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)

4. 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 <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. For clang-tidy config examples, see 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