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

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

> On Jul 1, 2021, at 1:48 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Wed, Jun 30, 2021 at 9:15 PM Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> 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.
> 
> It's not a bug in SSA, it's at most a missed optimization there.

I still think that SSA cannot handle block-scoped variable correctly in this case, it should not move the variable out side of the block scope. -:)

>  But
> with -ftrivial-auto-var-init it
> becomes a correctness issue.  I think the idea avoiding the USE of the
> variable in .DEFERRED_INIT
> and instead passing the init size is a good one and should avoid this
> case (hopefully).


Agreed, I have made such change yesterday, and this work around the current issue.

temp = .DEFERRED_INIT (temp/WITH_SIZE_EXPR(temp), init_type)

To:

temp = .DEFERRED_INIT (SIZE_of_temp, init_type)

Thanks.

Qing
> 
> Richard.
> 
>> 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.
>>> 
>>