[cfe-dev] Clang on Windows targteing gcc compiles very slowly

Sun May 31 14:10:44 PDT 2015

Another thing to consider is that clang-cl doesn't implement /MP, the cl
flag for multiprocessing. If you are comparing cl with MSBuild to clang-cl
MSBuild, clang will go slower because it doesn't run multiple compiles in
parallel.

This, however, doesn't explain the 2x slowdown vs gcc from Edward's
experimentation. So far all we have are theories on that.

On Sun, May 31, 2015 at 12:34 AM, Dennis Luehring <dl.soluz at gmx.net> wrote:

> another example:
> https://github.com/RuntimeCompiledCPlusPlus/RuntimeCompiledCPlusPlus
>
> compiles in a minute in VS2010 - take years with cl-clang, with
> compile-bug reports in cl-clang
>
> so my "dirty" testbase for finding speed difference and bugs in cl-clang
> would be
>
> -Xerces C++ 3.1.2 (SVN Tag)
> -DosBox (latest SVN)
> -RuntimeCompiledCPlusPlus (lastest Git)
> -ScummVM (SVN)
>
>
>
> Am 31.05.2015 um 09:06 schrieb Dennis Luehring:
>
>> >Huh, that's surprising. Do you happen to
>> >have VS2013 too? What are you
>> >compiling? Anything open-source?
>>
>> just VS2010 - and a very closed source project, 5 libs, ~50-100kLOC,
>> around 1200 Warnings (legacy code)
>>
>> latest Xerces C++ is another example (does not fully link under cl-clang)
>> - some warnings
>> svn co https://svn.apache.org/repos/asf/xerces/c/tags/Xerces-C_3_1_2
>> VC2010: ~1:59min
>> cl-clang: stopped at ~6:15 with link error
>>
>> or latests DosBox (gives compiler bug report) - many warnings
>> VC2010: ~1:25
>> cl-clang: stopped at ~3:19 with compiler-bug report
>>
>> it seems that the cl-clang guys should do compiletests against more
>> dirty projects - not against
>> the super-clean chrome sources :)
>>
>>
>> Am 31.05.2015 um 08:21 schrieb Nico Weber:
>> > Huh, that's surprising. Do you happen to have VS2013 too? What are you
>> > compiling? Anything open-source?
>> >
>> > That build is release but with asserts enabled. Disabling asserts saves
>> > ~30% of runtime, but that would still be 1:30min which is still slower
>> than
>> > VS. We only compared to 2013 though, maybe 2010 is faster than 2013?
>> >
>> > On Sat, May 30, 2015 at 11:14 PM, Dennis Luehring <dl.soluz at gmx.net>
>> wrote:
>> >
>> > > VS2010 ~1:12min
>> > > cl-clang (plugin download from 5.5.2015) ~2:15min
>> > > clang-238562 ~2:11min
>> > >
>> > >
>> > > Am 31.05.2015 um 07:48 schrieb Nico Weber:
>> > >
>> > >> Can you try the clang-cl.exe in
>> > >>
>> > >>
>> http://commondatastorage.googleapis.com/chromium-browser-clang/index.html?path=Win/
>> > >> ?
>> > >>
>> > >> On Sat, May 30, 2015 at 10:46 PM, Dennis Luehring <dl.soluz at gmx.net>
>> > >> wrote:
>> > >>
>> > >> > im using the cl-clang vstudio plugin binary from the homepage and
>> it
>> > >> > compiles slower (nearly 2 times i think) then cl
>> > >> >
>> > >> >
>> > >> > Am 31.05.2015 um 07:40 schrieb Nico Weber:
>> > >> >
>> > >> >> Can you post the cmake invocation you use?
>> > >> >>
>> > >> >> In general, we see clang being faster than gcc on linux/mac, and
>> about
>> > >> as
>> > >> >> fast as cl.exe on windows on our bots. It sounds like you don't
>> > >> configure
>> > >> >> your build correctly. (clang-built-by-clang is a bit faster than
>> > >> >> clang-built-by-cl on windows; I don't know how
>> clang-built-by-clang and
>> > >> >> clang-built-by-gcc on windows compare. But if you're seeing a 2-3x
>> > >> >> slowdown, you're probably accidentally doing a debug build of
>> clang
>> > >> >> somehow.)
>> > >> >>
>> > >> >> On Sat, May 30, 2015 at 9:57 PM, Francois Pichet <
>> pichet2000 at gmail.com
>> > >> >
>> > >> >> wrote:
>> > >> >>
>> > >> >> > I just noticed that assert is turned on even in Release on
>> Windows
>> > >> >> > Is that on purpose? (seems like a bug to me)
>> > >> >> >
>> > >> >> > Anyway if I build clang with "LLVM_ENABLE_ASSERTIONS:BOOL=OFF"
>> the
>> > >> build
>> > >> >> > get stuck at verifier.cpp from LLVMCore. CL.exe is stuck in an
>> > >> infinite
>> > >> >> > loop.
>> > >> >> >
>> > >> >> > I am using MSVC 2013 Update 5.
>> > >> >> >
>> > >> >> > On Sat, May 30, 2015 at 10:30 AM, Reid Kleckner <rnk at google.com
>> >
>> > >> wrote:
>> > >> >> >
>> > >> >> >> You said you configured in release mode, but assertions might
>> still
>> > >> be
>> > >> >> >> on. That accounts for a 2x slowdown. Otherwise I'd say that we
>> > >> haven't
>> > >> >> >> profiled and optimized clang on windows very much.
>> > >> >> >>
>> > >> >> >> Sent from phone
>> > >> >> >> On May 29, 2015 4:51 PM, "Edward Diener" <
>> > >> >> eldlistmailingz at tropicsoft.com>
>> > >> >> >> wrote:
>> > >> >> >>
>> > >> >> >>> I test Boost libraries mostly on Windows using various
>> version of
>> > >> gcc
>> > >> >> >>> 4.3 and up, VC++ 8.0 and up, and the latest version of clang
>> which
>> > >> I
>> > >> >> build
>> > >> >> >>> from source using mingw/gcc-4.8.1 in release mode.
>> > >> >> >>>
>> > >> >> >>> While clang is a great compiler when it comes to testing code,
>> > >> getting
>> > >> >> >>> intelligent error message, and implementing the latest C++
>> > >> standard,
>> > >> >> it is
>> > >> >> >>> noticably slower ( 2x or 3x at minimum ) than the other
>> compilers I
>> > >> >> use.
>> > >> >> >>>
>> > >> >> >>> The clang command line parameters usually being used are
>> usually
>> > >> along
>> > >> >> >>> the lines of:
>> > >> >> >>>
>> > >> >> >>> -c -x c++ -O0 -g -fno-inline -Wall -g -march=i686 -m32
>> > >> >> >>>
>> > >> >> >>> with the addition of 'std=c++11' occasionally depending on the
>> > >> test.
>> > >> >> >>>
>> > >> >> >>> Does anybody have any idea why clang is so much slower than
>> any of
>> > >> the
>> > >> >> >>> other compilers I use ? It does not seem to matter whether it
>> is in
>> > >> >> C++03
>> > >> >> >>> mode or C++11 mode, it is noticeably slower than the other
>> > >> compilers.
>> > >> >> >>>
>> > >> >> >>> _______________________________________________
>> > >> >> >>> cfe-dev mailing list
>> > >> >> >>> cfe-dev at cs.uiuc.edu
>> > >> >> >>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> > >> >> >>>
>> > >> >> >>
>> > >> >> >> _______________________________________________
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>> > >> >> >>
>> > >> >> >>
>> > >> >> >
>> > >> >> > _______________________________________________
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>> > >> >> >
>> > >> >> >
>> > >> >>
>> > >> >>
>> > >> >>
>> > >> >> _______________________________________________
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>> > >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> > >> >>
>> > >> >
>> > >> > _______________________________________________
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>> > >> >
>> > >>
>> > >>
>> > >
>> >
>>
>>
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