GCC 5 optimizer produces incorrect code

[Egor Suvorov]

Inspired by the following blog post: http://codeforces.com/blog/entry/50291

Attached file is a snippet of code which, when compiled with `g++ a.cpp` produces an executable `a.out`, which runs correctly. But when compiled with `g++ a.cpp -O1` the resulting executable crashes in line 10 (according to GDB). Looks like array out-of-bounds error happens despite corresponding check being present in the code.

We was able to reproduce it on two independent installations of Ubuntu 16.04, compiler version is `gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)` (looks like it's the most recent package available as of February 7, 2017).

Steps to reproduce:
1. Download `a.cpp` attached.
2. Run `g++ a.cpp -O1` and ensure that it finished without any errors.
3. You can optionally run `g++ a.cpp -O1 -Wall -Wextra -Werror -pedantic` and ensure that this command finishes without any errors as well.
4. Run `./a.out`.

Expected behavior: program finishes correctly.

Real behavior: program crashes with segmentation fault.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Egor Suvorov (yeputons)]

Sidenote: we were unable to reproduce that bug on:
1. Ideone (they claim to use GCC 5.1, but do not specify which flavor it is).
2. tdm-gcc 5.1.0 on Windows.
3. gcc version 6.1.0 (Gentoo 6.1.0 p1.0)

And we were able to reproduce it on (in addition to what's mentioned in the report):
1. g++ (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
2. gcc version 5.4.1 20160904 (Ubuntu 5.4.1-2ubuntu1~14.04)
3. gcc version 6.2.0 20160901 (Ubuntu 6.2.0-3ubuntu11~14.04)

That's why it's submitted to Ubuntu's bugtracker first.

[Matthias Klose (doko)]

fixed in gcc-7