mohitmishra786/llvm

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

# 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/compiler-internals/llvm-passes for writing and testing custom LLVM passes
• Use skills/compiler-internals/compiler-frontend for generating LLVM IR from an AST
• Use skills/compiler-internals/jit-compilation for ORC JIT execution of LLVM IR
• 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