[libcxx-dev] Fwd: libcxx performance issue

[Yan Yichen via libcxx-dev]

Sorry for missing that, it's one (randomly selected) STL #include <vector> with an empty main function as below:
$ perf stat -a -- clang++ -o /dev/null --compile -std=c++11  test.cc<http://test.cc>

 Performance counter stats for 'system wide':

       6286.898000      cpu-clock (msec)          #    7.984 CPUs utilized
             1,029      context-switches          #    0.164 K/sec
                23      cpu-migrations            #    0.004 K/sec
             5,069      page-faults               #    0.806 K/sec
     3,359,820,123      cycles                    #    0.534 GHz
     3,011,112,580      instructions              #    0.90  insn per cycle
       565,470,025      branches                  #   89.944 M/sec
         3,667,572      branch-misses             #    0.65% of all branches

       0.787455804 seconds time elapsed

$ perf stat -a -- clang++ -o /dev/null --compile -std=c++11 -stdlib=libc++ test.cc<http://test.cc>

 Performance counter stats for 'system wide':

      20095.100787      cpu-clock (msec)          #    7.995 CPUs utilized
             1,128      context-switches          #    0.056 K/sec
                44      cpu-migrations            #    0.002 K/sec
             7,716      page-faults               #    0.384 K/sec
    11,861,997,273      cycles                    #    0.590 GHz
    10,930,520,024      instructions              #    0.92  insn per cycle
     2,064,359,090      branches                  #  102.729 M/sec
        10,959,608      branch-misses             #    0.53% of all branches

       2.513417717 seconds time elapsed

$ clang++ -o libstdcxx.o -std=c++11 test.cc<http://test.cc>
$ clang++ -o libcxx.o -std=c++11 -stdlib=libc++ test.cc<http://test.cc>

$ ldd libstdcxx.o
        linux-vdso.so.1 (0x00007fff3efc7000)
        libstdc++.so.6 => /home/yiyan/.local/lib64/libstdc++.so.6 (0x00007fb320d6f000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb3209d1000)
        libgcc_s.so.1 => /home/yiyan/.local/lib64/libgcc_s.so.1 (0x00007fb3207b9000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb3203c8000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fb32114e000)

$ ldd libcxx.o
        linux-vdso.so.1 (0x00007fffd8fe4000)
        libc++.so.1 => /home/yiyan/.local/lib/libc++.so.1 (0x00007f01bbcba000)
        libc++abi.so.1 => /home/yiyan/.local/lib/libc++abi.so.1 (0x00007f01bba65000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f01bb6c7000)
        libgcc_s.so.1 => /home/yiyan/.local/lib64/libgcc_s.so.1 (0x00007f01bb4af000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f01bb0be000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f01bae9f000)
        librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f01bac97000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f01bbfc8000)

$ cat test.cc<http://test.cc>
#include <vector>

int main() {}

Yichen

On Aug 25, 2019, at 10:47 PM, Eric Fiselier <eric at efcs.ca<mailto:eric at efcs.ca>> wrote:

You would need to tell us what you're actually compiling before we can guess at the problem.

/Eric

On Sat, Aug 24, 2019 at 2:28 PM Yan Yichen via libcxx-dev <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>> wrote:
Dear developers,
  Sorry to bother you, but I’ve met a wired problem using clang with libcxx.
  I’ve built clang and libcxx with clang, and trying to compile c++ program with libc++, but compilation is extremely slow when using libcxx, as below:

$ time clang++ -o /dev/null -std=c++11 -stdlib=libc++ test.cc<http://test.cc>
clang++ -o /dev/null -std=c++11 -stdlib=libc++ test.cc<http://test.cc>  4.29s user 0.06s system 97% cpu 4.476 total
$ time clang++ -o /dev/null -std=c++11 test.cc<http://test.cc>
clang++ -o /dev/null -std=c++11 test.cc<http://test.cc>  1.41s user 0.06s system 92% cpu 1.582 total

  I wonder if you have any idea where should I start to address this issue, or anything I probably configured wrong? Thanks in advanced.

Best,
Yichen
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