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<p class="MsoNormal">(CCing Chris and Petr, who’ve done the most work on the runtimes build)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">At least for me on Linux, using LLVM_ENABLE_PROJECTS is actually the unusual way of building libc++; I use LLVM_ENABLE_RUNTIMES. The reason is, my host compiler is often gcc, but I want to build, test, and ship libc++ with the clang I just
built.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The runtimes build is when you use LLVM_ENABLE_RUNTIMES. It sets up the build of all runtimes (compiler-rt, libc++, libc++abi, libunwind, etc.) as a CMake ExternalProject which depends on the build of clang and other toolchain tools. In
other words, if I run the following:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">cmake -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi' path/to/my/llvm-project/llvm<o:p></o:p></p>
<p class="MsoNormal">ninja cxx<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The build system will automatically build clang and other toolchain tools (e.g. llvm-ar), run the ExternalProject configuration with e.g. CMAKE_C_COMPILER and CMAKE_CXX_COMPILER set to the just-built clang, and then build libc++ with that
configuration (so with the just-built clang). It’s a pretty convenient workflow for my setup. It also takes care of e.g. automatically rebuilding libc++ if you make changes to clang and then run `ninja cxx` again.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As for why the runtimes build use the “standalone build” setup, it’s because there’s a separate CMake configuration happening for the runtimes in this setup (which is necessary in order to be able to configure them to use the just-built
toolchain), so e.g. clang isn’t available as an in-tree target. See <a href="https://reviews.llvm.org/D62410">
https://reviews.llvm.org/D62410</a> for more details. Your top-level CMakeLists.txt in the runtimes build is llvm/runtimes/CMakeLists.txt and not libcxx/CMakeLists.txt (as it would be in a fully standalone build), but it’s also not llvm/CMakeLists.txt (as it
would be with LLVM_ENABLE_PROJECTS).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">At the CMake round table at the dev meeting last October, we’d discussed the runtimes builds, and Chris had advanced that there should be two supported ways to build the runtimes:<o:p></o:p></p>
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<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">If you’re building as part of the LLVM build and using LLVM_ENABLE_RUNTIMES<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1">If you’re building fully standalone (as in you’re pointing CMake to libc++’s source directory, so that your top-level CMakeLists is libcxx/CMakeLists.txt)<o:p></o:p></li></ol>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I agree with that position. In particular, I think LLVM_ENABLE_PROJECTS is definitely the wrong thing to use for compiler-rt, which is strongly tied to your just-built compiler. I think it’s arguably the wrong thing to use for libc++/libc++abi/libunwind
as well, where you either want to use your just-built Clang (which LLVM_ENABLE_RUNTIMES was made for), or a different compiler (in which case you’d do a fully standalone build), but silently using your host compiler for them is probably not what you want.
(The LLVM_ENABLE_PROJECTS workflow for libc++/libc++abi/libunwind probably works out better on macOS, where your host compiler is a recent Clang anyway so the difference between it and a just-built Clang aren’t as marked, but I’ve had issues even on macOS
in the past where using the host compiler via LLVM_ENABLE_PROJECTS gave me weird libc++ test errors, and LLVM_ENABLE_RUNTIMES just worked.)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What do you think?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Louis Dionne via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Reply-To: </b>Louis Dionne <ldionne@apple.com><br>
<b>Date: </b>Wednesday, April 8, 2020 at 8:46 AM<br>
<b>To: </b>Libc++ Dev <libcxx-dev@lists.llvm.org><br>
<b>Cc: </b>llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject: </b>[llvm-dev] Clarifying the supported ways to build libc++, libc++abi and libunwind<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">[Cross-post to llvm-dev to make sure everybody relevant sees this]<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Hi,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">I'm currently trying to simplify the libc++/libc++abi/libunwind build systems and testing setup. In doing so, I am encountering issues related to "unusual" ways of building them. By unusual, I just mean "not the
usual monorepo build with LLVM_ENABLE_PROJECTS". I would like to pin down what the set of supported use cases for building the runtime libraries are. In particular, the world I would like to live in is one where the only way to build libc++/libc++abi/libunwind
is:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Monaco"> $ mkdir build</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Monaco"> $ cd build</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Monaco"> $ cmake <monorepo-root>/llvm -DLLVM_ENABLE_PROJECTS=libcxx;libcxxabi;libunwind <options></span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Monaco"> $ ninja -C build install-{cxx,cxxabi,unwind}</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">The "runtimes" build would be built on top of this -- it would be just a driver for building these libraries using documented options against the just-built Clang. I think it already does so in essence, however
if I'm not mistaken it uses the "Standalone build" and it definitely sets some magic and undocumented CMake variables (like HAVE_LIBCXXABI) that we have to be really careful not to break.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">So, to better understand what people use today, I have some questions. I know the answer to some of those, but I want to see what others have to say:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">1. What is a "Standalone build"? What does it enable that a normal monorepo build can't?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">2. What is the "Runtimes" build? How does it work, what is it used for, and what does it expect from libc++/libc++abi/libunwind?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">3. Are there other "hidden" ways to build the runtime libraries?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Cheers,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Louis<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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