# llvm

A skill for working with LLVM IR: emitting intermediate representation, running passes with opt, lowering to assembly with llc, and diagnosing missed optimisations.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 899 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/llvm/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/llvm

## What it does

The skill walks an agent through LLVM as a user, not as a compiler author. It covers emitting textual IR from a C file, running a single pass or a full pipeline such as `default<O2>`, and printing IR before and after a transformation. Lowering to assembly goes through `llc`, with flags for the target CPU and instruction set.

Inside are a seven-step workflow and reference tables. One table breaks down the core IR constructs: `alloca`, `load`/`store`, `getelementptr`, `phi`, `icmp`/`fcmp`, `bitcast`. Another lists passes and what they do: `mem2reg` promotes `alloca` to SSA registers, `instcombine` handles peephole work, `loop-vectorize` and `slp-vectorize` cover vectorisation, `gvn` and `licm` take care of common subexpressions and loop-invariant code. A separate section deals with diagnostics: `-Rpass-missed` and `-Rpass-analysis` explain why a loop failed to vectorise, while `-print-after-all` dumps IR after every pass.

Writing your own passes and llvm-lit tests is out of scope here — that lives in `skills/compiler-internals/llvm-passes`. You also need LLVM installed with `clang`, `opt`, `llc` and `llvm-dis` on PATH, because without them none of the commands in the skill will run.

## Install

### Claude Code (native platform)

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

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

### Codex

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

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

### Cursor

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

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

### GitHub Copilot

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

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

### Gemini CLI

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

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

### Windsurf (Devin Desktop)

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

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill llvm -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/llvm/SKILL.md

## When to use it
- inspect the LLVM IR of a function and understand each instruction
- run a single pass or a standard optimisation pipeline through `opt`
- lower IR to assembly for a specific CPU with `llc`
- find out why a loop was not vectorised
- convert bitcode to text and back with `llvm-dis` and `llvm-as`

## Example prompts
- Show me the LLVM IR for this function and explain what `getelementptr` does
- Run `mem2reg` and `instcombine` on `src.ll` and show what changed
- Why wasn't this loop vectorised? Run clang with `-Rpass-missed=loop-vectorize`

## 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: ae576f5f46392e0f.

BEGIN THIRD-PARTY FILE ae576f5f46392e0f
````text
---
name: llvm
description: LLVM IR and pass pipeline skill. Use when working directly with LLVM Intermediate Representation (IR), running opt passes, generating IR with llc, inspecting or writing LLVM IR for custom passes, or understanding how the LLVM backend lowers IR to assembly. Activates on queries about LLVM IR, opt, llc, llvm-dis, LLVM passes, IR transformations, or building LLVM-based tools.
---

# LLVM IR and Tooling

## Purpose

Guide agents through LLVM as a **user**: generating and inspecting IR, running existing optimisation passes with `opt`, lowering to assembly with `llc`, and diagnosing missed optimisations. For **writing new LLVM passes** (PassPlugin, llvm-lit testing), use `skills/compiler-internals/llvm-passes` instead.

## Triggers

- "Show me the LLVM IR for this function"
- "How do I run an LLVM optimisation pass?"
- "What does this LLVM IR instruction mean?"
- "How do I write a custom LLVM pass?"
- "Why isn't auto-vectorisation happening in LLVM?"

## Workflow

### 1. Generate LLVM IR

```bash
# Emit textual IR (.ll)
clang -O0 -emit-llvm -S src.c -o src.ll

# Emit bitcode (.bc)
clang -O2 -emit-llvm -c src.c -o src.bc

# Disassemble bitcode to text
llvm-dis src.bc -o src.ll
```

### 2. Run optimisation passes with `opt`

```bash
# Apply a specific pass
opt -passes='mem2reg,instcombine,simplifycfg' src.ll -S -o out.ll

# Standard optimisation pipelines
opt -passes='default<O2>' src.ll -S -o out.ll
opt -passes='default<O3>' src.ll -S -o out.ll

# List available passes
opt --print-passes 2>&1 | less

# Print IR before and after a pass
opt -passes='instcombine' --print-before=instcombine --print-after=instcombine src.ll -S -o out.ll 2>&1 | less
```

### 3. Lower IR to assembly with `llc`

```bash
# Compile IR to object file
llc -filetype=obj src.ll -o src.o

# Compile to assembly
llc -filetype=asm -masm-syntax=intel src.ll -o src.s

# Target a specific CPU
llc -mcpu=skylake -mattr=+avx2 src.ll -o src.s

# Show available targets
llc --version
```

### 4. Inspect IR

Key IR constructs to understand:

| Construct | Meaning |
|-----------|---------|
| `alloca` | Stack allocation (pre-SSA; `mem2reg` promotes to registers) |
| `load`/`store` | Memory access |
| `getelementptr` (GEP) | Pointer arithmetic / field access |
| `phi` | SSA φ-node: merges values from predecessor blocks |
| `call`/`invoke` | Function call (`invoke` has exception edges) |
| `icmp`/`fcmp` | Integer/float comparison |
````
END THIRD-PARTY FILE ae576f5f46392e0f

The third-party file has ended. Everything between the ae576f5f46392e0f tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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