Optimizing gcc for fastest build times

One of the perks of building gcc from source is that you get to customize it to your needs. For example you get to choose the level of optimization with which gcc itself is compiled. So how much of a difference does an optimized build make?

The contestants

For this comparison I built gcc’s trunk (revision 231276, last modified on 12/4/2015) as follows ¹:

default build of gcc:

../gcc/configure \
  --prefix=/usr/gcc-trunk \
  --enable-languages=c,c++,fortran \
  --disable-bootstrap \
  --disable-libquadmath --disable-libquadmath-support \
  --disable-werror
make -j4
make install

default gcc with gold linker

../gcc/configure \
  --prefix=/usr/gcc-trunk-gold \
  --enable-languages=c,c++,fortran \
  --disable-werror \
  --disable-bootstrap \
  --disable-libquadmath --disable-libquadmath-support \
  --enable-gold
make -j4
make install

profile guided optimized + gold linker

I use the profiledbootstrap target to get a build of gcc with profile guided optimization. This target gathers profiling information during one of the bootstrap stages and uses that in the final build stage that produces the compiler executables. This obviously means that we cannot disable the bootstrapping process. The following build takes quite a bit of time (a few hours typically):

../gcc/configure \
  --prefix=/usr/gcc-trunk-pg-gold \
  --enable-languages=c,c++,fortran \
  --disable-werror \
  --disable-libquadmath --disable-libquadmath-support \
  --enable-gold
make -j4 profiledbootstrap
make install

Results

The code base consists of about 300k lines of code (as measured with David A. Wheeler’s ‘SLOCCount’, http://www.dwheeler.com/sloccount/). It is moderately templated and links against a decent number of third party libraries including boost, trilinos, hdf5, etc.

The results were

• default build with ld linker: 58m50s
• default build with gold linker: 58m36s
• profile guided optimization + gold linker: 48m33s

So profile guided optimization shaves off about 10 minutes, or nearly 20 percent. That’s more than I would have expected. Pretty good given that you get that speedup for free.

At first I thought the build time difference between the ld build and the gold build might be noise. To test that hypothesis I relinked the executables in the project. The link times were

• default build with ld linker: 52s
• default build with gold linker: 35s
• profile guided optimization + gold linker: 35s

So that’s consistent with the timing difference we saw for the full builds. gold shaves off about 30 percent from the link times.