> On Jun 30, 2021, at 1:59 PM, Richard Biener <rguent...@suse.de> wrote:
>
> On June 30, 2021 8:07:43 PM GMT+02:00, Qing Zhao <qing.z...@oracle.com> wrote:
>>
>>
>>> On Jun 30, 2021, at 12:36 PM, Richard Biener <rguent...@suse.de>
>> wrote:
>>>
>>> On June 30, 2021 7:20:18 PM GMT+02:00, Andrew Pinski
>> <pins...@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
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.
>
