llvm-ir-and-passes

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