# LLVMBuild Guide

1. Introduction
2. Project Organization
3. Build Integration
4. Component Overview
5. Format Reference

## Introduction

This document describes the `LLVMBuild` organization and files which we use to describe parts of the LLVM ecosystem. For description of specific LLVMBuild related tools, please see the command guide.

LLVM is designed to be a modular set of libraries which can be flexibly mixed together in order to build a variety of tools, like compilers, JITs, custom code generators, optimization passes, interpreters, and so on. Related projects in the LLVM system like Clang and LLDB also tend to follow this philosophy.

In order to support this usage style, LLVM has a fairly strict structure as to how the source code and various components are organized. The `LLVMBuild.txt` files are the explicit specification of that structure, and are used by the build systems and other tools in order to develop the LLVM project.

## Project Organization

The source code for LLVM projects using the LLVMBuild system (LLVM, Clang, and LLDB) is organized into components, which define the separate pieces of functionality that make up the project. These projects may consist of many libraries, associated tools, build tools, or other utility tools (for example, testing tools).

For the most part, the project contents are organized around defining one main component per each subdirectory. Each such directory contains an `LLVMBuild.txt` which contains the component definitions.

The component descriptions for the project as a whole are automatically gathered by the LLVMBuild tools. The tools automatically traverse the source directory structure to find all of the component description files. NOTE: For performance/sanity reasons, we only traverse into subdirectories when the parent itself contains an `LLVMBuild.txt` description file.

## Build Integration

The LLVMBuild files themselves are just a declarative way to describe the project structure. The actual building of the LLVM project is handled by another build system (currently we support both Makefiles and CMake.

The build system implementation will load the relevant contents of the LLVMBuild files and use that to drive the actual project build. Typically, the build system will only need to load this information at "configure" time, and use it to generative native information. Build systems will also handle automatically reconfiguring their information when the contents of the LLVMBuild.txt files change.

Developers generally are not expected to need to be aware of the details of how the LLVMBuild system is integrated into their build. Ideally, LLVM developers who are not working on the build system would only ever need to modify the contents of the LLVMBuild.txt description files (although we have not reached this goal yet).

For more information on the utility tool we provide to help interfacing with the build system, please see the llvm-build documentation.

## Component Overview

As mentioned earlier, LLVM projects are organized into logical components. Every component is typically grouped into its own subdirectory. Generally, a component is organized around a coherent group of sources which have some kind of clear API separation from other parts of the code.

LLVM primarily uses the following types of components:

- Libraries - Library components define a distinct API which can be independently linked into LLVM client applications. Libraries typically have private and public header files, and may specify a link of required libraries that they build on top of.
- Build Tools - Build tools are applications which are designed to be run as part of the build process (typically to generate other source files). Currently, LLVM uses one main build tool called TableGen to generate a variety of source files.
- Tools - Command line applications which are built using the LLVM component libraries. Most LLVM tools are small and are primarily frontends to the library interfaces.

Components are described using LLVMBuild.txt files in the directories that define the component. See the Format Reference section for information on the exact format of these files.

## LLVMBuild Format Reference

LLVMBuild files are written in a simple variant of the INI or configuration file format (Wikipedia entry). The format defines a list of sections each of which may contain some number of properties. A simple example of the file format is below:

    ; Comments start with a semi-colon.

    ; Sections are declared using square brackets.
    [component_0]

    ; Properties are declared using '=' and are contained in the previous section.
    ;
    ; We support simple string and boolean scalar values and list values, where
    ; items are separated by spaces. There is no support for quoting, and so
    ; property values may not contain spaces.
    property_name = property_value
    list_property_name = value_1 value_2 ... value_n
    boolean_property_name = 1 (or 0)

LLVMBuild files are expected to define a strict set of sections and properties. An typical component description file for a library component would look typically look like the following example:

    [component_0]
    type = Library
    name = Linker
    parent = Libraries
    required_libraries = Archive BitReader Core Support TransformUtils

A full description of the exact sections and properties which are allowed follows.

Each file may define exactly one common component, named "common". The common component may define the following properties:

- subdirectories [optional]

  If given, a list of the names of the subdirectories from the current subpath to search for additional LLVMBuild files.

Each file may define multiple components. Each component is described by a section who name starts with "component". The remainder of the section name is ignored, but each section name must be unique. Typically components are just number in order for files with multiple components ("component_0", "component_1", and so on).

Section names not matching this format (or the "common" section) are currently unused and are disallowed.

Every component is defined by the properties in the section. The exact list of properties that are allowed depends on the component type. Components may not define any properties other than those expected by the component type.

Every component must define the following properties:

- type [required]

  The type of the component. Supported component types are detailed below. Most components will define additional properties which may be required or optional.

- name [required]

  The name of the component. Names are required to be unique across the entire project.

- parent [required]

  The name of the logical parent of the component. Components are organized into a logical tree to make it easier to navigate and organize groups of components. The parents have no semantics as far as the project build is concerned, however. Typically, the parent will be the main component of the parent directory.

  Components may reference the root pseudo component using '$ROOT' to indicate they should logically be grouped at the top-level.

Components may define the following properties:

- dependencies [optional]

  If specified, a list of names of components which must be built prior to this one. This should only be exactly those components which produce some tool or source code required for building the component.

  NOTE: Group and LibraryGroup components have no semantics for the actual build, and are not allowed to specify dependencies.

The following section lists the available component types, as well as the properties which are associated with that component.

- type = Group

  Group components exist purely to allow additional arbitrary structuring of the logical components tree. For example, one might define a "Libraries" group to hold all of the root library components.

  Group components have no additionally properties.

- type = Library

  Library components define an individual library which should be built from the source code in the component directory.

  Components with this type use the following properties:

  - library_name [optional]

    If given, the name to use for the actual library file on disk. If not given, the name is derived from the component name itself.

  - required_libraries [optional]

    If given, a list of the names of Library or LibraryGroup components which must also be linked in whenever this library is used. That is, the link time dependencies for this component. When tools are built, the build system will include the transitive closure of all required_libraries for the components the tool needs.

  - add_to_library_groups [optional]

    If given, a list of the names of LibraryGroup components which this component is also part of. This allows nesting groups of components. For example, the X86 target might define a library group for all of the X86 components. That library group might then be included in the all-targets library group.

- type = LibraryGroup

  LibraryGroup components are a mechanism to allow easy definition of useful sets of related components. In particular, we use them to easily specify things like "all targets", or "all assembly printers".

  Components with this type use the following properties:

  - required_libraries [optional]

    See the Library type for a description of this property.

  - add_to_library_groups [optional]

    See the Library type for a description of this property.

- type = TargetGroup

  TargetGroup components are an extension of LibraryGroups, specifically for defining LLVM targets (which are handled specially in a few places).

  The name of the component should always be the name of the target.

  Components with this type use the LibraryGroup properties in addition to:

  - has_asmparser [optional] [boolean]

    Whether this target defines an assembly parser.

  - has_asmprinter [optional] [boolean]

    Whether this target defines an assembly printer.

  - has_disassembler [optional] [boolean]

    Whether this target defines a disassembler.

  - has_jit [optional] [boolean]

    Whether this target supports JIT compilation.

- type = Tool

  Tool components define standalone command line tools which should be built from the source code in the component directory and linked.

  Components with this type use the following properties:

  - required_libraries [optional]

    If given, a list of the names of Library or LibraryGroup components which this tool is required to be linked with. NOTE: The values should be the component names, which may not always match up with the actual library names on disk.

    Build systems are expected to properly include all of the libraries required by the linked components (i.e., the transitive closer of required_libraries).

    Build systems are also expected to understand that those library components must be built prior to linking -- they do not also need to be listed under dependencies.

- type = BuildTool

  BuildTool components are like Tool components, except that the tool is supposed to be built for the platform where the build is running (instead of that platform being targetted). Build systems are expected to handle the fact that required libraries may need to be built for multiple platforms in order to be able to link this tool.

  BuildTool components currently use the exact same properties as Tool components, the type distinction is only used to differentiate what the tool is built for.