[llvm-dev] Using C++14 code in LLVM

[Hubert Tong via llvm-dev]

On Tue, Oct 31, 2017 at 6:26 PM, Zachary Turner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On Tue, Oct 31, 2017 at 3:19 PM Justin Bogner <mail at justinbogner.com>
> wrote:
>
>> Zachary Turner <zturner at google.com> writes:
>>
>> > If 3 months later we started requiring C++17, someone could build clang
>> 6
>> > with the system compiler and then build Clang ToT.
>> >
>> > If 3 months later we started requiring C++20, someone could still build
>> > clang 6 with the system compiler and then build Clang ToT.
>> >
>> > Every relaxation of the kind of code we can use in LLVM does not
>> > necessitate an extra hop in the bootstrapping process, because existing
>> > versions of clang can already compile through C++20.
>>
> 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).


>
>> I hate to exaggerate, but this sounds almost like an argument for using
>> new C++ features the day after we implement them. There obviously has to
>> be some balance here.
>>
>
> 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 :)
>
> 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.
>
> For C++14 and C++17 though, I think the argument still holds.  System
> Compiler -> {GCC 7 or Clang 5-6} -> ToT
>
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.


>
> should require only 1 hop regardless of language standard, and that covers
> us for quite a while.
>
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