# clang

Guides coding agents through Clang and LLVM compiler flags, static analysis, and ThinLTO. Useful for optimizing and debugging C/C++ builds.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 722 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/compilers/clang/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/clang

## What it does

This skill equips agents with detailed workflows for `clang` and `clang++`. It directs flag choices for diagnostics, inline fix-it hints, and template error trees. Agents can quickly pick flags like `-Oz` for code size or `-ferror-limit` to curb noise.

It outlines workflows for optimization remarks, ThinLTO linking, and instrumentation-based PGO via `llvm-profdata`. Remarks reveal why specific loops were not vectorized or functions skipped for inlining. Note that switching to `-fuse-ld=lld` will fail if the `lld` package is missing.

The guide also details `clang-tidy` checks and macOS Apple LLVM nuances.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill clang -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/clang/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/clang/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill clang -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/clang/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill clang -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/clang/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills clang
```

2. **Manually: put the files in place**
- File: `.github/skills/clang/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill clang -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/compilers/clang
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/clang/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill clang -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/compilers/clang/SKILL.md

## When to use it
- Diagnose why loops fail to vectorize using Clang optimization remarks
- Configure ThinLTO builds with the lld linker
- Run clang-tidy checks with automated code fixes
- Set up profile-guided optimization pipelines using llvm-profdata

## Example prompts
- Explain why loop vectorization failed in matrix.c using Clang remarks
- Configure a build with ThinLTO and the lld linker
- Run clang-tidy on src/main.cpp with bugprone checks and auto-fix

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 366307e5f5b4a231.

BEGIN THIRD-PARTY FILE 366307e5f5b4a231
````text
---
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
````
END THIRD-PARTY FILE 366307e5f5b4a231

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First 3184 of 5520 bytes. © mohitmishra786 · MIT · https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/compilers/clang/SKILL.md

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