[llvm-dev] A Short Policy Proposal Regarding Host Compilers

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Fri May 11 09:54:20 PDT 2018

I'd be opposed to 6/5, given where it would leave us.
It's simply hard to see a compelling reason to leave things that long.

In particular, given this is about what it takes to produce a binary
release of clang/llvm from trunk (and not what it takes to use one), i'd
like to see some evidence/argument that using 3/1.5 would actually have a
material affect on the number of contributions, etc.
(I have doubts it would have any affect on the abliity of new developers to
start contributing, etc).
All of the clang/llvm based tools i have around (cquery, rtags, you name
it) all download and ship binary releases of clang/llvm (and FWIW, they
ship and use 1-2 year old releases).
It's also unclear to me it makes sense to try to make sure any user can
compile the latest version - for example, researchers using it almost never
keep up with trunk, even with our current policy that supports things for
longer.  They stick with the version that existed when they started.
So it's unclear that we are doing a thing users actually want in practice
anyway :)

Finally,  given the rate of support for newer C++ standards in LLVM/GCC
seems to be accelerating and not slowing down (AFAICT), keeping a time
period this long will just put you farther and farther behind over time.

It may be better to simply express it in terms of releases, and say "we
support the past 2/3 major gcc releases, the past 2/3 major clang releases,
and the past 2 major msvc releases"

On Fri, May 11, 2018 at 8:58 AM, Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I second this proposal, and I make a motion to lengthen 3/1.5 to 6/5.
> On Fri, May 11, 2018 at 9:37 AM, Keane, Erich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi All-
>> As we all know, the C++14 discussion is flaring up again.  Chandler
>> brought up that he would like a concrete plan to switch.  In my opinion,
>> this is insufficient, as it will result in us simply having this discussion
>> AGAIN next release.  Instead, I would prefer us to have a concrete Policy
>> on our host compilers.  That way, changes like this are unsurprising to our
>> users, and advance our codebase sufficiently.  I believe the arguments
>> for/against upgrading have been made repeatedly, so I won't repeat them
>> here.  My proposal is thus:
>> Starting with the Clang 7.0 release, we will officially support any major
>> release of our host compilers (MSVC, GCC, Clang, ?ICC?) released in the
>> past 3* years from our previous branch date to give trunk-developers time
>> to transition (so for 7.0, 3 years before January 3, 2018).  This will be
>> enforced via the CMake CheckCompilerVersion script (ala
>> https://reviews.llvm.org/D46723).  ADDITIONALLY, a CMake warning will be
>> issued for any major release less than 1.5* years old to give our users
>> sufficient time to transition/upgrade their compilers.  Finally, our
>> dependent C++ version will be the best released standard officially
>> supported by the collection of compilers (for example, we'd support -C++20
>> if all compilers had std=c++20 or eqiv, but NOT std=c++2a).
>> The 3-years/1.5 years would result in our minimum GCC/Clang becoming:
>> GCC5.1/Clang3.6.  We would WARN on anything older than GCC7.1/Clang3.8
>> /End Proposal
>> *: To Be Bikeshed
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