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

Puyan Lotfi via cfe-dev cfe-dev at lists.llvm.org
Fri Apr 5 14:35:26 PDT 2019

A few of us (Alex, Saleem and I) have discussed ideas for ELF generation.
It sounds like aside from yaml2obj, lld, and MC's ELFObjectWriter there is
also all of the ELF emission done by llvm-objcopy.
Alex tells me he's actually been intending to extract a lot of the ELF
emission code from llvm-objcopy (llvm/tools/llvm-objcopy/ELF/Object.cpp/h)
into a library somewhere in lib/Object.
We think it's probably best to do this refactoring, then eventually have
llvm-elfabi, clang-ifso, yaml2obj, etc all do their ELF writing through it
in some capacity (either going through one of the text formats, or

As far as clang-ifso goes, I think it should be a standalone tool that is
capable of generating multiple ifso formats (yaml, tbe, raw elf, etc).
At some point soon I will probably put up a phabricator patch that only
includes the yaml emission along with some tests; I think that could work
for now.



On Fri, Apr 5, 2019 at 9:47 AM Puyan Lotfi <puyan.lotfi.llvm at gmail.com>

> CC'ing George and Alex:
> I was thinking along these lines, that it would be nice to have some kind
> of common code base for things like yaml2obj as well. I spent some time at
> first looking at and trying to decouple yaml2elf from the yaml parts until
> I realized that it'd be a lot better to just generate the yaml in memory
> and pass it on to yaml2elf.
> Is llvm-elfabi planning to interop with yaml2obj any? As far as I can
> tell, currently in the monorepo the only places that are creating ELF .dyn*
> sections are in yaml2obj and lld. I agree it'd be nice to figure out some
> kind of code reuse plan.
> PL
> On Thu, Apr 4, 2019 at 6:21 PM Petr Hosek <phosek at chromium.org> wrote:
>> llvm-elfabi is going to generate ELF binaries directly as well (we
>> originally considered extending lld to support tbe files directly but we
>> later decided not to do that to avoid introducing the additional
>> complexity). It'd be great if clang-ifso could emit tbe files and use the
>> same implementation for generating ELF binaries to avoid duplication
>> between the two tools.
>> On Thu, Apr 4, 2019 at 5:42 PM Puyan Lotfi <puyan.lotfi.llvm at gmail.com>
>> wrote:
>>> Shoaib, Petr:
>>> I don’t yet know the full details of elf-tapi, but the two ways I
>>> believe these differs are:
>>> 1) clang-ifso is a clang based tool that scans from the source level and
>>> uses visibility attributes to put together a set of symbols to expose in
>>> the ifso file.
>>> 2) clang-ifso primarily aims to generate the ELF binary directly, but
>>> also can generate yaml that yaml2obj can generate the ifso from. Currently
>>> the aim is to avoid introducing linker changes.
>>> It’s possible clang-ifso could also support generating said text stubs
>>> (tbe’s) too.
>>> PL
>>> On Thu, Apr 4, 2019 at 5:23 PM Petr Hosek <phosek at chromium.org> wrote:
>>>> To expand on Shoaib's answer, llvm-elfabi
>>>> <https://github.com/llvm/llvm-project/tree/master/llvm/tools/llvm-elfabi> is
>>>> a very similar tool that we've been working on for the past few months in
>>>> upstream. The current implementation is still incomplete, but once
>>>> D56569 <https://reviews.llvm.org/D56569>, D55839
>>>> <https://reviews.llvm.org/D55839> and D55864
>>>> <https://reviews.llvm.org/D55864> land (hopefully within the next few
>>>> weeks), it should be possible to use llvm-elfabi to produce text stubs
>>>> (called .tbe files) and generate ELF interface libraries out of those.
>>>> On Thu, Apr 4, 2019 at 5:18 PM Shoaib Meenai via cfe-dev <
>>>> cfe-dev at lists.llvm.org> wrote:
>>>>> 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
>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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