> On Jul 1, 2021, at 1:48 AM, Richard Biener <richard.guent...@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 <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.
>
> 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.
>>>
>>
