Re: Enabling -ftree-slp-vectorize on -O2/Os

[Allan Sandfeld Jensen <linux at carewolf dot com>]

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