[CMake] CMake adds -qalias=noansi by for XLC

[Todd Gamblin]

Hi,

I noticed while poking around CMake's compiler modules that the defaults for
XL are kind of inconsistent.  Specifically, If you look here:

https://github.com/Kitware/CMake/blob/master/Modules/Compiler/XL-C.cmake

CMake adds -qalias=noansi by default, which prevents a bunch of
optimizations.  CMake does not add that for XL C++.  It also doesn't tweak
any default aliasing settings for other compilers.  Why XLC?

At LLNL, we don't like qalias=noansi because it prevents a lot of
optimizations, so we typically recommend that users NOT use that flag.  It's
been surprising for us that CMake adds it by default when it's not a default
for XL, which uses -qalias=ansi (type-based aliasing) by default.

Is there some reason CMake adds it?  Could it be removed in future versions
for consistency with the C++ settings?  I can push a feature branch if
people agree that this is a good idea.

-Todd


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