https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61619
Bug ID: 61619
Summary: Benefits from -ftree-vectorize lost easily when
changing unrelated code
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: shmueller2 at gmail dot com
Created attachment 33011
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33011&action=edit
Uncommenting (1), (2) or (3) significantly accelerates this code
When compiling the attached code with:
g++-4.9 -std=c++11 -O3 -Wall -Wextra -pedantic bug.cpp -o bug
I found that minor changes in seemingly irrelevant aspects of the code had a
strong effect on performance. When run with:
time ./bug
the code as attached gave the following best-of-10 timing on a 2011 Macbook
Air:
real 0m1.718s
user 0m1.395s
sys 0m0.306s
Minor changes by uncommenting any of the lines marked with (1), (2), (3)
(replacing the line immediately above) yielded significantly better results:
Uncomment line (1):
real 0m1.343s
user 0m1.029s
sys 0m0.312s
Uncomment line (2):
real 0m1.364s
user 0m1.062s
sys 0m0.297s
Uncomment line (3):
real 0m1.332s
user 0m1.016s
sys 0m0.315s
The generated assembly code (-S) differs significantly in all cases. When using
-fno-tree-vectorize
the performance is similar to the first (slow) result for all variations.
The bug I'm reporting is that the optimization benefits from -ftree-vectorize
are apparently lost easily and non-transparently when changing seemingly
unrelated parts of the code on a high level, which should not affect
performance.
I would have expected that none of the changes (1), (2) and (3) would have
resulted in a difference in the generated assembly code, and it was very
surprising to me that such details mattered so much.
