Re: HELP!! How to inhibit optimizations applied to .DEFERRED_INIT argument?

[Andrew Pinski <pinskia@gmail.com>]

On Wed, Jun 30, 2021 at 12:14 PM Qing Zhao <qing.zhao@oracle.com> wrote:
>
>
>
> > On Jun 30, 2021, at 1:59 PM, Richard Biener <rguenther@suse.de> wrote:
> >
> > On June 30, 2021 8:07:43 PM GMT+02:00, Qing Zhao <qing.zhao@oracle.com> wrote:
> >>
> >>
> >>> On Jun 30, 2021, at 12:36 PM, Richard Biener <rguenther@suse.de>
> >> wrote:
> >>>
> >>> On June 30, 2021 7:20:18 PM GMT+02:00, Andrew Pinski
> >> <pinskia@gmail.com> wrote:
> >>>> On Wed, Jun 30, 2021 at 8:47 AM Qing Zhao via Gcc-patches
> >>>> <gcc-patches@gcc.gnu.org> wrote:
> >>>>>
> >>>>> I came up with a very simple testing case that can repeat the same
> >>>> issue:
> >>>>>
> >>>>> [qinzhao@localhost gcc]$ cat t.c
> >>>>> extern void bar (int);
> >>>>> void foo (int a)
> >>>>> {
> >>>>> int i;
> >>>>> for (i = 0; i < a; i++) {
> >>>>>   if (__extension__({int size2;
> >>>>>       size2 = 4;
> >>>>>       size2 > 5;}))
> >>>>>   bar (a);
> >>>>> }
> >>>>> }
> >>>>
> >>>> You should show the full dump,
> >>>> What we have is the following:
> >>>>
> >>>>
> >>>>
> >>>> size2_3 = PHI <size2_1(D), size2_13>
> >>>> <bb 3> :
> >>>>
> >>>> size2_12 = .DEFERRED_INIT (size2_3, 2);
> >>>> size2_13 = 4;
> >>>>
> >>>> So CCP decides to propagate 4 into the PHI and then decides
> >> size2_1(D)
> >>>> is undefined so size2_3 is then considered 4 and propagates it into
> >>>> the .DEFERRED_INIT.
> >>>
> >>> Which means the DEFERED_INIT is inserted at the wrong place.
> >>
> >> Then, where is the correct place for “.DEFERRED_INIT(size2,2)?
> >>
> >> The variable “size2” is a block scope variable which is declared inside
> >> the “if” condition:
> >
> > But that's obviously not how it behaves
> > During into SSA phase since we're inserting a PHI for it - and we're inserting it because of the use in the DEFERED_INIT call. I suppose you need to fiddle with the SSA rewrite and avoid treating the use as a use but only for the purpose of inserting PHIs...
>
> Please see my other email on the new small testing case without -ftrivial-auto-var-init.  The same issue in SSA with that testing case even without -ftrivial-auto-var-init.
> It looks like an existing bug to me in SSA.
>
> Let me know if I still miss anything

Yes you missed it is unspecified what the value if the auto variable
is used uninitialized. Isn't that the point of what you are trying to
fix in the first place?
So ccp takes PHI<a_1(D), 4> and says since a_1(D) is uninitialized,
the value is 4.

Thanks,
Andrew

>
> Qing
> >
> > You might be able to construct a testcase which has a use before the real init where then the optimistic CCP propagation will defeat the DEFERED_INIT otherwise.
> >
> > I'd need to play with the actual patch to find a good solution to this problem.
> >
> > Richard.
> >
>