How to use
- Copy the installation command and review the source package before installing it in your own agent environment.
- Check the source document for required tools, credentials and supported hosts.
- Describe your input and desired deliverable; review the output against the source material.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill llvmBefore you start
- Installation does not include a model subscription, API allowance or permission to access your files.
- Repository stars and installation counts describe activity, not an independent quality or security test.
Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
LLVM IR and Tooling
Purpose
Guide agents through LLVM as a user: generating and inspecting IR, running existing optimisation passes with opt, lowering to assembly with llc, and diagnosing missed optimisations. For writing new LLVM passes (PassPlugin, llvm-lit testing), use skills/compiler-internals/llvm-passes instead.
Triggers
- "Show me the LLVM IR for this function"
- "How do I run an LLVM optimisation pass?"
- "What does this LLVM IR instruction mean?"
- "How do I write a custom LLVM pass?"
- "Why isn't auto-vectorisation happening in LLVM?"
Workflow
1. Generate LLVM IR
bash
# Emit textual IR (.ll)
clang -O0 -emit-llvm -S src.c -o src.ll
# Emit bitcode (.bc)
clang -O2 -emit-llvm -c src.c -o src.bc
# Disassemble bitcode to text
llvm-dis src.bc -o src.ll2. Run optimisation passes with opt
bash
# Apply a specific pass
opt -passes='mem2reg,instcombine,simplifycfg' src.ll -S -o out.ll
# Standard optimisation pipelines
opt -passes='default<O2>' src.ll -S -o out.ll
opt -passes='default<O3>' src.ll -S -o out.ll
# List available passes
opt --print-passes 2>&1 | less
# Print IR before and after a pass
opt -passes='instcombine' --print-before=instcombine --print-after=instcombine src.ll -S -o out.ll 2>&1 | less3. Lower IR to assembly with llc
bash
# Compile IR to object file
llc -filetype=obj src.ll -o src.o
# Compile to assembly
llc -filetype=asm -masm-syntax=intel src.ll -o src.s
# Target a specific CPU
llc -mcpu=skylake -mattr=+avx2 src.ll -o src.s
# Show available targets
llc --version4. Inspect IR
Key IR constructs to understand:
| Construct | Meaning |
|---|---|
alloca | Stack allocation (pre-SSA; mem2reg promotes to registers) |
load/store | Memory access |
getelementptr (GEP) | Pointer arithmetic / field access |
phi | SSA φ-node: merges values from predecessor blocks |
call/invoke | Function call (invoke has exception edges) |
icmp/fcmp | Integer/float comparison |
br | Branch (conditional or unconditional) |
ret | Return |
bitcast | Reinterpret bits (no-op in codegen) |
ptrtoint/inttoptr | Pointer↔integer (avoid where possible) |
5. Key passes
| Pass | Effect |
|---|---|
mem2reg | Promote alloca to SSA registers |
instcombine | Instruction combining / peephole |
simplifycfg | CFG cleanup, dead block removal |
loop-vectorize | Auto-vectorisation |
slp-vectorize | Superword-level parallelism (straight-line vectorisation) |
inline | Function inlining |
gvn | Global value numbering (common subexpression elimination) |
licm | Loop-invariant code motion |
loop-unroll | Loop unrolling |
argpromotion | Promote pointer args to values |
sroa | Scalar Replacement of Aggregates |
6. Debugging missed optimisations
bash
# Why was a loop not vectorised?
clang -O2 -Rpass-missed=loop-vectorize -Rpass-analysis=loop-vectorize src.c
# Dump pass pipeline
clang -O2 -mllvm -debug-pass=Structure src.c -o /dev/null 2>&1 | less
# Print IR after each pass (very verbose)
opt -passes='default<O2>' -print-after-all src.ll -S 2>&1 | less7. Useful llvm tools
| Tool | Purpose |
|---|---|
llvm-dis | Bitcode → textual IR |
llvm-as | Textual IR → bitcode |
llvm-link | Link multiple bitcode files |
llvm-lto | Standalone LTO |
llvm-nm | Symbols in bitcode/object |
llvm-objdump | Disassemble objects |
llvm-profdata | Merge/show PGO profiles |
llvm-cov | Coverage reporting |
llvm-mca | Machine code analyser (throughput/latency) |
For binutils equivalents, see skills/binaries/binutils.
Related skills
- Use
skills/compiler-internals/llvm-passesfor writing and testing custom LLVM passes - Use
skills/compiler-internals/compiler-frontendfor generating LLVM IR from an AST - Use
skills/compiler-internals/jit-compilationfor ORC JIT execution of LLVM IR - Use
skills/compilers/clangfor source-level Clang flags - Use
skills/binaries/linkers-ltofor LTO at link time - Use
skills/profilers/linux-perfcombined withllvm-mcafor micro-architectural analysis
