LLVMLinux for the Real World

Bryce Adelstein-Lelbach, LSU Undergraduate

blelbach@cct.lsu.edu

llvm.linuxfoundation.org

Overview

• I ask you some questions

• Introduction to LLVM and Clang

• History of the LLVMLinux project

• Applications of LLVMLinux

– Diagnostics

– Source Transformation

– Cross Compilation

• You ask me some questions

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Have you ever used a search-and-replace tool to update deprecated or incorrect code?

How big was the codebase?

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Have you ever manually performed a tedious update of deprecated or incorrect code?

How big was the codebase?

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Raise your hand if you’ve used a cross compiler

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LLVM and Clang

• LLVM

– A modular and extensible compiler framework

– Uses a common intermediate representation known as LLVM IR

• Clang

– An LLVM frontend for C, C++ and other C-like languages

– Highly standard compliant

– GCC compatible

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Open Source (BSD style license)

Library Architecture

• Both LLVM and Clang are designed with a library architecture – Instead of traditional, monolithic drivers, LLVM

and Clang provide a series of libraries which can be used to implement compiler tools

• This design makes LLVM and Clang modular and extensible

• Facilitates the development of domain-specific solutions to toolchain problems

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Beginning

• October, 2010 – First working Clang-compiled Linux kernel

• December 2010 – Most of the major initial bugs are resolved

• April, 2011 – Work from initial collaborators is unified on Github

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Linux Foundation

• May, 2011 – Linux Foundation begins supporting development

• May, 2012 – All existing work is consolidated under the LLVMLinux project (hosted by LF)

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Diagnostics

• Superb diagnostics

– Column numbers for all diagnostics

– Caret/Point diagnostics

• Range highlighting

– Intelligent typedef preservation and unwrapping

– Macro expansion

– Wide range of CFG-based warnings

• Example: –Wtautological-compare warns about checking if an unsigned integer is less than 0

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Diagnostics

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Suggested fix

Carrot and range

Source line Flag

Source location

Diagnostic type

Static Analyzer

• The Clang Static Analyzer is a tool built with the Clang libraries that detects bugs – Supports plugins

• A static analyzer uses techniques similar to those used to generate compiler warnings – Static analyzers perform far more exhaustive

analysis

– Static analyzers can detect bugs that would typically only manifest at runtime

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Static Analyzer

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Fixits

• Clang’s diagnostic framework can suggest solutions (fixits) to common syntactical mistakes and even simple semantic errors

– Fixits can fix the source files itself

– Can copy the source files and apply the fixes to the copy

• Fixits are the first iteration of a still evolving framework for automatic refactoring

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Refactoring

• Clang developers are currently developing infrastructure for automatic source refactoring

• Automatic refactoring with a compiler framework is more powerful than refactoring with regex-based tools – Most refactoring tools only refactor syntax, Clang-

based tools can refactor semantics and syntax

• The framework could potentially be utilized to replace tools like coccinelle with something more robust

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Universal Driver

• Clang has, from the start, been capable of generating code for multiple platforms

• The universal driver model is a new initiative which Clang is pursuing to make cross compiling easier – The basic idea is to have one driver application for all

targets

– The user would specify a target via a command line option and the driver would handle the rest

• This technique would simplify the deployment of Linux to embedded platforms such as smartphones and tablets

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Acknowledgments

• Linux Foundation – Mike Woster – Jan-Simon Möller – Behan Webster

• Early Colloborators – PaX Team – Alp Toker – Török Edwin – Sedat Dilek

• Mark Charlebois • Various LLVM/Clang developers • Various Linux kernel developers

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