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

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:

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)

1. Static analysis

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

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)

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

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 . For clang-tidy config examples, see references/clang-tidy.md .