Okay, I think I can withdraw the suggestion. It is apparently not providing a stable end performance.
I would like to end with sharing the measurements I made that motivated me to suggest the change. Hopefully it can be useful if tree-slp-vectorize gets improved and the suggestion comes up again. As I said previously, the benchmarks I ran were not affected, probably because most thing we benchmark in Qt often is hand-optimized already, but the binary size with tree-slp-vectorize was on average a one or two percent smaller, though was not universal, and many smaller libraries were unaffected. ---------------------------- gcc-8 version 8.1.0 (Debian 8.1.0-4) gcc-7 version 7.3.0 (Debian 7.3.0-19) Qt version 5.11.0 (edited to override selective use of -O3) size library g++-7 -march=corei7 -O2 8015632 libQt5Widgets.so.5.11.0 6194288 libQt5Gui.so.5.11.0 760016 libQt5DBus.so.5.11.0 5603160 libQt5Core.so.5.11.0 g++-7 -march=corei7 -O2 -ftree-slp-vectorize 8007440 libQt5Widgets.so.5.11.0 6182000 libQt5Gui.so.5.11.0 760016 libQt5DBus.so.5.11.0 5603224 libQt5Core.so.5.11.0 g++-8 -O2 8062520 libQt5Widgets.so.5.11.0 6232160 libQt5Gui.so.5.11.0 765584 libQt5DBus.so.5.11.0 5848528 libQt5Core.so.5.11.0 g++-8 -O2 -ftree-slp-vectorize 8058424 libQt5Widgets.so.5.11.0 6219872 libQt5Gui.so.5.11.0 769680 libQt5DBus.so.5.11.0 5844560 libQt5Core.so.5.11.0 g++-8 -march=corei7 -O2 8062520 libQt5Widgets.so.5.11.0 6215584 libQt5Gui.so.5.11.0 765584 libQt5DBus.so.5.11.0 5844440 libQt5Core.so.5.11.0 g++-8 -march=corei7 -O2 -ftree-slp-vectorize 8046136 libQt5Widgets.so.5.11.0 6191008 libQt5Gui.so.5.11.0 765584 libQt5DBus.so.5.11.0 5840472 libQt5Core.so.5.11.0 g++-8 -march=haswell -O2 8046136 libQt5Widgets.so.5.11.0 6170408 libQt5Gui.so.5.11.0 765584 libQt5DBus.so.5.11.0 5852448 libQt5Core.so.5.11.0 g++-8 -march=haswell -O2 -ftree-slp-vectorize 8046136 libQt5Widgets.so.5.11.0 6158120 libQt5Gui.so.5.11.0 765584 libQt5DBus.so.5.11.0 5848480 libQt5Core.so.5.11.0 g++-8 -march=haswell -Os 6990368 libQt5Widgets.so.5.11.0 5030616 libQt5Gui.so.5.11.0 624160 libQt5DBus.so.5.11.0 4847056 libQt5Core.so.5.11.0 g++-8 -march=haswell -Os -ftree-slp-vectorize 6986272 libQt5Widgets.so.5.11.0 5018328 libQt5Gui.so.5.11.0 624160 libQt5DBus.so.5.11.0 4847120 libQt5Core.so.5.11.0 g++-8 -march=haswell -Os -flto 6785760 libQt5Widgets.so.5.11.0 4844464 libQt5Gui.so.5.11.0 593488 libQt5DBus.so.5.11.0 4688432 libQt5Core.so.5.11.0 g++-8 -march=haswell -Os -flto -ftree-slp-vectorize 6777568 libQt5Widgets.so.5.11.0 4836272 libQt5Gui.so.5.11.0 593488 libQt5DBus.so.5.11.0 4688472 libQt5Core.so.5.11.0
