llvm

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

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