Add a proposal for a libc project under the LLVM umbrella.

Reviewers: chandlerc, dlj, echristo, hfinkel, jfb, zturner

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64939

llvm-svn: 369012

docs/Proposals/LLVMLibC.rst

@@ -0,0 +1,125 @@
+==============================
+"llvm-libc" C Standard Library
+==============================
+
+.. contents:: Table of Contents
+  :depth: 4
+  :local:
+
+Introduction
+============
+
+This is a proposal to start *llvm-libc*, an implementation of the
+C standard library targeting C17 and above, as part of the LLVM project.
+llvm-libc will also provide platform specific extensions as relevant.
+For example, on Linux it also provides pthreads, librt and other POSIX
+extension libraries.
+
+Features
+========
+
+llvm-libc will be developed to have a certain minimum set of features:
+
+- C17 and upwards conformant.
+- A modular libc with individual pieces implemented in the "as a
+  library" philosophy of the LLVM project.
+- Ability to layer this libc over the system libc if possible and desired
+  for a platform.
+- Provide C symbols as specified by the standards, but take advantage
+  and use C++ language facilities for the core implementation.
+- Provides POSIX extensions on POSIX compliant platforms.
+- Provides system-specific extensions as appropriate. For example,
+  provides the Linux API on Linux.
+- Vendor extensions if and only if necessary.
+- Designed and developed from the start to work with LLVM tooling and
+  testing like fuzz testing and sanitizer-supported testing.
+- ABI independent implementation as far as possible.
+- Use source based implementations as far possible rather than
+  assembly. Will try to *fix* the compiler rather than use assembly
+  language workarounds.
+- Extensive unit testing and standards conformance testing. If relevant
+  and possible, differential testing: We want to be able
+  to test llvm-libc against another battle-tested libc. This is
+  essentially to understand how we differ from other libcs. Also if
+  relevant and possible, test against the testsuite of an another
+  battle-tested libc implementation.
+
+Why a new C Standard Library?
+=============================
+
+Implementing a libc is no small task and is not be taken lightly. A
+natural question to ask is, "why a new implementation of the C
+standard library?" There is no single answer to this question, but
+some of the major reasons are as follows:
+
+- Most libc implementations are monolithic. It is a non-trivial
+  porting task to pick and choose only the pieces relevant to one's
+  platform. The llvm-libc will be developed with sufficient modularity to
+  make picking and choosing a straightforward task.
+- Most libc implementations break when built with sanitizer specific
+  compiler options. The llvm-libc will be developed from the start to
+  work with those specialized compiler options.
+- The llvm-libc will be developed to support and employ fuzz testing
+  from the start.
+- Most libc implementations use a good amount of assembly language,
+  and assume specific ABIs (may be platform dependent). With the llvm-libc
+  implementation, we want to use normal source code as much as possible so
+  that compiler-based changes to the ABI are easy. Moreover, as part of the
+  LLVM project, we want to use this opportunity to fix performance related
+  compiler bugs rather than using assembly workarounds.
+- A large hole in the LLVM toolchain will be plugged with llvm-libc.
+  With the broad platform expertise in the LLVM community, and the
+  strong license and project structure, we think that llvm-libc will
+  be more tunable and robust, without sacrificing the simplicity and
+  accessibility typical of the LLVM project.
+
+Platform Support
+================
+
+We envision that llvm-libc will support a variety of platforms in the coming
+years. Interested parties are encouraged to participate in the design and
+implementation, and add support for their favorite platforms.
+
+ABI Compatibility
+=================
+
+As llvm-libc is new, it will not offer ABI stability in the initial stages.
+However, as we've heard from other LLVM contributors that they are interested
+in having ABI stability, llvm-libc code will be written in a manner which is
+amenable to ABI stability. We are looking for contributors interested in
+driving the design in this space to help us define what exactly does ABI
+stability mean for llvm-libc.
+
+Layering Over Another libc
+==========================
+
+When meaningful and practically possible on a platform, llvm-libc will be
+developed in a fashion that it will be possible to layer it over the system
+libc. This does not mean that one can mix llvm-libc with the system-libc. Also,
+it does not mean that layering is the only way to use llvm-libc. What it
+means is that, llvm-libc can optionally be packaged in a way that it can
+delegate parts of the functionality to the system-libc. The delegation happens
+internal to llvm-libc and is invisible to the users. From the user's point of
+view, they only call into llvm-libc.
+
+There are a few problems one needs to be mindful of when implementing such a
+delegation scheme in llvm-libc. Examples of such problems are:
+
+1. One cannot mix data structures from llvm-libc with those from the
+system-libc. A translation from one set of data structures to the other should
+happen internal to llvm-libc.
+2. The delegation mechanism has to be implemented over a related set of
+functions. For example, one cannot delegate just the `fopen` function to the
+system-libc. One will have to delegate all `FILE` related functions to the
+system-libc.
+
+Current Status
+==============
+
+llvm-libc development is still in the planning phase.
+
+Build Bots
+==========
+
+Once the development starts, there will be llvm-libc focused builders added to
+the LLVM BuildBot.