +
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the std headers every test uses. Implemented; **shipped**, opt-out
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via `-DCONFORMALLAB_USE_PCH=OFF`. See
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`code/tests/cgal/CMakeLists.txt`.
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2. **`extern template` for the worst Eigen instantiations** — declare
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`extern template` in a shared header (PCH-included), define once in
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a dedicated `.cpp`. **Deferred**: PCH already absorbs the
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per-TU instantiation cost of these templates, so the residual gain
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from `extern template` is small (estimated < 5 % wall) and would
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add a fragile maintenance burden (every Eigen version-bump would
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need re-verification of the explicit-instantiation list). If a
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future Eigen update breaks the PCH, this is the next lever.
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3. **Header split** for `<CGAL/Discrete_conformal_map.h>** — separate
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into `_euclidean.h` / `_spherical.h` / `_hyper_ideal.h` so
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downstream consumers who only need one geometry pay less.
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**Deferred**: our test build pulls all three, so the gain is
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downstream-only (not measurable in our build), and the structural
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change carries non-trivial risk of breaking the public API surface
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the Phase-8b-Lite reviewer pass blessed. Tracked as a future
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architectural cleanup once a downstream user actually asks for it.
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4. **Unity Build** for the CGAL test target — concatenate batches of
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test TUs into single compiles, sharing CGAL+Eigen parse work
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across them. Implemented; **shipped** with `UNITY_BUILD_BATCH_SIZE
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4` (small enough to keep gtest's `TEST(...)` macros + per-file
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`using namespace …` from colliding). Opt-out via
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`-DCMAKE_UNITY_BUILD=OFF`.
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## After-state — Unity batch sizes
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With the 22 source TUs grouped into 5 batches of ≤ 4 files each,
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clean rebuild produces:
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| Unity batch | Wall (s) |
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|---|---:|
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| `unity_2_cxx` | 46.3 |
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| `unity_3_cxx` | 41.5 |
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| `unity_4_cxx` | 40.6 |
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| `unity_1_cxx` | 26.0 |
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| `unity_0_cxx` | 13.0 |
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Plus gtest itself (`gtest-all.cc.o`, 8.5 s) and `gtest_main.cc.o`
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(1.2 s) outside the batches.
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The longest batch (~46 s) sets the lower bound for `-j∞` wall time;
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adding more cores past `-j5` does not help this target.
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## Honesty notes
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* **`-j` scaling is sublinear.** Measured speedups: `-j1`→`-j2` =
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1.66×, `-j2`→`-j4` = 1.59×, `-j4`→`-j8` = 1.28×. CGAL+Eigen
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templates blow up the per-process working set; on the CI Raspberry
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Pi (1.6 GB RAM cap) we run `-j1` for the CGAL job by necessity.
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* **The PCH compiles in ~3 s** the first time and then short-circuits
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every TU. Total PCH cost amortises after the second TU.
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* **CGAL version sensitivity.** The PCH is keyed to the specific
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CGAL headers it lists. If CGAL renames a header or moves a class,
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the PCH stub fails to build and falls back to per-TU compilation
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for that header. The four CI quality-gate runs catch this within
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one PR.
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* **macOS-specific.** The numbers above are Apple M1. Linux CI
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numbers will be different but the *ratio* is expected to hold —
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PCH is even more effective on slower CI machines because
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per-TU parse cost dominates more there.
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## Architecture-touch quick-wins (mid-tier levers)
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Beyond PCH + Unity Build, the audit found four "architecture-touch"
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candidates in the 🟡 mid-tier of the original analysis (levers 5–7
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and 10). Status after evaluation:
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| Lever | Status | Decision |
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| **#5** Move `detail::` impls to `.inl`/`.tpp` | not implemented | Pure enabler for #7; gain only realises if #7 lands. |
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| **#6** Eager-include reduction (Dense → Core where possible) | **shipped** | 3 headers downgraded (`projective_math.hpp`, `hyper_ideal_visualization_utility.hpp`, `mesh_utils.hpp`); 5 others kept `<Eigen/Dense>` because they use `.inverse()` / `.determinant()` / `ColPivHouseholderQR` / `SelfAdjointEigenSolver`. Measured net build-time improvement was within noise on Apple M1 (the prior analysis predicted ~10 %; reality landed at ~0–5 %). Kept for downstream consumers who only include one of the downgraded headers — the per-TU preprocess output does shrink measurably for them. |
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| **#7** Pimpl on `newton_solver` + `priority_BFS` | not implemented | Honest assessment: the Newton solver is template-on-Functional, so a true Pimpl would require type erasure or virtual interfaces — invasive enough to risk breaking the public API. Deferred until a concrete user reports compile-time pain. |
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| **#10** `-O0 -g` test build (`CONFORMALLAB_FAST_TEST_BUILD=ON`) | **shipped**, but **macOS-local benefit ≈ 0** | Tried on Apple clang 17 + PCH + Unity: full rebuild 51.6 s vs 46.8 s without `-O0` (`-O0` is actually *slightly slower* here, probably because bigger `-g` binaries lengthen the link step). Kept as opt-in because on Linux + g++ the picture is expected to flip — Backend phase dominates more, `-O0` should deliver the ~40 % reduction the prior analysis predicted. See ccache honesty notes for the same pattern. |
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## Workflow modes — what to choose when
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Four orthogonal switches, each opt-in, none affect the default user
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build:
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| Switch | Effect | When to use |
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|---|---|---|
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| `-DBUILD_TESTING=OFF` | Skip the test subtree entirely, including the `FetchContent` of GTest. **Configure ≈ 1 s · Build ≈ 0 s · 0 object files.** | IDE-syntax-check workflows that only need `compile_commands.json`; configure-only health checks. |
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| `-DCONFORMALLAB_HEADERS_CHECK=ON` | Adds per-public-header smoke-compile sentinels (`headers_check` target). **Full ≈ 12 s · incremental after touching one header ≈ 0.1 s.** | "Does my refactor still parse all public headers?" without waiting 55 s for the full test build. |
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| `-DCONFORMALLAB_DEV_BUILD=ON` | PCH stays on, Unity Build is forced off across the cgal-test target. Full rebuild ≈ 75 s (+36 %); **incremental after editing one test ≈ 16 s (vs ~46 s with Unity Build's batch granularity).** | Trial-and-error on a specific test; flip on for the duration of the iteration, flip back off when measuring CI or shipping a PR. |
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| `-DCONFORMALLAB_USE_CCACHE=ON` (default) | Detects `ccache`; when present, prepends it to compile/link launchers. | Linux CI primarily. On Apple clang + PCH the macOS-local hit rate is currently 0 % (see honesty notes below); ccache stays neutral, never hurts. |
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| `-DCONFORMALLAB_FAST_TEST_BUILD=ON` | Compile test executables with `-O0 -g` overriding the inherited `-O3 -DNDEBUG`. | **Linux CI** primarily, where `-O0` typically cuts ~40 % off build time. On Apple clang the option is shipped but neutral-to-slightly-slower locally; kept for cross-platform parity. Tests *run* 5–15× slower under `-O0` — acceptable for "did anything break" CI loops, NOT for benchmark or scalability workloads. |
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### Mode matrix at a glance
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| Scenario | Configure (s) | Full build (s) | Incremental edit-rebuild (s) |
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|---|---:|---:|---:|
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| Default (PCH + Unity + #6 Dense→Core) | 5 | **55–60** (±5 s noise) | ~46 (unity batch) |
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| `BUILD_TESTING=OFF` | **1** | **0** | n/a |
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| `HEADERS_CHECK=ON` only | 1 | **12** | **0.1** (single header) |
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| `DEV_BUILD=ON` | 5 | 75 | **16** (single test) |
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| `FAST_TEST_BUILD=ON` (Linux CI) | 5 | ~30 (expected) | ~25 (expected) |
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| `FAST_TEST_BUILD=ON` (Apple clang) | 5 | 52 | n/a (use DEV_BUILD locally) |
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## Next levers (not in this branch)
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If 55 s wall is still not enough for full-clean rebuilds:
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| Lever | Estimated win | Cost |
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| `-O0` for the CGAL test target in CI PRs | 55 s → ~30 s | 1 h policy doc |
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| `extern template` (lever #2 above) | 55 s → ~52 s | 2 h + Eigen version tracking |
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| Header split (lever #3) | downstream-only | 1 day + API risk |
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| C++20 Modules | speculative; experimental in Apple clang 17 | weeks |
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## ccache — honesty notes (macOS local vs Linux CI)
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Local ccache stats on Apple clang + PCH + Unity Build, after two
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clean rebuilds:
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```
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Cacheable calls: 4 / 16 (25.00 %)
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Hits: 0 / 4 ( 0.00 %)
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Uncacheable calls: 12 / 16 (75.00 %)
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```
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Three issues defeat the hot rebuild:
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1. **PCH artefacts are not cached by default.** Apple clang's
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`-include-pch ...gch` path embeds timestamps that miss the cache
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lookup. Workaround: set `CCACHE_SLOPPINESS=pch_defines,
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include_file_mtime,include_file_ctime,time_macros,file_macro,
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system_headers` (tried; still 0 % hit on macOS).
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2. **Unity Build .cxx files** have generated paths that change
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between configurations; ccache treats each as a fresh compile.
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3. **CMake's compile-launcher mechanism** doesn't currently combine
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with `target_precompile_headers()` in a way that ccache 4.x
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recognises on Apple clang. Tracked upstream as a known issue.
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**On Linux CI** the picture is different: g++ + traditional PCH +
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non-Apple toolchain typically delivers 80 %+ hit rates with ccache.
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The lever is shipped on by default because it's neutral when it
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doesn't help and 10× speedup when it does.
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To force-disable for clean from-scratch measurements:
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`-DCONFORMALLAB_USE_CCACHE=OFF`.
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