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

[Nikodemus Siivola via cfe-dev]

Is there a way to build LLVM libraries with debug info and clang with
release flags?

(I'm working on a frontend toy and figuring out what I've done to to crash
LLVM is much easier with a debug build...)


On Thu, Jun 1, 2017 at 10:47 AM, Hahnfeld, Jonas <
Hahnfeld at itc.rwth-aachen.de> 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.
>
>
>
> 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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