[cfe-dev] clang-cl.exe (32bit) dog slow, capping at ~16.5% CPU

[David Blaikie via cfe-dev]

On Thu, Jun 1, 2017 at 12:48 AM Hahnfeld, Jonas via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> > cmake -G "MinGW Makefiles" \
>
>   -DCMAKE_INSTALL_PREFIX=C:\LLVM \
>
>   -DCMAKE_BUILD_TYPE=Debug \
>
>   -DLLVM_ENABLE_ASSERTIONS=On \
>
>   -DLLVM_TARGETS_TO_BUILD=X86 \
>
>   -DLLVM_ENABLE_CXX1Y=On \
>
>   ../
>
>
>
> CMAKE_BUILD_TYPE=Debug will result in a non-optimized compiler. Although I
> don’t have numbers at hand, you might want to set CMAKE_BUILD_TYPE=Release.
>

Also  -DLLVM_ENABLE_ASSERTIONS=On

The build system at some point had a warning informing users that this
might produce an order of magnitude (or more) slower compiler.

I'd suggest you keep an optimized and unoptimized compiler around. Though
even then, Hello World doesn't take me 10 seconds to compile... so maybe
there's other things going on. Sounds like you'd want one set of LLVM stuff
with all the debugging knobs turned on (exactly as you've built) & use that
with your frontend, and another, release build, for running clang, etc for
practical porpoises.




>
>
> Regards,
>
> Jonas
>
>
>
> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of *Nikodemus
> Siivola via cfe-dev
> *Sent:* Thursday, June 1, 2017 9:43 AM
> *To:* cfe-dev at lists.llvm.org
> *Subject:* [cfe-dev] clang-cl.exe (32bit) dog slow, capping at ~16.5% CPU
>
>
>
> I was wondering why compiles were so slow and noticed that clang-cl.exe is
> never consuming more than ~16.5% CPU on my laptop. This is an i7-7700HQ,
> 32GB memory, running Windows 10 Pro.
>
>
>
> Memory and disk use appear trivial as well, and the computer was otherwise
> mostly idle at the time.
>
>
>
> Compiling a trivial hello-world takes 11 seconds:
>
>
>
> $ clang --version
>
> clang version 4.0.1
>
> Target: i686-pc-windows-msvc
>
> Thread model: posix
>
> InstalledDir: C:\LLVM\bin
>
>
>
> $ cat foo.cpp
>
> #include <iostream>
>
>
>
> int main()
>
> {
>
>     std::cout << "Yo" << std::endl;
>
>     return 0;
>
> }
>
>
>
> $ time clang-cl foo.cpp
>
>
>
> real    0m11.886s
>
> user    0m0.015s
>
> sys     0m0.015s
>
>
>
> Compiling the same program under the Linux subsystem and clang-3.5 takes
> 0.16seconds. The Visual Studio commandline compiler is likewise almost
> instant.
>
>
>
> This is release_40 from one of the git mirrors, build built as part of the
> LLVM tree using Visual Studio 2015 command prompt, with (I think, not 100%
> sure, don't know cmake well enough to figure out after the fact) the
> following cmake invovations:
>
>
>
> cmake -G "MinGW Makefiles" \
>
>   -DCMAKE_INSTALL_PREFIX=C:\LLVM \
>
>   -DCMAKE_BUILD_TYPE=Debug \
>
>   -DLLVM_ENABLE_ASSERTIONS=On \
>
>   -DLLVM_TARGETS_TO_BUILD=X86 \
>
>   -DLLVM_ENABLE_CXX1Y=On \
>
>   ../
>
> cmake --build .
>
>
>
> Did I pick spectacularly stupid build options or something?
>
>
> Any suggestions for figuring out what is going on? (Windows is not my
> regular platform so I'm a bit out of depth here.)
>
> Cheers,
>
>
>
>  -- nikodemus
>
>
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