llvm-ir-and-passes

该 Skill 面向 LLVM IR 阅读与优化流程，说明 SSA、基本块、PHI 与 poison/undef 语义，并给出用 clang 生成 IR、用 opt 检查 pass 管线、用 llvm-dis 反汇编 bitcode 及对比优化前后 IR 的方法；还包含常见 pass 排错与教学场景。

mohitmishra786

热度 0.00

最近更新 2026-10-19 04:15:29

分类：开发依赖：代码执行 / 读写文件应用场景：调试排错 / 概念解释 / 代码审查 / 教学辅导

安装技能

复制下面的话术发给你的 Agent，它会通过 secure-skills 检索并安装该技能。

执行命令安装此 skill： npx secure-skills-cli add mohitmishra786/low-level-dev-skills/llvm-ir-and-passes

Metadata

name: llvm-ir-and-passes
description: LLVM IR and passes skill for reading compiler IR. Use when analyzing LLVM IR, understanding SSA, pass pipeline order, or running opt on bitcode. Activates on queries about LLVM IR, SSA form, opt passes, llvm-dis, pass pipeline, or reading .ll files.

LLVM IR and Passes

Purpose

Teach agents to read and reason about LLVM IR (SSA, types, terminators), navigate the standard pass pipeline, and use opt/llvm-dis for inspection. Complements skills/compilers/llvm (toolchain) and skills/compiler-internals/llvm-passes (writing plugins) — not merged.

When to Use

Understanding what -O2 changed in generated IR
Triaging miscompiles between Clang versions
Preparing to write a custom pass
Teaching SSA and dominance without writing C++ passes yet

Workflow

1. LLVM IR structure

; Function in SSA form
define i32 @add(i32 %a, i32 %b) {
entry:
  %sum = add i32 %a, %b
  ret i32 %sum
}

Key concepts: basic blocks, PHI nodes at merges, typed values (i32, ptr, vectors).

2. Emit IR from Clang

clang -S -emit-llvm -O0 -o foo.ll foo.c
clang -c -emit-llvm -O2 -o foo.bc foo.c
llvm-dis foo.bc -o foo.ll

3. Standard optimization pipeline (New PM)

Clang -O2 roughly runs:

SimplifyCFG → InstCombine → GVN → LICM → LoopVectorize → ...

Inspect with:

opt -passes='default<O2>' -S foo.ll -o foo-opt.ll
opt -passes='default<O2>' -print-passes foo.ll 2>&1 | head

4. Useful analysis passes

Pass	Shows
`-passes=print<domtree>`	Dominator tree
`-passes=print<loops>`	Loop nests
`-passes=print-alias-sets`	Alias sets

opt -passes='print<domtree>' -disable-output foo.ll

5. Reading PHI and UB

merge:
  %v = phi i32 [ %a, %then ], [ %b, %else ]

undef and poison in IR model LLVM poison semantics — distinct from C UB but related.

6. Compare before/after

opt -passes='instcombine,simplifycfg' -S foo.ll -o - | diff -u foo.ll -

7. Agent usage

/llvm-ir-and-passes Explain this PHI node and which pass likely created it

Common Problems

Symptom	Cause	Fix
Pass not found	LLVM version rename	`opt --print-passes` for your build
IR mismatch	Different LLVM major	Match clang/opt versions
"optnone" blocks opts	`-O0` attribute	Compile with `-O1+`
Huge IR	Inlined headers	`-fno-discard-value-names` off; filter function
Wrong pipeline	Legacy vs NPM	Use `-passes=` syntax

Related Skills

skills/compilers/llvm — toolchain overview
skills/compiler-internals/llvm-passes — writing PassPlugins
skills/compiler-internals/compiler-optimizations-deep — RA and ISel
skills/compiler-internals/compiler-frontend — IR generation
skills/rust/rustc-basics — rustc --emit=llvm-ir