GCC and C vs C++ Speed, Measured.

With the imminent release of gcc 4.8, GCC has finally switched to C++ as the implementation language.  As usual, LWN has excellent coverage.  Those with long memories will remember Linux trying to use g++ back in 1992 and retreating in horror at the larger, slower code.  The main benefit was stricter typechecking, particularly for enums (a great idea: I had -Wstrict-enum patches for gcc about 12 years ago, which was a superset of the -Wenum-compare we have now, but never got it merged).

With this in mind, and Ian Taylor’s bold assertion that “The C subset of C++ is as efficient as C”, I wanted to test what had changed with some actual measurements.  So I grabbed gcc 4.7.2 (the last release which could do this), and built it with C and C++ compilers:

  1. ../gcc-4.7.2/configure –prefix=/usr/local/gcc-c –disable-bootstrap –enable-languages=c,c++ –disable-multiarch –disable-multilib
  2. ../gcc-4.7.2/configure –prefix=/usr/local/gcc-cxx –disable-bootstrap –enable-languages=c,c++ –disable-multiarch –disable-multilib –enable-build-with-cxx

The C++-compiled binaries are slightly larger, though that’s mostly debug info:

  1. -rwxr-xr-x 3 rusty rusty 1886551 Mar 18 17:13 /usr/local/gcc-c/bin/gcc
    text       data        bss        dec        hex    filename
    552530       3752       6888     563170      897e2    /usr/local/gcc-c/bin/gcc
  2. -rwxr-xr-x 3 rusty rusty 1956593 Mar 18 17:13 /usr/local/gcc-cxx/bin/gcc
    text       data        bss        dec        hex    filename
    552731       3760       7176     563667      899d3    /usr/local/gcc-cxx/bin/gcc

Then I used them both to compile a clean Linux kernel 10 times:

  1. for i in `seq 10`; do time make -s CC=/usr/local/gcc-c/bin/gcc 2>/dev/null; make -s clean; done
  2. for i in `seq 10`; do time make -s CC=/usr/local/gcc-cxx/bin/gcc 2>/dev/null; make -s clean; done

Using stats –trim-outliers, which throws away best and worse, and we have the times for the remaining 8:

  1. real    14m24.359000-35.107000(25.1521+/-0.62)s
    user    12m50.468000-52.576000(50.912+/-0.23)s
    sys    1m24.921000-27.465000(25.795+/-0.31)s
  2. real    14m27.148000-29.635000(27.8895+/-0.78)s
    user    12m50.428000-52.852000(51.956+/-0.7)s
    sys    1m26.597000-29.274000(27.863+/-0.66)s

So the C++-compiled binaries are measurably slower, though not noticably: it’s about 865 seconds vs 868 seconds, or about .3%.  Even if a kernel compile spends half its time linking, statting, etc, that’s under 1% slowdown.

And it’s perfectly explicable by the larger executable size.  If we strip all the gcc binaries, and do another 10 runs of each (… flash forward to the next day.. oops, powerfail, make that 2 days later):

  1. real    14m24.659000-33.435000(26.1196+/-0.65)s
    user    12m50.032000-57.701000(50.9755+/-0.36)s
    sys    1m26.057000-28.406000(26.863+/-0.36)s
  2. real    14m26.811000-29.284000(27.1308+/-0.17)s
    user    12m51.428000-52.696000(52.156+/-0.39)s
    sys    1m26.157000-27.973000(26.869+/-0.41)s

Now the difference is 0.1%, pretty much in the noise.

Summary: so whether you like C++ or not, the performance argument is moot.