[cfe-dev] [libc++] much longer build time of my project using libc++ 5.0 vs 3.5.2

[Eric Fiselier via cfe-dev]

For my sanity, and because this has come up before more than once, I need
to ask:

If you're testing on OS X, can you confirm that the old clang setup isn't
defaulting to libstdc++ 4.2?
This happens often --- and accidentally -- with older platforms or with
older Clang toolsets.

Other users have reported a ~20% compile time slowdown after switching from
libstdc++ 4.2 to libc++.

That being said, I'm going to try and get some builds set up to attempt to
reproduce this.

/Eric


On Mon, Nov 20, 2017 at 4:29 PM, chen hsu <chenxuz at outlook.com> wrote:

> merging thread.. I have precompilation configuration for my project,
> and by enabling it, the build time is 100 minutes. but since precompilation
> introduces some other issues (cmake doesn't support precompilation so we
> have to use some custom script, which is not perfect), we still prefer
> to building the code without precompilation, if any other option is
> available.
>
>
> I don't have a test case yet, but our use of c++ stl is quite common,
> vector, wstring, map, set, etc. wstring is used a lot since the code was
> ported from Windows to linux. On windows side we use msbuild, which only
> takes 50 minutes to build the whole project. Of course this is not
> comparable due to compiler difference, but we are targeting to make Linux
> build time to be close to that of Windows.
> ------------------------------
> *From:* Eric Fiselier <eric at efcs.ca>
> *Sent:* Saturday, November 18, 2017 8:20:37 PM
> *To:* Dimitry Andric
> *Cc:* chen hsu; cfe-dev
> *Subject:* Re: [cfe-dev] [libc++] much longer build time of my project
> using libc++ 5.0 vs 3.5.2
>
>
>
> On Sat, Nov 18, 2017 at 6:15 AM, Dimitry Andric via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Do you have some sort of test case?  For example, a standalone .cpp file
> which includes a bunch of libc++ headers, and which now takes much more
> time to compile?
>
> It would be interesting to see where this regressed, if it did.
>
>
> +1. I would love to see a test case here.
>
>
>
> -Dimitry
>
> On 17 Nov 2017, at 21:04, chen hsu via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> Hi, my project has been using libc++/c++abi 3.5.2 for two years, and
> recently I am upgrading it to libc++/c++abi 5.0, only to find that the
> build time for our project increased dramatically, from 100 minutes to 160
> minutes on a 6-core intel-CPU, same situation for both debug and release
> build. The compiler is clang 5.0. The comparison is pretty fair - same
> machine and compiler, same codebase, the only difference is the C++
> headers. Is there a way to improve? It's kind of blocker for our adoption
> of libc++/c++abi 5.0, as it takes too long for our developers to build.
>
> Thanks a lot.
>
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