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

[Qing Zhao <qing.zhao@oracle.com>]

> 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 

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.
>