Clang -print-stats diff for libjingle_peerconnection.channel.o with libstdc++ and libc++
There doesn't seem to be any one thing causing the difference on the Clang side: there is simply more of everything in the libc++ build. See attachment: nearly 4x as many types, 10x as many implicit special members, 2.5x as many declarations, and so on.

Compilation time comparisons

I had tested compilation time for all combinats of clang, gcc with libcxx and libstdc++ . The compiled code was short:

#include

int main() {
std::cout<<"hello cout\n";
return 0;
}

I repeated every run few times and got consistent results. The results attached.

• clang is about 50% slower than gcc for all three cases.
• Windows.h doubles compilation time and not required. Remove it out asap!
• Even w/o windows.h, libcxx compilation time is twice the libstdc++ compilation time with either compiler.
• Comparing each compiler with its own library, clang+libcxx compilation is three times as much slower as gcc+libstdc++ !!

• clang is about 50% slower than gcc for all three cases.
• Windows.h doubles compilation time and not required. Remove it out asap!
• Even w/o windows.h, libcxx compilation time is twice the libstdc++
  compilation time with either compiler.
• Comparing each compiler with its own library, clang+libcxx compilation is
  three times as much slower as gcc+libstdc++ !!

Which gcc/libstdc++ are you comparing against?

Looks like things haven't improved much:

λ clang -v
clang version 3.7.0 (trunk 236946)
Target: x86_64-suse-linux
Thread model: posix
Found candidate GCC installation: /usr/lib64/gcc/x86_64-suse-linux/4.8
Selected GCC installation: /usr/lib64/gcc/x86_64-suse-linux/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

λ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib64/gcc/x86_64-suse-linux/4.8/lto-wrapper
Target: x86_64-suse-linux
Configured with: ../configure --prefix=/usr --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64 --enable-languages=c,c++,objc,fortran,obj-c++,java,ada --enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.8 --enable-ssp --disable-libssp --disable-plugin --with-bugurl=http://bugs.opensuse.org/ --with-pkgversion='SUSE Linux' --disable-libgcj --disable-libmudflap --with-slibdir=/lib64 --with-system-zlib --enable-__cxa_atexit --enable-libstdcxx-allocator=new --disable-libstdcxx-pch --enable-version-specific-runtime-libs --enable-linker-build-id --enable-linux-futex --program-suffix=-4.8 --without-system-libunwind --with-arch-32=i586 --with-tune=generic --build=x86_64-suse-linux --host=x86_64-suse-linux
Thread model: posix
gcc version 4.8.3 20141208 [gcc-4_8-branch revision 218481] (SUSE Linux)

λ cat t.cpp
#include

int main() {
std::cout<<"hello cout\n";
return 0;
}

λ time clang++ t.cpp
clang++ -std=c++14 -stdlib=libc++ -fuse-ld=gold t.cpp 0.67s user 0.02s system 99% cpu 0.692 total

λ time g++ t.cpp
g++ t.cpp 0.09s user 0.01s system 96% cpu 0.107 total

It would be unfair to compare g++ w/ libstdc++ and clang w/ libc++. I'll look into comparing the same clang with both libstdc++ and libc++. However this is very low on my priority list.

It would be unfair to compare g++ w/ libstdc++ and clang w/ libc++. I'll
look into comparing the same clang with both libstdc++ and libc++. However
this is very low on my priority list.

Well thats only a 1x slowdown:

λ time clang++ -std=c++11 -stdlib=libstdc++ t.cpp
clang++ -std=c++14 -stdlib=libc++ -fuse-ld=gold -std=c++11 -stdlib=libstdc++ 0.29s user 0.03s system 99% cpu 0.316 total

λ time clang++ -std=c++11 -stdlib=libc++ t.cpp
clang++ -std=c++14 -stdlib=libc++ -fuse-ld=gold -std=c++11 -stdlib=libc++ 0.60s user 0.03s system 99% cpu 0.633 total

I don't really think that this is a bug. I think that we have been comparing different things.

First, some timings that I just did. (All of these, I ran several times)

$ time totclang -c -std=c++11 -stdlib=libstdc++ bug.cpp

real 0m0.187s
user 0m0.156s
sys 0m0.025s

$ time totclang -c -std=c++11 -I /Sources/LLVM/llvm/projects/libcxx/include bug.cpp

real 0m0.346s
user 0m0.311s
sys 0m0.030s

Seems pretty straightforward, right?

But that's comparing libstdc++ 4.2.1, a C++03 standard library implementation, to ToT libc++ a C++11 library implementation. The C++11 standard library is much larger.

So, let's compare apples to apples. Let's look at more modern libstdc++ versions - ones that implement the full C++11 standard library. (Note that libstdc++ didn't get most of the C++11 stuff until 4.8.x, and didn't really have all of it until 5.1)

$ time totclang -c -std=c++11 -I /Sources/gcc/gcc-4.9.2/bin/include/ bug.cpp

real 0m0.344s
user 0m0.309s
sys 0m0.032s

$ time totclang -c -std=c++11 -I /Sources/gcc/gcc-5.1.0/bin/include/ bug.cpp

real 0m0.345s
user 0m0.304s
sys 0m0.032s

Now the difference is about 1 ms - probably less than the uncertainty in the timing.

I would appreciate it if others would try this, and see if it matches my findings; but I'm tempted to close this as "works for me".

I would appreciate it if others would try this, and see if it matches my
findings; but I'm tempted to close this as "works for me".

Similar results on Linux with ToT clang and gcc 5-branch.

Closing because no one disputed my "works for me" examples in a year.