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Reviewers: chandlerc, dlj, echristo, hfinkel, jfb, zturner Subscribers: dexonsmith, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D64939 llvm-svn: 369012
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==============================
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"llvm-libc" C Standard Library
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==============================
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.. contents:: Table of Contents
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:depth: 4
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:local:
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Introduction
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============
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This is a proposal to start *llvm-libc*, an implementation of the
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C standard library targeting C17 and above, as part of the LLVM project.
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llvm-libc will also provide platform specific extensions as relevant.
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For example, on Linux it also provides pthreads, librt and other POSIX
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extension libraries.
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Features
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========
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llvm-libc will be developed to have a certain minimum set of features:
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- C17 and upwards conformant.
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- A modular libc with individual pieces implemented in the "as a
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library" philosophy of the LLVM project.
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- Ability to layer this libc over the system libc if possible and desired
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for a platform.
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- Provide C symbols as specified by the standards, but take advantage
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and use C++ language facilities for the core implementation.
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- Provides POSIX extensions on POSIX compliant platforms.
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- Provides system-specific extensions as appropriate. For example,
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provides the Linux API on Linux.
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- Vendor extensions if and only if necessary.
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- Designed and developed from the start to work with LLVM tooling and
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testing like fuzz testing and sanitizer-supported testing.
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- ABI independent implementation as far as possible.
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- Use source based implementations as far possible rather than
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assembly. Will try to *fix* the compiler rather than use assembly
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language workarounds.
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- Extensive unit testing and standards conformance testing. If relevant
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and possible, differential testing: We want to be able
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to test llvm-libc against another battle-tested libc. This is
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essentially to understand how we differ from other libcs. Also if
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relevant and possible, test against the testsuite of an another
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battle-tested libc implementation.
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Why a new C Standard Library?
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=============================
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Implementing a libc is no small task and is not be taken lightly. A
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natural question to ask is, "why a new implementation of the C
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standard library?" There is no single answer to this question, but
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some of the major reasons are as follows:
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- Most libc implementations are monolithic. It is a non-trivial
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porting task to pick and choose only the pieces relevant to one's
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platform. The llvm-libc will be developed with sufficient modularity to
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make picking and choosing a straightforward task.
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- Most libc implementations break when built with sanitizer specific
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compiler options. The llvm-libc will be developed from the start to
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work with those specialized compiler options.
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- The llvm-libc will be developed to support and employ fuzz testing
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from the start.
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- Most libc implementations use a good amount of assembly language,
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and assume specific ABIs (may be platform dependent). With the llvm-libc
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implementation, we want to use normal source code as much as possible so
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that compiler-based changes to the ABI are easy. Moreover, as part of the
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LLVM project, we want to use this opportunity to fix performance related
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compiler bugs rather than using assembly workarounds.
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- A large hole in the LLVM toolchain will be plugged with llvm-libc.
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With the broad platform expertise in the LLVM community, and the
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strong license and project structure, we think that llvm-libc will
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be more tunable and robust, without sacrificing the simplicity and
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accessibility typical of the LLVM project.
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Platform Support
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================
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We envision that llvm-libc will support a variety of platforms in the coming
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years. Interested parties are encouraged to participate in the design and
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implementation, and add support for their favorite platforms.
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ABI Compatibility
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=================
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As llvm-libc is new, it will not offer ABI stability in the initial stages.
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However, as we've heard from other LLVM contributors that they are interested
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in having ABI stability, llvm-libc code will be written in a manner which is
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amenable to ABI stability. We are looking for contributors interested in
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driving the design in this space to help us define what exactly does ABI
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stability mean for llvm-libc.
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Layering Over Another libc
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==========================
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When meaningful and practically possible on a platform, llvm-libc will be
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developed in a fashion that it will be possible to layer it over the system
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libc. This does not mean that one can mix llvm-libc with the system-libc. Also,
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it does not mean that layering is the only way to use llvm-libc. What it
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means is that, llvm-libc can optionally be packaged in a way that it can
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delegate parts of the functionality to the system-libc. The delegation happens
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internal to llvm-libc and is invisible to the users. From the user's point of
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view, they only call into llvm-libc.
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There are a few problems one needs to be mindful of when implementing such a
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delegation scheme in llvm-libc. Examples of such problems are:
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1. One cannot mix data structures from llvm-libc with those from the
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system-libc. A translation from one set of data structures to the other should
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happen internal to llvm-libc.
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2. The delegation mechanism has to be implemented over a related set of
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functions. For example, one cannot delegate just the `fopen` function to the
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system-libc. One will have to delegate all `FILE` related functions to the
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system-libc.
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Current Status
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==============
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llvm-libc development is still in the planning phase.
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Build Bots
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==========
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Once the development starts, there will be llvm-libc focused builders added to
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the LLVM BuildBot.
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