clang

Q: How do I install clang?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill clang in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/clang/ (personal) or .claude/skills/clang/ (project).

by mohitmishra786

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.

Installs: 727
Last 24h: —
Trending rank: —
Security: Not audited

Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill clang

01
Install
Run the command in your project. The CLI asks which agents to install for.
02
Describe the task
Ask your agent for something this skill covers. It loads clang automatically when relevant.
03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

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

FAQ

What does the clang skill do?

How do I install clang?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill clang in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/clang/ (personal) or .claude/skills/clang/ (project).

Which AI agents can use clang?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is clang safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains clang?

It is published by mohitmishra786 in mohitmishra786/low-level-dev-skills. It has 727 recorded installs.