[cfe-dev] [RFC] clang-ifso: A new clang-based tool for generating interface libraries.

[Shoaib Meenai via cfe-dev]

How does this compare to the ELF TAPI implementation in LLVM (http://lists.llvm.org/pipermail/llvm-dev/2018-September/126472.html and https://reviews.llvm.org/D53051)?

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of cfe-dev <cfe-dev at lists.llvm.org>
Reply-To: Puyan Lotfi <puyan.lotfi.llvm at gmail.com>
Date: Thursday, April 4, 2019 at 4:52 PM
To: cfe-dev <cfe-dev at lists.llvm.org>
Cc: Puyan Lotfi <puyan at puyan.org>
Subject: [cfe-dev] [RFC] clang-ifso: A new clang-based tool for generating interface libraries.

Hi All,

The following is our proposal for clang-ifso, we hope to get some valuable feedback from the community on this and we hope this work helps SDK authors in the future:

On platforms such as Darwin and Windows, there exist library interface files (TAPI tbd files, Windows Import Library files) that appear to the compile-time linker as just another library file (i.e. dylib, dll, etc) but are in-fact only empty listings of symbols to interface functions that were intended for exposure by the library writer (i.e. their `.text` sections are stripped, but they have the API symbols needed for linking). These library interfaces can be used to both limit access to internals of a library at static compile time, and can be used to speedup link time in the case of linking with extremely large dynamic libraries. Aside from providing more controlled API exposure and reduced memory usage in linking, there is also the benefit of having a much smaller distribution size for development SDKs (in the case where you’ve got an SDK with applications that are built and linked on a PC but then deployed to run on a totally different device). Finally, these interface libraries can in many cases be used to break up build dependencies as well if they can be cached or generated quickly in some way prior to building all the different libraries in a build.

clang-ifso is a tool that intends to bring the concept of interface libraries to ELF shared objects. We call it ifso as a shorthand for InterFace-Shared-Object: as in, we intend to support ELF by producing a .so that looks just like a regular .so file to the linker but has most of the .text and other contents dropped and has only the intended API interface symbols populated. That means a .so file generated by clang-ifso contains only a stripped .text section plus the `.dynsym` and `.dynstr` sections along with the global symbols populated. Currently it is in a prototype state and is capable of generating object-yaml or ELF, and is implemented using the ClangTool interface.

As mentioned at the beginning, there is prior art in this area with things like TBD files on Darwin as well as Microsoft’s import libraries. As an aside, there is evidence that ELF versions of ifsos are also deployed in production in some settings, so this is not completely new with ELF either but the effort to make all of the work upstream is. One other way that clang-ifso does differ from a lot of the prior work is that it uses the clang parser to glean what visible NamedDecls are present in the library headers.

Because clang-ifso operates from the clang level we can and do enable the programmer to use visibility attributes to expose or hide whatever interface libraries they want in their actual code rather than relying on a separate file to strip symbols as a post-build step. And since all compile-time linking will be done with the ifso rather than the .so, all clients of the library will need to completely rely on whatever was exposed through visibility attributes.

We are eager to to hear feedback and ideas in this space. Code for clang-ifso is available at:

https://github.com/plotfi/llvm-project/tree/master/clang/tools/clang-ifso.

PL


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