Clang Skill: Оптимизация и анализ C/C++ проектов

--- name: clang description: Clang/LLVM compiler skill for C/C++ projects. Use when working with clang or clang++ for diagnostics, sanitizer instrumentation, optimization remarks, static analysis with clang-tidy, LTO via lld, or when migrating from GCC to Clang. Activates on queries about clang flags, clang-tidy, clang-format, better error messages, Apple/FreeBSD toolchains, or LLVM-specific optimizations. Covers flag selection, diagnostic tuning, and integration with LLVM tooling. --- # Clang ## Purpose Guide agents through Clang-specific features: superior diagnostics, sanitizer integration, optimization remarks, static analysis, and LLVM tooling. Covers divergences from GCC and Apple/FreeBSD specifics. ## Triggers - "I want better compiler diagnostics/errors" - "How do I use clang-tidy / clang-format?" - "How do I see what the compiler optimised or didn't?" - "I'm on macOS / FreeBSD using clang" - "clang-cl for MSVC-compatible builds" — see `skills/compilers/msvc-cl` - Sanitizer queries — see `skills/runtimes/sanitizers` ## Workflow ### 1. Build mode flags (identical to GCC) Clang accepts most GCC flags. Key differences: | Feature | GCC | Clang | |---------|-----|-------| | Min size | `-Os` | `-Os` or `-Oz` (more aggressive) | | Optimise only hot | — | `-fprofile-instr-use` (LLVM PGO) | | Thin LTO | `-flto` | `-flto=thin` (faster) | | Static analyser | `-fanalyzer` | `clang --analyze` or `clang-tidy` | ### 2. Clang-specific diagnostic flags ```bash # Show fix-it hints inline clang -Wall -Wextra --show-fixits src.c # Limit error count clang -ferror-limit=5 src.c # Verbose template errors (disable elision) clang -fno-elide-type src.cpp # Show tree diff for template mismatch clang -fdiagnostics-show-template-tree src.cpp ``` Clang's diagnostics include exact range highlighting and fix-it suggestions that GCC lacks. ### 3. Optimization remarks Optimization remarks let you see what Clang did or refused to do: ```bash # Inliner decisions clang -O2 -Rpass=inline src.c # Missed vectorisation clang -O2 -Rpass-missed=loop-vectorize src.c # Why a loop was not vectorized clang -O2 -Rpass-analysis=loop-vectorize src.c # Save all remarks to YAML for post-processing clang -O2 -fsave-optimization-record src.c # Produces src.opt.yaml ``` Interpret remarks: - `remark: foo inlined into bar` — inlining happened; good for hot paths - `remark: loop not vectorized: loop control flow is not understood` — restructure the loop - `remark: not vectorized: cannot prove it is safe to reorder...` — add `__restrict__` or `#pragma clang loop vectorize(assume_safety)` ### 4. Static analysis ```bash # Built-in analyser (CSA) clang --analyze -Xanalyzer -analyzer-output=text src.c # clang-tidy (separate tool, richer checks) clang-tidy src.c -- -std=c++17 -I/usr/include # Enable specific check families clang-tidy -checks='clang-analyzer-*,modernize-*,bugprone-*' src.cpp -- # Apply fixits automatically clang-tidy -fix src.cpp -- ``` Common `clang-tidy` check families: - `bugprone-*`: real bugs (use-after-move, dangling, etc.) - `clang-analyzer-*`: CSA checks (memory, null deref) - `modernize-*`: C++11/14/17 modernisation - `performance-*`: unnecessary copies, move candidates - `readability-*`: naming, complexity ### 5. LTO with lld ```bash # Full LTO clang -O2 -flto -fuse-ld=lld src.c -o prog # Thin LTO (faster link, nearly same quality) clang -O2 -flto=thin -fuse-ld=lld src.c -o prog # Check lld is available clang -fuse-ld=lld -Wl,--version 2>&1 | head -1 ``` For large projects, ThinLTO is preferred: link times 5-10x faster than full LTO with comparable code quality. ### 6. PGO (LLVM instrumentation) ```bash # Step 1: instrument clang -O2 -fprofile-instr-generate prog.c -o prog_inst # Step 2: run with representative input ./prog_inst < workload.input # Generates default.profraw # Step 3: merge profiles llvm-profdata merge -output=prog.profdata default.profraw # Step 4: use profile clang -O2 -fprofile-instr-use=prog.profdata prog.c -o prog ``` AutoFDO (sampling-based, less intrusive): collect with `perf`, convert with `create_llvm_prof`, use with `-fprofile-sample-use`. See `skills/profilers/linux-perf`. ### 7. GCC compatibility Clang is intentionally GCC-compatible for driver flags. Key differences: - Clang does not support all GCC-specific attributes; check with `__has_attribute(foo)` - `-Weverything` enables all Clang warnings (no GCC equivalent); too noisy for production, useful for one-off audits - Some GCC intrinsics need `#include <x86intrin.h>` on Clang too - `__int128` is supported; `__float128` requires `-lquadmath` on some targets ### 8. macOS specifics On macOS, `clang` is the system compiler (Apple LLVM). Key points: - `ld64` is the default linker; `lld` requires explicit `-fuse-ld=lld` and Homebrew LLVM - Use `-mmacosx-version-min=X.Y` to set deployment target - Sanitizers on macOS use `DYLD_INSERT_LIBRARIES`; do not strip the binary - `xcrun clang` resolves to the Xcode toolchain clang For flag reference, see [references/flags.md](references/flags.md). For clang-tidy config examples, see [references/clang-tidy.md](references/clang-tidy.md). ## Related skills - Use `skills/compilers/gcc` for GCC-equivalent flag mapping - Use `skills/runtimes/sanitizers` for `-fsanitize=*` workflows - Use `skills/compilers/llvm` for IR-level work (`opt`, `llc`, `llvm-dis`) - Use `skills/compilers/msvc-cl` for `clang-cl` on Windows - Use `skills/binaries/linkers-lto` for linker-level LTO details