[PATCH] c++: Consider addresses of heap artificial vars always non-NULL [PR98988]

[Jakub Jelinek]

On Tue, Feb 09, 2021 at 03:00:20PM -0500, Jason Merrill wrote:
> > With -fno-delete-null-pointer-checks which is e.g. implied by
> > -fsanitize=undefined or default on some embedded targets, the middle-end
> > folder doesn't consider addresses of global VAR_DECLs to be non-NULL, as one
> > of them could have address 0.
> 
> Hmm, are these VAR_DECLs going into the symtab?  That seems undesirable.

They are not in the symtab from the C++ FE.  And we don't allow those VAR_DECLs or their
addresses to leak into the IL anyway.
But, they are TREE_STATIC because pretending they are automatic didn't
really work (e.g. address of automatic variables is not a constant expression).
The generic folding code uses maybe_nonzero_address predicate which
apprently creates a symtab entry for them but as nothing uses them they
won't be emitted.
If we wanted to avoid that, we'd need to either add some generic VAR_DECL
bit for those vars or add a target hook that would return true for them and
then handle them in the middle-end.

	Jakub