/fast-rust

Compilation Optimization

Goals

• Reduce edit-build-test latency in day-to-day development.
• Improve release performance with safe, measurable build changes.

Measure First

• Use cargo build --timings to visualize compile time hot spots.
• For Rust 1.59 or earlier: cargo +nightly build -Ztimings.
• Inspect the HTML Gantt chart to decide on crate splitting or codegen-units tuning.

Dependency Hygiene

• Remove unused dependencies; cargo-udeps is accurate but slow, cargo-machete is fast but heuristic.
• Sparse index is the default for crates.io in modern Cargo; only configure it explicitly for older toolchains or when forcing git.

Build Configuration

• Keep toolchains current; upstream improvements often reduce compile time.
• Prefer CI caches that are reliable (e.g., sccache); incremental builds often hurt clean CI due to extra IO and dependency tracking.
• Use a fast linker (for example, mold) for large dependency graphs:

[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]

• On macOS, set split-debuginfo = "unpacked" to skip dsymutil.
• For release builds, codegen-units = 1 improves optimization; override per crate if codegen is too slow:

[profile.release.package]
arrow2 = { codegen-units = 4 }

Workspace Structure and Tests

• Split oversized crates to improve parallelism and reduce cross-dependencies.
• Organize integration tests under tests/it/ to avoid many small binaries.
• Consider disabling doc tests if they dominate compile time, but weigh the API visibility cost.
• Prefer golden-file or logic-test frameworks for large query/result suites.

Minimal PGO Workflow

1. Install LLVM tools: rustup component add llvm-tools-preview (ensure llvm-profdata is on PATH).
2. Clean old data: rm -rf /tmp/pgo-data.
3. Use release builds for both instrumentation and final compile.
4. Build instrumented binaries:

RUSTFLAGS="-Cprofile-generate=/tmp/pgo-data" cargo build --release --target=x86_64-unknown-linux-gnu

5. Run a representative workload to generate .profraw files.
6. Merge profiles and rebuild:

llvm-profdata merge -o /tmp/pgo-data/merged.profdata /tmp/pgo-data
RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata -Cllvm-args=-pgo-warn-missing-function" \
  cargo build --release --target=x86_64-unknown-linux-gnu

Keep the workload realistic; PGO quality depends on the fidelity of the profile data.