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

[Hahnfeld, Jonas via cfe-dev]

> 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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