diff --git a/llvm/docs/Proposals/LLVMLibC.rst b/llvm/docs/Proposals/LLVMLibC.rst new file mode 100644 --- /dev/null +++ b/llvm/docs/Proposals/LLVMLibC.rst @@ -0,0 +1,123 @@ +============================== +"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 +================= + +Questions around how to achieve ABI stability across different versions of +llvm-libc are currently open and will be taken up at a future point in time. +To begin with, llvm-libc will be developed in such a way that it is independent +of ABI aspects like calling convention, stack layout etc. + +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.