unexpected result with -O2 solved via "volatile"

[Marc Glisse]

On Sun, 19 Sep 2021, Allin Cottrell via Gcc wrote:

> Should this perhaps be considered a bug? Below is a minimal test case for a 
> type of calculation that occurs in my real code. It works as expected when 
> compiled without optimization, but produces what seems like a wrong result 
> when compiled with -O2, using both gcc 10.3.1 20210422 on Fedora and gcc 
> 11.1.0-1 on Arch Linux. I realize there's a newer gcc release but it's not 
> yet available for Arch, and looking at 
> https://gcc.gnu.org/gcc-11/changes.html I didn't see anything to suggest that 
> something relevant has changed.

https://gcc.gnu.org/bugs/ says that you should first try compiling your 
code with -fsanitize=undefined, which tells you at runtime that your code 
is broken.

Apart from that, bug reports should go to https://gcc.gnu.org/bugzilla/ 
and questions to gcc-help@gcc.gnu.org.

-- 
Marc Glisse