Compilation speed notes

Compilation speed notes

[Paul Smith]

Hi all; thanks for the help recently with sysroot compilers.  I was
able to get things building following advice from Joseph and Jonathan.

I did want to mention that I noticed that GCC got non-trivially slower
between my previous version (8.1) and this new version (9.3).

I have a decently-sized C++14 or so code base, configured to use pre-
compiled headers and unity builds (we pass 20 .cpp files to each
invocation of the compiler).  The system is GNU/Linux with 8 cores and
32G memory, on an older but still usable Intel Xeon E3-1270 3.5GHz and
I use "make -j8" to build it.

I built it 5 times in a row (although I can't believe caching would be
an issue with code this size) with a git clean before each build, and
no ccache or anything.

The GCC 8.1 builds took about 14m35s.

The GCC 9.3 builds took about 16m40s, so that's a drop of about 13%.

I used the same binutils for both.  Both compiles used the same compile
options of course; in particular they used -ggdb3 -O2.

I did discover that if I build GCC 9.3 with profiledbootstrap-lean, I
gained back that loss and got the compile times back down to almost the
same as GCC 8.1... but I assume that if I'd built GCC 8.1 with
profiledbootstrap then that would have been even faster.

Note I haven't tried to compare the performance of the resulting code
(yet...)  This is just looking at compile speed.


Anyway I know that detail-free messages like this are more annoying
than helpful and I don't mean to be annoying.  If anyone wants to ask
for more details or for me to do more testing I'm amenable.  Otherwise
this is just one data point to consider, or not.

Thanks for the new release!