[Bug middle-end/83215] C++: struct with char-array assumed to alias with everything

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83215

Richard Biener <rguenth at gcc dot gnu.org> changed:

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

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
The key here is that "mytest" has alias-set zero.  It has that because
aggregate copying of it has to transfer whatever is in 'buf'.  But if "mytest"
has alias-set zero that means any component of it has that as well.

So this correctness fix is really tied to how we need to handle aggregate
copying
of structs.

I don't like that (and I simply blame the C++ standards commitee).  It's a
really bad choice for optimization.

Note making the C++ FE do sth "special" for aggregate copying works only
sofar as the middle-end also sometimes generates such copying (argument
copying during inlining, etc).