https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107323
Bug ID: 107323
Summary: Problem with GCC optimizition
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: fatemetmhr at gmail dot com
Target Milestone: ---
Created attachment 53732
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53732&action=edit
The code
Hi!
I faced a problem with GCC optimizitions on codeforces.com. I used GNU G++20
11.2.0 (64 bit, winlibs) for my code but the bug can be found in GCC 11.2.0
anywhere else (and even with some other versions of GCC).
Here's the compilation command used in Codeforces:
g++ -Wall -Wextra -Wconversion -static -DONLINE_JUDGE -Wl,--stack=268435456 -O2
-std=c++20 <source>
There's no error or warning while compiling the code.
The code has different results while using GCC 11.2.0 and GCC 9.2.0.
There's a problem with -ftree-loop-distribute-patterns. The flag made GCC
generate a wrong code, including a memset after the `for` part ends, so the
variables will be set to 0 at the end of the process.
This flag is obviously doing wrong while optimizing the code. And, somehow, it
can be fixed while using -ftree-vectorize. Which isn't turned on automatically
in -O2 with GCC 11.2.0 .
Anyway some other versions of GCC (like 12) seems to still have the problem
with -ftree-loop-distribute-patterns, but just because of using
-ftree-vectorize, they works correctly.
I posted many other codes here, using different optimizitions flags:
https://codeforces.com/blog/entry/108168
