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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Not to rain on the GCC parade, but all this loose talk about bootstrapping our way into using C++20 is making me real nervous.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I think the MSVC limiting factor is a real one. My company delivers a pile of products to a whole lot of game developers, and those products are (nearly) all
built with MSVC. MSVC is a solid toolchain with a long track record, and actually supports the Windows environment moderately well. ;-) With all due respect to the folks working on Clang/LLVM's Windows support (and I've even done a tiny bit of it myself),
we are nowhere near willing to build our products with such a relatively untried Windows-target tool as Clang itself.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In support of Windows Clang, I will say it has advanced to the point where my team has floated the thought of having an agenda item on a planning meeting to
talk about what it would take to work up an experimental build/test pipeline to try out the idea of evaluating the possibility that we could potentially consider adopting Clang as the build compiler for our toolchain. I am probably stating that too strongly.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">More concisely: Given the two-version policy for MSVC, what MSVC(-1) supports is an upper bound on what version of C++ the LLVM project can really adopt.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Now I do understand this is not what zturner is actually asking; he's asking, how do we decide when to move the *lower* bound, which is somehow caught up in
the practicalities of what's delivered with which distros and how important is that really.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In order to advance the lower bound to C++14, the issue is, can we require people to have GCC 5? End of problem statement.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks for listening,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Zachary Turner via llvm-dev<br>
<b>Sent:</b> Tuesday, October 31, 2017 4:01 PM<br>
<b>To:</b> Hubert Tong<br>
<b>Cc:</b> Zachary Turner via llvm-dev<br>
<b>Subject:</b> Re: [llvm-dev] Using C++14 code in LLVM<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Oct 31, 2017 at 3:45 PM Hubert Tong <<a href="mailto:hubert.reinterpretcast@gmail.com">hubert.reinterpretcast@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Tue, Oct 31, 2017 at 6:26 PM, Zachary Turner via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Oct 31, 2017 at 3:19 PM Justin Bogner <<a href="mailto:mail@justinbogner.com" target="_blank">mail@justinbogner.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Zachary Turner <<a href="mailto:zturner@google.com" target="_blank">zturner@google.com</a>> writes:<br>
<br>
> If 3 months later we started requiring C++17, someone could build clang 6<br>
> with the system compiler and then build Clang ToT.<br>
><br>
> If 3 months later we started requiring C++20, someone could still build<br>
> clang 6 with the system compiler and then build Clang ToT.<br>
><br>
> Every relaxation of the kind of code we can use in LLVM does not<br>
> necessitate an extra hop in the bootstrapping process, because existing<br>
> versions of clang can already compile through C++20.<o:p></o:p></p>
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<p class="MsoNormal">The feature set of C++20 is not closed yet by the committee. I don't see where this is coming from (but if you would like to share, then please do tell).<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal"><br>
I hate to exaggerate, but this sounds almost like an argument for using<br>
new C++ features the day after we implement them. There obviously has to<br>
be some balance here.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Someone could probably use that line of reasoning to argue for using new features immediately after implementing them, but that someone wouldn't be me :)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In any case, the point was simply to illustrate that, in general, you do not need to add a hop to the bootstrapping process every time you bump the language standard. C++20 is 4-6 years out before we're even discussing it though, and any
discussion we have about if, when, or how to move to it now will probably be irrelevant by that time.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">For C++14 and C++17 though, I think the argument still holds. System Compiler -> {GCC 7 or Clang 5-6} -> ToT<o:p></o:p></p>
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<p class="MsoNormal">Copies of C++17 are not available for sale yet. There will likely be tweaks in the wording and in the implementation. It would be quite unfortunate to pick up C++17 for development on ToT only to discover 3 years afterwards that some range
of revisions can only be built properly with some specific build compilers with a particular bug/interpretation of the feature.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> Technically correct (the best kind). But I don't have a CL up for review that I'm waiting to commit which changes the standard, nor am I even proposing moving to C++17 in the immediate future (indeed, in the original post I mentioned
at least one compiler does not have *any* feature complete C++17 implementation available, and is actually a ways out from having one, and certainly we couldn't consider adopting C++17 until every supported compiler has *some* version that is feature complete).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">C++14 is a different story, and that was part of the reason for the original post. There is definitely interest from many people to at least --have a plan-- for bumping the standard. So the goal of this is to work this plan out.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">With MSVC we have a very clear, tangible policy of "we support the two latest major releases of MSVC". This is what I'm trying to get at.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">---What is the policy for GCC?---<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It seems like we don't have one. I would like to have an actual, concrete, specific, well-defined rule that people can refer to decide when we can bump the minimum required GCC version.<o:p></o:p></p>
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