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

[Eric Fiselier via cfe-dev]

So I've managed to roughly reproduce similar results on Linux building LLVM
w/ libc++ 3.5 and libc++ ToT.
On my machine I saw build times of 700 vs 800 seconds respectively (to
build a small portion of LLVM).

Initially I though the cause may have been the Lib C header wrappers, but
taking them out had no effect.
I'll keep looking...

/Eric

On Tue, Nov 21, 2017 at 10:59 AM, Eric Fiselier <eric at efcs.ca> wrote:

> 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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>
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