[libc-commits] [PATCH] D152028: [libc][Docs] Add some motivation for the GPU libc

[Joseph Huber via Phabricator via libc-commits]

jhuber6 updated this revision to Diff 527953.
jhuber6 added a comment.

Clean up leftover change


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152028/new/

https://reviews.llvm.org/D152028

Files:
  libc/docs/gpu/index.rst
  libc/docs/gpu/motivation.rst
  libc/docs/gpu/support.rst
  libc/docs/gpu/testing.rst


Index: libc/docs/gpu/testing.rst
===================================================================
--- libc/docs/gpu/testing.rst
+++ libc/docs/gpu/testing.rst
@@ -19,7 +19,7 @@
 
 The testing support in LLVM's libc implementation for GPUs is designed to mimic
 the standard unit tests as much as possible. We use the `remote procedure call
-<libc_gpu_rpc>` support to provide the necessary utilities like printing from
+<libc_gpu_rpc>`_ support to provide the necessary utilities like printing from
 the GPU. Execution is performed by emitting a ``_start`` kernel from the GPU
 that is then called by an external loader utility. This is an example of how
 this can be done manually:
Index: libc/docs/gpu/support.rst
===================================================================
--- libc/docs/gpu/support.rst
+++ libc/docs/gpu/support.rst
@@ -12,7 +12,7 @@
 
 The following functions and headers are supported at least partially on the
 device. Some functions are implemented fully on the GPU, while others require a
-`remote procedure call <libc_gpu_rpc>`.
+`remote procedure call <libc_gpu_rpc>`_.
 
 ctype.h
 -------
Index: libc/docs/gpu/motivation.rst
===================================================================
--- /dev/null
+++ libc/docs/gpu/motivation.rst
@@ -0,0 +1,58 @@
+.. _libc_gpu_motivation:
+
+==========================
+Motivation and Limitations
+==========================
+
+.. contents:: Table of Contents
+  :depth: 4
+  :local:
+
+Motivation
+==========
+
+This project aims to provide a subset of the C standard library to users of GPU
+accelerators. We deliberately choose to only implement a subset of the C library
+as some features are not expressly useful or easily implemented on the GPU. This
+will be discussed further in `Limitations <libc_gpu_limitations>`_. The main
+motivation behind this project is to provide a common implementation of host
+services for the GPU.
+
+The main idea behind this project is that programming GPUs can be as
+straightforward as programming on CPUs. This project aims to validate the GPU as
+a more general-purpose target and to simplify the incidental complexity of
+existing device libraries. The implementations here will also enable more
+complex implementations of other libraries on the GPU, such as ``libc++``.
+
+Currently, host services and C library features are provided sparsely by the
+different GPU vendors. We wish to provide these functions more completely and
+make their implementations available in a common format. This is useful for
+targets like OpenMP offloading or SYCL which wish to unify the offloading
+toolchain. We also aim to provide these functions in a format compatible with
+offloading in ``Clang`` so that we can treat the C library for the GPU as a
+standard static library.
+
+For this reason, we primarily are focused on functions that are provided by
+existing device libraries such as ``printf``, ``malloc``, and ``free``. We also
+compile the ``libc`` source code directly without any offloading runtime. The
+goal is to provide a library without any existing dependencies that can be used
+to stand up various services for any target capable of linking the bitcode.
+
+We also wish to use the existing LLVM C library unit test suite to expand test
+coverage targeting GPUs. For this reason we build `tests on the GPU
+<libc_gpu_testing>`_ that can run a unit test as if it were being run on the
+CPU. This also helps users port applications that traditionally were run on the
+CPU.
+
+.. _libc_gpu_limitations:
+
+Limitations
+===========
+
+We currently only implement a subset of the standard C library. The GPU does not
+currently support thread local variables so variables like ``errno`` are not
+provided. Furthermore, the GPU under the OpenCL model cannot safely provide a
+mutex interface, nor would it be profitable if implemented. This means that
+features like file buffering are not implemented on the GPU. We can also not
+easily provide threading features on the GPU due to the execution model so these
+will be ignored, as will features like ``locale`` or ``time``.
Index: libc/docs/gpu/index.rst
===================================================================
--- libc/docs/gpu/index.rst
+++ libc/docs/gpu/index.rst
@@ -16,3 +16,4 @@
    support
    testing
    rpc
+   motivation


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