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From: "trippels at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/64953] Compiling sourcecode for STM32F103 causes USB errors with some optimization settings
Date: Thu, 05 Feb 2015 19:44:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64953

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |trippels at gcc dot gnu.org

--- Comment #2 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
(In reply to manuel.reimer from comment #0)
> After some trying I've found out that the key in this case is the
> optimization setting. If I use "-O3", then the resulting binary works again
> if I use the 4.9.2 compiler.
> 
> So "something" is wrong with the optimization.
> 
> Can someone give me a hint about what changed between 4.8 and 4.9.2? Is this
> a bug in the gcc compiler or do I just need some new "compiler switch" to
> disable "some new feature" and get a valid binary, again?

Try to narrow the issue down to a single source file. Once you have found
the file in question, you may narrow the problem down to a single function
by using __attribute__((optimize("-Os"))).

You could also try to build the application with -fsanitize=undefined
and see if any issue pops up.