diff --git a/libc/docs/entrypoints.rst b/libc/docs/entrypoints.rst --- a/libc/docs/entrypoints.rst +++ b/libc/docs/entrypoints.rst @@ -1,3 +1,5 @@ +.. _entrypoints: + Entrypoints in LLVM libc ------------------------ diff --git a/libc/docs/index.rst b/libc/docs/index.rst --- a/libc/docs/index.rst +++ b/libc/docs/index.rst @@ -78,6 +78,7 @@ api_test mechanics_of_public_api source_layout + porting .. toctree:: :hidden: diff --git a/libc/docs/porting.rst b/libc/docs/porting.rst new file mode 100644 --- /dev/null +++ b/libc/docs/porting.rst @@ -0,0 +1,116 @@ +.. _porting: + +======================================= +Bringup on a New OS or Architecture +======================================= + +.. contents:: Table of Contents + :depth: 4 + :local: + +An OS specific config directory +=============================== + +If you are starting to bring up LLVM's libc on a new operating system, the first +step is to add a directory for that OS in the ``libc/config`` directory. Both +`Linux `_ and +`Windows `_, +the two operating systems on which LLVM's libc is being actively developed, +have their own config directory. + +.. note:: Windows development is not as active as the development on Linux. + There is a + `Darwin `_ + config also which is in a similar state as Windows. + +The api.td file +--------------- + +If the :ref:`fullbuild_mode` is to be supported on the new operating system, +then a file named ``api.td`` should be added in it's the config directory. It +is written in the +`LLVM tablegen language `_. +It lists all the relevant macros and type definitions we want in the +public libc header files. See the existing Linux +`api.td `_ +file as an example to prepare the ``api.td`` file for the new operating system. + +.. note:: In future, LLVM tablegen will be replaced with a different DSL to list + config information. So, the use of the ``api.td`` might be replaced with a + different methodology. + +Architecture Subdirectory +========================= + +There are parts of the libc which are implemented differently for different +architectures. The simplest example of this is the ``syscall`` function and +its internal implementation - its Linux implementation differs for different +architectures. Since a large part of the libc makes use of syscalls (or an +equivalent on non-Linux like platforms), it might be simpler and convenient to +bring up the libc for one architecture at a time. In such cases, wherein the +support surface of LLVM's libc differs for each target architecture, one will +have to add a subdirectory (within the config directory os the operating +system) for each target architecture, and list the relevant config information +separately in those subdirectories. For example, for Linux, the x86_64 and +aarch64 configs are in separate directories, named +`x86_64 `_ +and `aarch64 `_. +The libc CMake machinery looks for subdirectories named after the target +architecture. + +The entrypoints.txt file +======================== + +One of the important pieces of config information is listed in a file named +``entrypoints.txt``. This file lists the targets for the entrypoints (see +:ref:`entrypoints`) you want to include in the static archive of the libc (for +the :ref:`overlay_mode` and/or the :ref:`fullbuild_mode`.) If you are doing an +architecture specific bring up, then an ``entrypoints.txt`` file should be +created in the architecture subdirectory for each architecture. Else, having a +single ``entrypoints.txt`` in the operating system directory is sufficient. + +The Linux config has an ``entrypoint.txt`` for each individual target +architecture separately: `aarch64 `_, +`arm32 `_ and +`x86_64 `_. On the +other hand, the Windows config has a single ``entrypoints.txt`` +`file `_. + +A typical bring up procedure will normally bring up a small group of entrypoints +at a time. The usual practice is to progressively add the targets for those +entrypoints to the ``entrypoints.txt`` file as they are being brought up. The +same is the case if one is implementing a new entrypoint - the target for the +new entrypoint should be added to the relevant ``entrypoints.txt`` file. If +the implementation of the new entrypoint supports multiple operating systems and +target architectures, then multiple ``entrypoints.txt`` files will have to be +updated. + +The headers.txt file +==================== + +Another import piece of config informtion is listed in a file named +``headers.txt``. It lists the targets for the set of public headers that are to +provided by the libc. This is relevant only if the libc is to be used in the +:ref:`fullbuild_mode` on the target operating system and architecture. As with +the ``entrypoints.txt`` file, one ``headers.txt`` file should be listed for +each individual target architecture if you are doing an architecture specific +bring up. The Linux config has ``headers.txt`` file listed seperately for the +`aarch64 `_ +config and the +`x86_64 `_ +config. + +CI builders +=========== + +If you are contributing a port for a operating system or architecture which +is not covered by existing CI builders, you will also have to present a plan +for testing and contribute a CI builder. See +`this guide `_ for information +on how to add new builders to the +`LLVM buildbot `_. +You will either have to extend the existing +`Linux script `_ +and/or +`Windows script `_ +or add a new script for your operating system.