This document provides usage information about the LLVM makefile system. While loosely patterned after the BSD makefile system, LLVM has taken a deparature from BSD in order to implement additional features needed by LLVM.

Although makefile systems such as automake were attempted at one point, it has become clear that the variations requried by LLVM from any Makefle norm are too many to strictly use a more limited tool. Consequently, LLVM requires simply GNU Make 3.79, a widely portably makefile processor. LLVM unabashedly makes heavy use of the features of GNU Make so the dependency on GNU Make is firm. If you're not familiar with make, it is recommended that you read the GNU Makefile Manual.

While this document is rightly part of the LLVM Programmer's Manual, it is treated separately here because of the volume of content and because it is often an early source of bewilderment for new developers.

The LLVM makefile system is the component of LLVM that is responsible for building the software, testing it, generating distributions, rpms and other packages, installing and uninstalling, etc.

The LLVM Makefile System is quite generous. It not only builds its own software, but it can build yours too. Built into the system is knowledge of the llvm/projects directory. Any directory under projects that has both a configure script and a Makefile is assumed to be a project that uses the LLVM Makefile system. This allows your project to get up and running quickly by utilizing the built-in features that are used to compile LLVM.

Each directory to participate in the build needs to have a file named Makefile. This is the file first read by make. It has three sections:

1. Settable Variables - Required that must be set first.
2. include $(LEVEL)/Makefile.common - include the LLVM Makefile system.
3. Overridable Variables - Override variables set by the LLVM Makefile system.

Every project must have a Makefile.common file at its top source directory. This file serves three purposes:

1. It includes the project's configuration makefile to obtain values determined by the configure script. This is done by including the $(LEVEL)/Makefile.config file.
2. It specifies any other (static) values that are needed throughout the project. Only values that are used in all or a large proportion of the project's directories should be placed here.
3. It include's the standard rules for the LLVM Makefile system, $(LLVM_SRC_ROOT)/Makefile.rules. This file is the "guts" of the LLVM Makefile system.

Every project must have a Makefile.config at the top of its build directory. This file is generated by the configure script from the pattern provided by the Makefile.config.in file located at the top of the project's source directory. The contents of this file depend largely on what configuration items the project uses, however most projects can get what they need by just relying on LLVM's configuration found in $(LLVM_OBJ_ROOT)/Makefile.config.

This file, located at $(LLVM_SRC_ROOT)/Makefile.rules is the heart of the LLVM Makefile System. It provides all the logic, dependencies, and rules for building the targets supported by the system. What it does largely depends on the values of make variables that have been set before Makefile.rules is included.

User Makefiles need not have comments in them unless the construction is unusual or it doesn't strictly follow the rules and patterns of the LLVM makefile system. Makefile comments are invoked with the pound (#) character. The # character and any text following it, to the end of the line, are ignored by make.

This section describes each of the targets that can be built using the LLVM Makefile system. Any target can be invoked from any directory but not all are applicabe to a given directory (e.g. "dist" and "install" will always operate as if invoked from the top level directory).

When you invoke make with no arguments, you are implicitly instructing it to seek the "all" target (goal). This target is used for building the software and will do different things in different directories. For example, in a lib directory, the "all" target will compile source files and generate libraries. But, in a tools directory, it will link libraries and generate executables.

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Variables are used to tell the LLVM Makefile System what to do and to obtain information from it. The sections below describe the three kinds of variables.

Variables listed in the table below should be set before the inclusion of $(LEVEL)/Makefile.common. These variables provide input to the LLVM make system that tell it what to do for the current directory.

Variables listed in the table below can be used to override the default values provided by the LLVM makefile system. These variables should be set after the inclusion of $(LEVEL)/Makefile.common.

Variables listed in the table below can be used by the user's Makefile but should not be changed. Changing the value will generally cause the build to go wrong, so don't do it.