[Interest] building Qt 4.8.7 with gcc 5 and link-time optimisation on Linux

René J. V. Bertin rjvbertin at gmail.com
Wed Jul 22 23:20:40 CEST 2015

Thiago Macieira wrote:

> $ git name-rev --tags 1bd27f24fb7f7d82f4b67cffe43e0af644709106
> 1bd27f24fb7f7d82f4b67cffe43e0af644709106 tags/v5.4.0-alpha1~525
> It should work on 5.4 then. The fix for static libraries came with 5.4.1,
> though.

Ha, indeed, it's a little bit more complicated when doing out-of-source builds.
With an explicit reference to the .pro file -config was accepted, but I'm not 
yet sure that it actually changed anything when executed in a build directory on 
my Mac that had already seen a full build. It looks like all code was being 
recompiled, but I didn't see any flto options except in the 1st link command, 
which probably explains why it told me

LLVM ERROR: Cannot select: intrinsic %llvm.x86.sse42.crc32.64.64

(I'm using -march=native on an i7).

Is it possible to negate/cancel/revert the effect of qmake -config ltcg? :)

> Check if you have mkspecs/features/ltcg.prf.

That one actually exists in Qt 4.8.7 too ...

> It takes noticeably longer to compile :-)

And even longer to link? ;)

> the use of -ffunction-sections -Wl,--gc-sections. The libraries are bigger,
> but that might be due to my use of -O3 (causes code expansion).

I'd think so; my initial impression with Qt 4.8.7 is that the difference is 

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