https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84658
--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Martin Liška from comment #14)
> (In reply to Richard Biener from comment #13)
> > So you can loses the TREE_ADDRESSABLE restriction somewhat in requiring at
> > most one decl to be TREE_ADDRESSABLE - that's the one you need to keep, the
> > others may become aliases. Given TREE_ADDRESSABLE isn't reliable for
> > !TREE_STATIC vars
> > we can't merge any exported decls that way. Consider
> >
> > const int foo1;
> > const int foo2;
> >
> > int *bar(); // in other TU, returns address of foo1
> >
> > int main ()
> > {
> > if (bar() != &foo2)
> > ...;
> > }
> >
> > not sure if we want to do hand-waving saying that we can't possibly have
> > points-to sets mentioning those without seeing the function IL. Well,
> > might be similarly hand-waving as the reference aliasing issue.
> >
> > So we could drop the TREE_STATIC restriction if TREE_ADDRESSABLE is reliable
> > within the current TU.
>
> Note that this issue happens only with -fmerge-all-constants. And we have
> caveat
> in documentation that using the option, pointer comparison can be broken:
>
> Languages like C or C++ require each variable,
> including multiple instances of the same variable in recursive
> calls, to have distinct locations, so using this option results in
> non-conforming behavior.
>
> Thus:
>
> $ cat test.c
> const int foo1 = 3;
> const int foo2 = 3;
>
> const int *
> __attribute__((noinline))
> bar()
> {
> return &foo1;
> }
>
> int main ()
> {
> if ((bar() - &foo2) == 0)
> __builtin_abort ();
>
> return 0;
> }
>
> $ gcc test.c -O2 -fno-ipa-icf -fmerge-all-constants && ./a.out
> Aborted (core dumped)
>
> That said, would be Richi your comment in c#12 a sufficient fix?
Hum, isn't the bug INVALID then? Shouldn't we restrict -fmerge-all-constants
to not address-taken variables to not make the option useless?
The suggested fix in c#12 is a band-aid only.
What's the impact of not merging TREE_ADDRESSABLE decls into other decls?
I presume -fmerge-all-constants pre-dates ICF?
