Llvm Ir And Passes

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

4. Useful analysis passes

| Pass | Shows | |------|-------| | -passes=print | Dominator tree | | -passes=print | Loop nests | | -passes=print-alias-sets | Alias sets |

opt -passes='print' -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

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: mohitmishra786
• Source: mohitmishra786/low-level-dev-skills
• License: MIT
• Homepage: https://www.lowleveldevskills.com

Install and usage instructions live in the source repository linked above.