mohitmishra786/llvm-ir-and-passes
skills/compiler-internals/llvm-ir-and-passes/SKILL.md
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.
$ install --global
skillsauthnpx skillsauth add mohitmishra786/low-level-dev-skills llvm-ir-and-passesInstall this skill globally with one command. Works with Claude Code, Cursor, and Windsurf.
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TrivyContainer and dependency vulnerability scanner
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SemgrepStatic code analysis for vulnerabilities
95%
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mcp-scan (Snyk)Model Context Protocol security validation
95%
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Snyk (dep)Open source security scanning
50%
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Socket.devSupply chain security analysis
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Last scanned: Jun 28, 2026, 6:41 AM387.5s1 file scanned
SKILL.md
- 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
-O2changed 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 overviewskills/compiler-internals/llvm-passes— writing PassPluginsskills/compiler-internals/compiler-optimizations-deep— RA and ISelskills/compiler-internals/compiler-frontend— IR generationskills/rust/rustc-basics—rustc --emit=llvm-ir
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