llvm

name	llvm
description	LLVM IR and pass pipeline skill. Use when working directly with LLVM Intermediate Representation (IR), running opt passes, generating IR with llc, inspecting or writing LLVM IR for custom passes, or understanding how the LLVM backend lowers IR to assembly. Activates on queries about LLVM IR, opt, llc, llvm-dis, LLVM passes, IR transformations, or building LLVM-based tools.

LLVM IR and Tooling

Purpose

Guide agents through the LLVM IR pipeline: generating IR, running optimisation passes with opt, lowering to assembly with llc, and inspecting IR for debugging or performance work.

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

# 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.ll

2. Run optimisation passes with opt

# 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 | less

3. Lower IR to assembly with llc

# 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 --version

4. 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

# 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 | less

7. 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/compilers/clang for source-level Clang flags
• Use skills/binaries/linkers-lto for LTO at link time
• Use skills/profilers/linux-perf combined with llvm-mca for micro-architectural analysis