# compiler-optimizations-deep

Explains vectorization failures, register spills, and profiling pipelines with PGO or BOLT when default -O3 flags fail to optimize critical C and C++ loops.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 311 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/compiler-internals/compiler-optimizations-deep/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/compiler-optimizations-deep

## What it does

This skill covers compiler internals beyond static flags. It walks through LLVM IR and GCC GIMPLE stages: loop-invariant code motion (LICM), instruction selection, and greedy register allocation. When loops fail to vectorize, it helps analyze diagnostic output from Clang flag `-Rpass-missed=loop-vectorize` to spot aliasing or unknown trip counts.

The guide provides step-by-step commands for Clang profile-guided optimization and post-link optimization using BOLT. Running BOLT requires passing `-Wl,--emit-relocs` during linking and keeping binary symbols intact. A stripped binary will crash.

## Install

### Claude Code (native platform)

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

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

### Codex

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

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

### Cursor

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

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

### GitHub Copilot

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

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

### Gemini CLI

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

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/compiler-internals/compiler-optimizations-deep
```

### Windsurf (Devin Desktop)

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

### OpenCode

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

## When to use it
- diagnose loop vectorization misses using Clang optimization remarks
- shorten variable live ranges to avoid costly register stack spills
- configure profile-guided optimization pipelines using llvm-profdata
- reorder machine code blocks with LLVM BOLT after linking

## Example prompts
- /compiler-optimizations-deep Why did LLVM fail to vectorize this reduction loop?
- /compiler-optimizations-deep How can I reduce register spills in this inner loop?
- /compiler-optimizations-deep Show the compilation commands for PGO and BOLT in Clang

## Source text · SKILL.md · MIT

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BEGIN THIRD-PARTY FILE 592473b2e4bd2f02
````text
---
name: compiler-optimizations-deep
description: Deep compiler optimizations skill for RA, ISel, and PGO. Use when explaining register allocation, instruction selection, LICM, vectorization limits, or profile-guided optimization beyond -O3. Activates on queries about register allocation, instruction selection, LICM, auto-vectorization failure, PGO, or BOLT.
---

# Compiler Optimizations (Deep)

## Purpose

Explain optimization phases beyond flags: mid-level IR opts, register allocation, instruction selection/scheduling, vectorization boundaries, PGO, and post-link BOLT — bridging `skills/compilers/pgo` and LLVM/GCC internals.

## When to Use

- `-O3` did not vectorize a hot loop
- Teaching why register pressure causes spills
- Planning PGO or BOLT deployment
- Understanding pass interaction (e.g., LICM before vectorize)

## Workflow

### 1. Compiler pipeline map

```
Frontend → LLVM IR / GCC GIMPLE
├── Mid-level: DCE, GVN, LICM, inlining
├── Loop opts: unroll, vectorize
├── Codegen prep: legalize types
├── Instruction selection (DAG → machine ops)
├── Register allocation (greedy, linear scan)
└── Peephole / scheduling
```

### 2. Vectorization failure triage

```bash
clang -O3 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize foo.c
```

| Miss reason | Typical fix |
|-------------|-------------|
| Unknown trip count | peel loop; assert count |
| Dependence | reorder / separate accumulators |
| Function call in loop | inline or outline |
| Alignment unknown | `__builtin_assume_aligned` |

### 3. Register allocation intuition

When live ranges exceed physical registers, the allocator **spills** to stack slots — costly loads/stores. Reducing live ranges (splitting variables, rematerialization) helps.

GCC/LLVM both use graph coloring variants (LLVM "greedy regalloc").

### 4. PGO workflow (Clang)

```bash
clang -fprofile-instr-generate -O2 -o app foo.c
./app   # training workload
llvm-profdata merge default.profraw -o default.profdata
clang -fprofile-instr-use=default.profdata -O2 -o app_pgo foo.c
```

Improves branch layout, inlining, and vectorization thresholds.

See `skills/compilers/pgo` for GCC and BOLT.

### 5. BOLT (post-link)

```bash
llvm-bolt -instrument app -o app.inst
./app.inst
llvm-bolt -data=perf.fdata -reorder-blocks=+ -o app.bolt app
```

Optimizes layout after linker — needs relocations (`-Wl,--emit-relocs`).

### 6. LICM example

Loop-invariant code motion hoists `x * scale` out of inner loop when legal — reduces work per iteration.

### 7. Agent usage

```
/compiler-optimizations-deep Why did LLVM fail to vectorize this reduction loop?
```

## Common Problems
````
END THIRD-PARTY FILE 592473b2e4bd2f02

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