[CMake] Default CMAKE_C_FLAGS value by OS

[Zan Lynx]

On 02/16/2017 08:07 AM, Chuck Atkins wrote:
> That being said, I do think we should revisit the use of O3 by default
> given the safety issues surounding it.

The following is just my opinion:

In my experience if O3 fails it is a bug in the compiler, which should
be fixed and somebody has to find them first. Or a bug in the code being
built with O3, usually something subtle with undefined behavior.

Code bugs that happen to work sort-of correctly under O2 but not O3
aren't really safe. Future compiler versions often include changes that
break the code anyway.

I build a couple of production release C++ projects under several
versions of GCC (Linux releases since 2010). I use O3 and profiling
feedback, which, yes, does reveal some bugs, even up to segfaulting the
compiler, so I have special checks in the Makefile for a few of those
versions.

The thing that makes this safe to do is testing. If the code isn't
working correctly because of O3 I know it because it fails to pass its
tests.

I haven't looked but I would hope CMake has tests that are run against
the optimized code...
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