<div dir="ltr"><div>CC'ing George and Alex:<br></div><div><br></div><div>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. <br></div><div><br></div><div>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.</div><div><br></div><div>PL</div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Apr 4, 2019 at 6:21 PM Petr Hosek <<a href="mailto:phosek@chromium.org" target="_blank">phosek@chromium.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>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.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Apr 4, 2019 at 5:42 PM Puyan Lotfi <<a href="mailto:puyan.lotfi.llvm@gmail.com" target="_blank">puyan.lotfi.llvm@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div dir="auto">Shoaib, Petr:</div><div dir="auto"><br></div><div dir="auto">I don’t yet know the full details of elf-tapi, but the two ways I believe these differs are:</div></div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">It’s possible clang-ifso could also support generating said text stubs (tbe’s) too. </div><div dir="auto"><br></div><div dir="auto">PL</div><div dir="auto"><br></div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Apr 4, 2019 at 5:23 PM Petr Hosek <<a href="mailto:phosek@chromium.org" target="_blank">phosek@chromium.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr">To expand on Shoaib's answer, <a href="https://github.com/llvm/llvm-project/tree/master/llvm/tools/llvm-elfabi" target="_blank">llvm-elfabi</a> 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 <a href="https://reviews.llvm.org/D56569" target="_blank">D56569</a>, <a href="https://reviews.llvm.org/D55839" target="_blank">D55839</a> and <a href="https://reviews.llvm.org/D55864" target="_blank">D55864</a> 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.</div><br><div class="gmail_quote"></div></div><div dir="ltr"><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Apr 4, 2019 at 5:18 PM Shoaib Meenai via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<br></div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">How does this compare to the ELF TAPI implementation in LLVM (<a href="http://lists.llvm.org/pipermail/llvm-dev/2018-September/126472.html" target="_blank">http://lists.llvm.org/pipermail/llvm-dev/2018-September/126472.html</a> and
<a href="https://reviews.llvm.org/D53051" target="_blank">https://reviews.llvm.org/D53051</a>)? <u></u>
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<p class="MsoNormal"><b><span style="font-size:12pt;color:black">From: </span></b><span style="font-size:12pt;color:black">cfe-dev <<a href="mailto:cfe-dev-bounces@lists.llvm.org" target="_blank">cfe-dev-bounces@lists.llvm.org</a>> on behalf of cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>><br>
<b>Reply-To: </b>Puyan Lotfi <<a href="mailto:puyan.lotfi.llvm@gmail.com" target="_blank">puyan.lotfi.llvm@gmail.com</a>><br>
<b>Date: </b>Thursday, April 4, 2019 at 4:52 PM<br>
<b>To: </b>cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>><br>
<b>Cc: </b>Puyan Lotfi <<a href="mailto:puyan@puyan.org" target="_blank">puyan@puyan.org</a>><br>
<b>Subject: </b>[cfe-dev] [RFC] clang-ifso: A new clang-based tool for generating interface libraries.<u></u><u></u></span></p>
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<p class="MsoNormal">Hi All,<u></u><u></u></p>
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<p class="MsoNormal">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:<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">We are eager to to hear feedback and ideas in this space. Code for clang-ifso is available at:<u></u><u></u></p>
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<p class="MsoNormal"><a href="https://github.com/plotfi/llvm-project/tree/master/clang/tools/clang-ifso" target="_blank">https://github.com/plotfi/llvm-project/tree/master/clang/tools/clang-ifso</a>.<u></u><u></u></p>
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<p class="MsoNormal">PL<u></u><u></u></p>
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