https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105363
Bug ID: 105363
Summary: -ftree-slp-vectorize decreases performance
significantly (x64)
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: mtzguido at gmail dot com
Target Milestone: ---
Created attachment 52857
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52857&action=edit
Source file and outputs
Hello,
I found this example where using -O2 (which implies -ftree-slp-vectorize)
decreases performance by about 4x wrt -O1. I've pinned it down to the
-ftree-slp-vectorize, and -O3 -fno-tree-slp-vectorize works very well.
$ gcc bug_opt.c -O3 -o bug_opt-O3
$ time ./bug_opt-O3
real 0m6.627s
user 0m6.619s
sys 0m0.005s
$ gcc bug_opt.c -O3 -fno-tree-slp-vectorize -o bug_opt-O3-novec
$ time ./bug_opt-O3-novec
real 0m1.703s
user 0m1.701s
sys 0m0.000s
I've verified this with the current HEAD (1ceddd7497) and with 11.2 (though in
that version -O2 does not imply -ftree-slp-vectorize, so the problem starts to
appear at -O3).
I've minimized the example into a pretty basic insertion sort.
I have not checked the generated assembly.
I'm attaching the .c source, which has some more comments with timings. Also
attaching my /proc/cpuinfo, and the temp files generated with -O3. I imagine
the .o and binary is not too helpful, but can send them if needed.
Thanks,
Guido
