[Interest] building Qt 5.9 on Linux - clang or GCC?
Thiago Macieira
thiago.macieira at intel.com
Mon Dec 18 23:41:25 CET 2017
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
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