[llvm-dev] Using C++14 code in LLVM (original) (raw)

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Tue Oct 31 13:57:45 PDT 2017

Last time we discussed this, the consensus was "I think we can survive another year without generalized constexpr and variable templates".

Well, we did indeed survive. And it's been exactly a year! So naturally, it only makes sense to revive this :)

There's an active conversation going on in IRC right now, and it seems like there is more desire than there was last year.

What are the main gains from allowing C++14?

What are the main gains from allowing C++17? [1]

First, it seems like if we want to enable C++14 we need GCC >= 5. And if we want to enable C++17 we need GCC >= 7.

With that out of the way, here were some of the issues that were raised last time:

Issue: Ubuntu 14.04 LTS is on GCC 4.8.x, and we have to support it until end of life. Resolution: LTS is right around the corner, in 6 more months.

Issue: Various other platforms have older GCCs as their system compiler, and it's annoying to upgrade. Question: Do any of these not have a port you can install? For example, NetBSD 7 appears to have GCC 5.3 as a port, if DistroWatch is any indication. It could be wrong though and I could also be misinterpreting it.

Issue: If we're going to make people bootstrap a compiler, we might as well go all the way to C++17. Comment: I'm not opposed.

Some questions / comments of my own:

OpenBSD - Ships with GCC 4.2.1 anyway. They are already having to bootstrap something, so the proposal here does not change anything, because even current LLVM doesn't compile with GCC 4.2.1

CentOS & RHEL - No version of Distro, including trunk, has GCC >= 4.8.5 (are there ports?)

Debian - Minimum version 9 for GCC >= 5 (are there ports for earlier releases?)

Fedora - Minimum version 24 for GCC >= 5, minimum version 26 for GCC >= 7

Ubuntu - Minimum LTS 16.04 for GCC >= 5

NetBSD - Version 7 has GCC 4.8.4 by default, but contains port for 5.3.0

FreeBSD - Minimum Version 11 for GCC >= 5

So, thoughts?

[1] - Note that we'd need to wait a few more revs for MSVC before allowing C++17, but given that it's becoming easier and easier to bump the minimum MSVC version, I'm discounting this as a factor, as MSVC will not really be the bottleneck in any real sense.

On Tue, Oct 4, 2016 at 2:15 PM Mehdi Amini <mehdi.amini at apple.com> wrote:

On Oct 4, 2016, at 2:10 PM, Reid Kleckner <rnk at google.com> wrote: On Tue, Oct 4, 2016 at 11:58 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

On Oct 4, 2016, at 8:40 AM, Reid Kleckner via llvm-dev <_ _llvm-dev at lists.llvm.org> wrote: On Tue, Oct 4, 2016 at 8:29 AM, Zachary Turner <zturner at google.com> wrote:

I ask because many of these LTS distros are notoriously slow at updating their packages. While some people may think C++14 doesn't provide enough bang for the buck to justify bumping to GCC 4.9, C++17 definitely does. But at that point we're going to be talking about GCC 6.1 or 6.2, which is going to be significantly harder unless we want to wait 5-7 years, and I suspect people won't.

If by "notoriously slow" you mean they don't bump their toolchain versions at all, then yeah. We just wait until the LTS release is at end-of-life before dropping it. That’s the first time I read about this policy: we support every linux LTS distribution till their end-of-life? Only Ubuntu? Do you have a pointer where it is documented / discussed? (Note that Ubuntu LTS is 5 years AFAIK.) Sorry, I didn't mean to refer to the LTS support lifetime. I just meant we support the last LTS until we can reasonably expect users to have upgraded to the new one. If there's an LTS release every two years, then we want to keep supporting them for at least three years to give people a year to upgrade. OK, got it. Thanks for clarifying! Mehdi -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20171031/8d5866c1/attachment.html>

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