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