[cfe-dev] RFC: A proposal to move toward using C++11 features in LLVM & Clang / bounding support for old host compilers

[Chris Lattner]

On Oct 27, 2013, at 9:07 PM, Chandler Carruth <chandlerc at google.com> wrote:

> On Sun, Oct 27, 2013 at 11:18 AM, Chris Lattner <clattner at apple.com> wrote:
> On Sun, Oct 27, 2013 at 2:23 AM, Chandler Carruth <chandlerc at google.com> wrote:> Concrete long term proposal:
> >
> > We support building with C++ toolchains which were released and widely available on their respective target platforms at least 2 years prior to the next 6-month-cycle release. Immediately after each branch of a release, mainline can move the bar forward by 6 months. Thus, each release should build with the four previous releases.
> 
> I don't think it makes sense to have a hard policy like this.  It is fine to have a goal that allows us to continue picking up new features over time, but I think each such change will have to be discussed (at length) with the specifics of which interesting host configurations we would be dropping.
> 
> I didn't really intend to indicate this would be a hard policy, more that it would be the guideline. Part of the idea is that after each release branch is cut we could evaluate what a reasonable baseline would look like.

Ah ok, makes perfect sense then.

> I do think that 2 years is a good guideline, in part because two of the three toolchains which impact us most already use time-driven release cycles.
> 
> My hope is that we could *strive* for no more than two years of support for older toolchains as a way to push ourselves to not let this slip.

Sure, so long as there is room to be pragmatic :-)

> An obvious other input to this decision would be exactly what Benjamin brought up: we should look at what toolchain versions various OSes are shipping with. However, I think the diversity of OS is too broad to be easily explained in our getting started docs, etc. I would rather use the OS situation to inform the particular versions of each toolchain that we try to support, but to word the decision in terms of the toolchain versions.

Right.

>  This seems overly aggressive to me.  Why not start by baselining at VC++ 2010, and bump up everything else to match?  That would give us some basic C++'11 features for 3.4, then we could bump it up to VS 2012 in 3.5 if that goes well.
> 
> As others have pointed out, our baseline is already VS 2010, but our C++11 features have been held back by concern over GCC. I think the time frame is right to jump all the way to GCC 4.7 as the target for our 3.5 release, and would like to bump VS to 2012 at the same time if at all possible.
> 
> This isn't just a bump either, it brings things that I think will be very useful:
> - Fixed semantics for r-value references that is relevant to using move semantics broadly in LLVM
> - Range-based for loops, because as much as people say it "just reduces boiler plate", omg it is so much nicer
> - better concurrency support, notably we may be able to use the standard atomics library (with some testing and restrictions) which have compiler assistance behind them.

I would also love to use standard types like std::unique_ptr, move semantics, etc without worrying about conditional code or reinventing them in the llvm ADT library.  I also have a personal love affair for range-based for loops, and I expect a number of awesome new adaptors to paper over the Module::global_begin() sorts of ranges.  It would also be great to get something like python's enumerate function etc.  In short, you don't have to convince me about the benefits of dropping old compilers! :-)

At root, my conservatism comes from not wanting to hurt adoption of LLVM.  The hosts that I specifically care about are not the problem here, they are windows and GCC users, and I am certainly not the expert on either of them, but it is pretty obvious that LLVM is used by folks outside the core development group, and making a huge change all at once is risky.

I suppose what I'm saying is that we are currently not using *any* C++'11 features.  It seems like conservatively great progress for LLVM 3.4 to bump the minimum GCC requirement up to enable use of VC 2010-era features (like rvalue refs and simple stdlib features), then jump to VC2012 features in LLVM 3.5 assuming that goes well.  We're talking about a 6 month delta between the two, and I think we'll learn a lot in the first step.  This also helps stage out the inevitable "lets rewrite the codebase" churn over more time.

-Chris

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