[PATCH] testsuite/102690 - XFAIL g++.dg/warn/Warray-bounds-16.C

Martin Sebor msebor@gmail.com
Thu Nov 11 23:18:20 GMT 2021


On 11/11/21 1:18 AM, Richard Biener wrote:
> On Wed, 10 Nov 2021, Martin Sebor wrote:
> 
>> On 11/10/21 3:09 AM, Richard Biener via Gcc-patches wrote:
>>> This XFAILs the bogus diagnostic test and rectifies the expectation
>>> on the optimization.
>>>
>>> Tested on x86_64-unknown-linux-gnu, pushed.
>>>
>>> 2021-11-10  Richard Biener  <rguenther@suse.de>
>>>
>>>   PR testsuite/102690
>>>   * g++.dg/warn/Warray-bounds-16.C: XFAIL diagnostic part
>>>   and optimization.
>>> ---
>>>    gcc/testsuite/g++.dg/warn/Warray-bounds-16.C | 6 +++---
>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
>>> b/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
>>> index 17b4d0d194e..89cbadb91c7 100644
>>> --- a/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
>>> +++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
>>> @@ -19,11 +19,11 @@ struct S
>>>        p = (int*) new unsigned char [sizeof (int) * m];
>>>    
>>>        for (int i = 0; i < m; i++)
>>> -      new (p + i) int ();
>>> +      new (p + i) int (); /* { dg-bogus "bounds" "pr102690" { xfail *-*-* }
>>> } */
>>>      }
>>>    };
>>>    
>>>    S a (0);
>>>    
>>> -/* Verify the loop has been eliminated.
>>> -   { dg-final { scan-tree-dump-not "goto" "optimized" } } */
>>> +/* The loop cannot be eliminated since the global 'new' can change 'm'.  */
>>
>> I don't understand this comment.  Can you please explain how
>> the global operator new (i.e., the one outside the loop below)
>> can change the member of the class whose ctor calls the new?
>>
>> The member, or more precisely the enclosing object, doesn't
>> yet exist at the time the global new is called because its
>> ctor hasn't finished, so nothing outside the ctor can access
>> it.  A pointer to the S under construction can be used (and
>> could be accessed by a replacement new) but it cannot be
>> dereferenced to access its members because the object it
>> points to doesn't exist until after the ctor completes.
> 
> Yes, that's the C++ legalise - which is why I XFAILed that
> part of the test rather than just removed it.  The middle-end
> sees the object *this as existing and being global, thus
> accessible and mutable by '::new' which when replaced by
> the user could access and alter *this.  Like maybe for
> 
> S s;
> 
> void *operator new(..) { s.m = 0; }
> 
> main()
> {
>    new (&s) (1);
> }
> 
> that may be invalid C++ but this detail of C++ is not
> reflected in the GIMPLE IL.  Before the change that regressed
> this if S::S() would call a global function foo() instead
> of new to do the allocation the behavior would be as after
> the change.  Isn't the call to new or foo part of the
> construction and as such obviously allowed to access
> and alter the in-construction object?

Here's my understanding.

The lifetime of an object ends when its storage is reused to
create another object of a class type, and the lifetime of
the other object begins when its initialization is complete.

In the window between the new ctor starting and completing
the new object can be accessed in limited ways, but not
the old object.  (If it were otherwise, imagine the new
object's ctor throwing in the middle of constructing the new
object.  What state would that leave the old object in?  Which
members could still be accessed?)

The only way to access a subobject of the new object while
it's still under construction is through a pointer to that
subobject or through this (or a pointer derive from this).

It seems that it should be possible to capture the constraint
in the middle end that no member of an object under construction
can be accessed unless a pointer to it has escaped that's derived
from the this pointer.

But this seems like a sufficiently obscure case that an expert
on the C++ object model should confirm it.

Martin

> 
>> I copy the test below:
>>
>> inline void* operator new (__SIZE_TYPE__, void * v)
>> {
>>    return v;
>> }
>>
>> struct S
>> {
>>    int* p;
>>    int m;
>>
>>    S (int i)
>>    {
>>      m = i;
>>      p = (int*) new unsigned char [sizeof (int) * m];
>>
>>      for (int i = 0; i < m; i++)
>>        new (p + i) int (); /* { dg-bogus "bounds" "pr102690" { xfail *-*-* } }
>> */
>>    }
>> };
>>
>> S a (0);
>>
>> Thanks
>> Martin
>>
> 



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