[Interest] building Qt 5.9 on Linux - clang or GCC?

[Thiago Macieira]

On Monday, 18 December 2017 12:13:42 PST Allan Sandfeld Jensen wrote:
> It is not really much slower anymore
> though. It used to be slower because the final compilation was all in a
> single process and single threaded, but that part is now multithreaded.

>From experience, it's *not* slower.

It's only slower if you forget to pass the number of threads to use during 
linking. When I was creating the numbers for my other email, I had forgotten 
that (I was modifying the Makefile manually), leading to increased build 
times. Once corrected, the build times were faster.

The drawback is that if you have 4 processors and link two libraries at the 
same time with -flto=4, they'll both compete for resources. CPU time is not 
the issue, but it could lead to swapping to keep both optimisers in memory. 
That could happen for QtNetwork and QtGui, for example, since they are built 
soon after QtCore.

There's -flto=observer to work with the GNU make jobserver, but that requires 
changes to the Makefile itself and that causes a lot use-cases with make to 
break, so it's not an option.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center