On Fri, Oct 20, 2023 at 1:48 PM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi, I'm trying to make pass_fre work better for me. But I got a
> problem. Like the code below:
>
> int glob;
>
> void __attribute__((oninline))
> foo() {
> // do nothing meaningful
> }
>
> int main() {
> if (glob) {
> foo();
> } else {
> // do something that won't change glob
> }
>
> if (glob) {
> foo();
> } else {
> // do something that won't change glob
> }
> }
>
> I'm trying to merge the two if blocks. And of course it won't work. I
> see the code in tree-ssa-structalias.cc, it will do this check:
>
> static bool
> pt_solution_includes_1 (struct pt_solution *pt, const_tree decl) {
> // xxxx
> if (pt->nonlocal
> && is_global_var (decl))
> return true;
> // xxxx
> }
>
> So the pt->nonlocal will prevent the merge. I would like to ask if I
> can make this check less rigid by providing more information about
> function foo(). Because obviously the foo function does not change the
> value of glob here, but it is caused by the fact that it is noinline.
>
> So if I can prove that foo won't change glob and pass this infomation
> to this check, my goal was achieved. Is this possible?
In case 'glob' were static IPA reference has this info and we'd already
use it from call_may_clobber_ref_p. There's IPA mod-ref which is
a bit more powerful than IPA reference but I think it does not record
this precise info.
Note that the problem with non-static globals is that we do not know
whether they get modified indirectly because they might have their
address take in another TU and that address passed back into the TU.
Usually using LTO helps here and we can promote the decl to hidden
visibility.
Richard.
>
> Thanks
> Hanke Zhang
