[cmake-developers] CMake adds -qalias=noansi by XLC

Todd Gamblin tgamblin at llnl.gov
Wed Jul 1 12:14:39 EDT 2015


Hi,

I asked this over on the regular cmake list and got no answer, so maybe
developers is the right place for it.

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.

So, two questions:
- Is there some reason CMake adds the -qalias=noansi arg for XLC?
- Could it be removed in future versions for consistency with
other compilers' C++ settings?

I can push a feature branch if people agree that this is a good idea.

-Todd



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