https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84327
Bug ID: 84327
Summary: Copy pasting the documented optimize flags is not
equal to -O1
Product: gcc
Version: 8.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: xyzdr4gon333 at googlemail dot com
Target Milestone: ---
Created attachment 43392
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43392&action=edit
Program showing that the single optimization flags don't work as well as O1
My program reduces its runtime from 20s to 5s when using -O1. So I wanted to
know which optimization is responsible for that and used the optimizations
flags found here: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
But not even when copy-pasting ALL flags up to -O3 listed there, can I
reproduce the speedup to 5s!
See the attached file which also contains code comments on how I did compile
it.
This seems to be a very long-standing bug (5+ years):
https://stackoverflow.com/questions/12769173/selecting-gcc-optimisation-flags-equivalent-to-o1
https://stackoverflow.com/questions/20246357/gcc-using-o1-and-spelling-the-o1-options-out-leads-to-different-result-one-w
And even after trying to find the difference by using -Q --help=optimizers
which showed this diff:
17d16
< -fdelayed-branch
100a100
> -ftree-builtin-call-dce
Even when adding -ftree-builtin-call-dce I still don't get the same speedup
?!?! In fact nothing changes...
g++ "${O1Flags[@]}" -ftree-builtin-call-dce -std=c++11 optimizeFlags.cpp &&
./a.out
Tested with:
g++ (Debian 7.3.0-3) 7.3.0
g++-8 (Debian 8-20180207-2) 8.0.1 20180207 (experimental) [trunk revision
257435]
