On Wednesday 22 July 2015 19:50:27 René J.V. Bertin wrote:
> Hi,
>
> I'm experimenting with gcc-5.1 and link-time optimisation on Linux. I
> configure with
>
> CC = gcc-5
> CXX = g++-5
> CFLAGS += -O3 -ftracer -ftree-vectorize -flto -fuse-linker-plugin
> CXXFLAGS += -O3 -ftracer -ftree-vectorize -flto -fuse-linker-plugin
> LDFLAGS += -O3 -ftracer -ftree-vectorize -flto -fuse-linker-plugin
>
> and get as far as libQtScript which fails to link:
No need to look further. Neither QtScript nor QtWebKit will link with LTO.
Turn it off for those two.
The problem is the same: it's the SquirrelFishExtreme JIT code and their use
of inline assembly. The symbols created by the inline assembly aren't visible
to the LTO code generator.
> Some quick googling only leads to the impression that building with LTO is
> supported (even suggested as a means to reduce start-up times), but I
> haven't yet found any explicit instructions on how to activate the feature.
Yes, for all the other modules, but not those two.
Also, for QtGui (in both Qt 4 and 5), you need to be careful with the code for
SSE2 and SSSE3. Until I applied certain fixes to Qt 5, those would cause the
*entire* *build* to suddenly require SSSE3. Or fail to compile, depending on
the order of the files.
Note that this was GCC 4.8 and it has since got better at LTO. Maybe those
problems won't affect you, but I'd verify that QtGui isn't using unprotected
SSSE3 instructions.
> Am I doing something wrong (missing an option to prevent the discarding of
> .text sections maybe)?
>
> If this were a local build I'd just skip the LTO for QtScript, but the build
> is to be handled by a LauncPad PPA ....
Add to script.pro:
QMAKE_CXXFLAGS -= -flto
As for Qt 5, don't change CXXFLAGS. Instead, just rerun qmake and ask it to do
it the right way:
qtbase:
./configure [...]
qmake -config ltcg
make
other modules (besides qtscript and qtwebkit):
qmake -config ltcg
make
qmake propagates the flag to sub instances when it creates their Makefiles.
ltcg.prf will properly pass the compiler optimisation flags to the linker, will
pass -fno-fat-lto to GCC for dynamic library builds, will use the proper ar
wrapper to create static libraries, it will activate link_ltcg.prf so linking
to LTO static libraries works, etc.
ltcg.prf also supports Clang LTO, ICC's IPO and MSVC LTCG. You don't want to
use ICC IPO unless you have at least 32 GB of RAM, though.
Does it work? Yes, my GCC release build of Qt has been done with LTCG for two
years.
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
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