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

[Nikodemus Siivola via cfe-dev]

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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