Peter Smith

Open-source tools based on LLVM that target rich software platforms such as Linux are in widespread use. A large proportion of the investment in upstream LLVM development is targeted at this use case. There is much less use of open-source C/C++ toolchains for embedded software development, with GCC toolchains being dominant. This can be explained by a number of factors: * Assembling an embedded toolchain is complicated, due to cross-compilation and missing components such as the C-library. * Differences in the toolchain interface and missing functionality compared to GCC. * Code-size and performance gaps on embedded targets.

There are efforts in the LLVM community to improve the suitability for embedded software development. This presentation will cover some of the gaps and what we are trying to do to fill them. Specifically: * Hosted versus embedded toolchains. * Why do we want to use LLVM for an embedded toolchain? * What components make up an embedded toolchain, and which of these can be supplied by the LLVM project. * The LLVM bare metal driver. * A comparison of a LLVM based toolchain against an equivalent GNU toolchain. * Work that is being done in the community to improve embedded development. * How to get involved!

The presentation does not require any in depth knowledge of LLVM or compilers. An awareness of the components of a toolchain such as compiler, linker and libraries will be helpful. The toolchains used for comparison are the LLVM Embedded Toolchain for Arm and the GNU Arm Embedded Toolchain.

Details of LLVM community meet-ups, including the Monthly LLVM Embedded Toolchains call are available at https://llvm.org/docs/GettingInvolved.html

Ever wondered how the LLVM project and community works? Want to get some advice on how to most effectively contribute? Now is your chance at FOSDEM to ask experienced developers directly. This panel will host a number of experienced LLVM developers, answering any relevant questions from the audience.

In case questions from the audience do not fill the entire slot, a set of prepared questions focussed on how to get started working with and contributing to LLVM will be raised for the panelists to answer.

The panel will consist of the following experienced contributors to LLVM: Kristof Beyls, Peter Smith, Jeroen Dobbelaere, Nick Desaulniers

One of the main features of LLD, the LLVM Linker, is its high performance, frequently outperforming existing linkers by a substantial margin. In this presentation we'll take a look at why this might be? Starting with a brief description of the linker and what it must do, we'll look at how LLD approaches the problem, compare and contrast with other open source linkers and see how it performs on a selection of programs. We'll conclude by looking at what you might be able to take from LLD's architecture, implementation, and development into your own programs.

LLD (https://lld.llvm.org/) is a drop in replacement for system linkers such as GNU ld and link.exe. Since LLVM release 4.0 it has been available in the official release. Speed of linking is especially important in large projects where a single change in an object file can result in minutes of link time. On a fast machine can bring the link time of even large projects like Chrome and Clang under a minute on a fast machine, permitting developers to iterate on their changes more rapidly.

All linkers have the same basic job, find and load all the object files and libraries that will form the output, lay the contents out in order, write the combined output. In many ways a glorified form of cat. There is no single reason why LLD is faster than other linkers, it is a combination of architectural and implementation factors as well as a focus on performance by the projects maintainers. In some cases these factors will only pertain to linking but in many cases they will generalize to similar projects.

The intended audience for this talk is made up of: - People interested in learning more about the linking process. - Software developers interested in performance of batch programs that deal with a large input size.

Knowledge of C++ for the implementation aspects will be helpful.

In theory the LLVM tools support cross-compilation out of the box, with all tools potentially containing support for all targets. In practice getting it to work is more complicated, configuration options have to be given and the missing parts of the toolchain need to be provided. In this presentation we will go through the steps needed to use an X86 linux host to cross-compile an application to run on an AArch64 target, using as many of the LLVM tools as possible. We'll cover: - Getting hold of the LLVM tools and libraries. - Providing the missing bits that LLVM doesn't provide. - The configuration options needed to make it work. - Running the application using an emulator.

This talks is primarily aimed at users of LLVM based tools on Linux, with no specific knowledge of LLVM internals required for the majority of the material.

The LLVM based tools and libraries we will be using are: - Clang (including the integrated assembler). - LLD. - Compiler-rt. - Libc++, libc++abi and libunwind.

Some of these components can be provided pre-built, for others such as the libraries we may need to build them ourselves.

The main missing part of the toolchain that we have to provide is the C-library such as libc provided by either a multiarch Linux distribution or a standalone Linaro GCC release. For the primary example we'll build and run a Linux application that can be run on the qemu user mode emulator.