Llvm

mohitmishra786/low-level-dev-skills/skills/compilers/llvm

by mohitmishra786bdc58472fa9fNo license253 starsListed Oct 9, 2026Updated Oct 9, 2026Repository updated 3 months ago

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.

Instructions onlySoftware Development

AI-generated overview

Guides agents through using LLVM tooling to generate, inspect, optimise and lower LLVM IR.

What it does: This skill explains how to work with LLVM Intermediate Representation as a user: emitting textual IR or bitcode with clang, running optimisation passes with opt, lowering IR to assembly or object files with llc, and inspecting IR constructs. It also covers diagnosing missed optimisations and lists useful LLVM utilities such as llvm-dis, llvm-as, llvm-link and llvm-mca. It produces command guidance and reference explanations rather than files or code of its own.
When to use it: Use it when you need to generate or read LLVM IR, run or debug opt pass pipelines, lower IR with llc, or understand why an optimisation such as auto-vectorisation did not happen. It is aimed at using LLVM tools, not at authoring new LLVM passes.
Requirements: Requires an LLVM toolchain (clang, opt, llc and related llvm-* tools) available in the environment. Instructions only; no scripts are shipped.

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 textllvm-dis src.bc -o src.ll

# 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 textllvm-dis src.bc -o src.ll

2. Run optimisation passes with `opt`

bash

# Apply a specific passopt -passes='mem2reg,instcombine,simplifycfg' src.ll -S -o out.ll
# Standard optimisation pipelinesopt -passes='default<O2>' src.ll -S -o out.llopt -passes='default<O3>' src.ll -S -o out.ll
# List available passesopt --print-passes 2>&1 | less
# Print IR before and after a passopt -passes='instcombine' --print-before=instcombine --print-after=instcombine src.ll -S -o out.ll 2>&1 | less

# Apply a specific passopt -passes='mem2reg,instcombine,simplifycfg' src.ll -S -o out.ll
# Standard optimisation pipelinesopt -passes='default<O2>' src.ll -S -o out.llopt -passes='default<O3>' src.ll -S -o out.ll
# List available passesopt --print-passes 2>&1 | less
# Print IR before and after a passopt -passes='instcombine' --print-before=instcombine --print-after=instcombine src.ll -S -o out.ll 2>&1 | less

3. Lower IR to assembly with `llc`

bash

# Compile IR to object filellc -filetype=obj src.ll -o src.o
# Compile to assemblyllc -filetype=asm -masm-syntax=intel src.ll -o src.s
# Target a specific CPUllc -mcpu=skylake -mattr=+avx2 src.ll -o src.s
# Show available targetsllc --version

# Compile IR to object filellc -filetype=obj src.ll -o src.o
# Compile to assemblyllc -filetype=asm -masm-syntax=intel src.ll -o src.s
# Target a specific CPUllc -mcpu=skylake -mattr=+avx2 src.ll -o src.s
# Show available targetsllc --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

bash

# Why was a loop not vectorised?clang -O2 -Rpass-missed=loop-vectorize -Rpass-analysis=loop-vectorize src.c
# Dump pass pipelineclang -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

# Why was a loop not vectorised?clang -O2 -Rpass-missed=loop-vectorize -Rpass-analysis=loop-vectorize src.c
# Dump pass pipelineclang -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

Source and attribution

Source:mohitmishra786/low-level-dev-skillsinskills/compilers/llvmat commitbdc5847

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

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Llvm

LLVM IR and Tooling

Purpose

Triggers

Workflow

1. Generate LLVM IR

2. Run optimisation passes with `opt`

3. Lower IR to assembly with `llc`

4. Inspect IR

5. Key passes

6. Debugging missed optimisations

7. Useful llvm tools

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Llvm

LLVM IR and Tooling

Purpose

Triggers

Workflow

1. Generate LLVM IR

2. Run optimisation passes with opt

3. Lower IR to assembly with llc

4. Inspect IR

5. Key passes

6. Debugging missed optimisations

7. Useful llvm tools

Related skills

Source and attribution

More from mohitmishra786/low-level-dev-skills

Zig Debugging

Simd Intrinsics

Zig Cross

Rustc Basics

Zig Cinterop

Zig Build System

More in Software Development

Playground

M5 Onboard

Cardputer Buddy

Web Design Guidelines

Vercel React Native Skills

Vercel React Best Practices

2. Run optimisation passes with `opt`

3. Lower IR to assembly with `llc`