Re: [PATCH 1/2] Handle component_ref to a structre/union field including flexible array member [PR101832]

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

> On Feb 3, 2023, at 2:49 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Thu, 2 Feb 2023, Qing Zhao wrote:
> 
>> 
>> 
>>> On Feb 2, 2023, at 8:54 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Thu, 2 Feb 2023, Qing Zhao wrote:
>>> 
>>>> 
>>>> 
> 
> [...]
> 
>>>>>>>> +	return flexible_size_type_p (TREE_TYPE (last));
>>>>>>> 
>>>>>>> For types with many members this can become quite slow (IIRC we had
>>>>>>> bugs about similar walks of all fields in types), and this function
>>>>>>> looks like it's invoked multiple times on the same type per TU.
>>>>>>> 
>>>>>>> In principle the property is fixed at the time we lay out a record
>>>>>>> type, so we might want to compute it at that time and record the
>>>>>>> result.
>>>>>> 
>>>>>> You mean in FE? 
>>>>> 
>>>>> Yes, either in the frontend or in the middle-ends layout_type.
>>>>> 
>>>>>> Yes, that?s better and cleaner.
>>>>>> 
>>>>>> I will add one more field in the TYPE structure to record this information and check this field during middle end.
>>>>>> 
>>>>>> I had the same thought in the beginning, but not sure whether adding a 
>>>>>> new field in IR is necessary or not, other places in middle end might 
>>>>>> not use this new field.
>>>>> 
>>>>> It might be interesting to search for other code walking all fields of
>>>>> a type to determine this or similar info.
>>>> 
>>>> There is one which is defined in tree.cc but only is referenced in c/c-decl.cc:
>>>> 
>>>> /* Determine whether TYPE is a structure with a flexible array member,
>>>>  or a union containing such a structure (possibly recursively).  */
>>>> flexible_array_type_p
>>>> 
>>>> However, this routine is a little different than the one I tried to add:
>>>> 
>>>> In the current routine ?flexible_array_type_p?,  only one level nesting in the structure is accepted, multiple nesting in structure is not permitted.
>>>> 
>>>> So, my question is:  shall we accept multiple nesting in structure? i.e.
>>> 
>>> If we don't reject the testcase with an error, then yes.
>> 
>> Gcc currently accepts the multiple nesting in structure without error.  
>> So, we will continue to accept such extension as long as the flex array 
>> is at the end of the structure. At the same time, for the case the flex 
>> array is in the middle of the structure, issue additional warnings now 
>> to discourage such usage, and deprecate this case in a future release.
>> 
>> Does this sound reasonable? 
> 
> Please don't mix several issues - I think the flex array in the
> middle of a structure is separate and we shouldn't report that
> as flexible_array_type_p or flexible_size_type_p since the size
> of the containing structure is not variable.
Agreed on this.

My major question here is (for documentation change, sorry for mixing this thread with the documentation change): do we need to document this case together with the case in which struct with flex array is embedded into another structure? (As a GCC extension?)
> 
> For diagnostic purposes the intended use case is to treat
> a pointer to a structure that appears to have a fixed size
> but has (recursive) a member with a flexible array at the end
> as having variable size.  Just the same as array_at_struct_end_p
> treats this for the case of accesses involving such a type.

Yes. 
> 
> For the middle position case that's not the case.
Yes. 

Thanks.

Qing
> 
> Richard.
> 
>> Qing
>>> 
>>>> struct A {
>>>> int n;
>>>> char data[];/* Content following header */
>>>> };
>>>> 
>>>> struct B {
>>>> int m;
>>>> struct A a;
>>>> };
>>>> 
>>>> struct C {
>>>> int q;
>>>> struct B b;
>>>> };
>>>> 
>>>> Qing
>>>>> 
>>>>>> thanks.
>>>>>> 
>>>>>> Qing
>>>>>> 
>>>>>>> 
>>>>>>>> +      return false;
>>>>>>>> +    case UNION_TYPE:
>>>>>>>> +      for (x = TYPE_FIELDS (type); x != NULL_TREE; x = DECL_CHAIN (x))
>>>>>>>> +	{
>>>>>>>> +	  if (TREE_CODE (x) == FIELD_DECL
>>>>>>>> +	      && flexible_array_type_p (TREE_TYPE (x)))
>>>>>>>> +	    return true;
>>>>>>>> +	}
>>>>>>>> +      return false;
>>>>>>>> +    default:
>>>>>>>> +      return false;
>>>>>>>> +  }
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> /* Compute __builtin_object_size for PTR, which is a ADDR_EXPR.
>>>>>>>> OBJECT_SIZE_TYPE is the second argument from __builtin_object_size.
>>>>>>>> If unknown, return size_unknown (object_size_type).  */
>>>>>>>> @@ -633,45 +669,68 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>>>>>>>> 		    v = NULL_TREE;
>>>>>>>> 		    break;
>>>>>>>> 		  case COMPONENT_REF:
>>>>>>>> -		    if (TREE_CODE (TREE_TYPE (v)) != ARRAY_TYPE)
>>>>>>>> +		    /* When the ref is not to an array, a record or a union, it
>>>>>>>> +		       will not have flexible size, compute the object size
>>>>>>>> +		       directly.  */
>>>>>>>> +		    if ((TREE_CODE (TREE_TYPE (v)) != ARRAY_TYPE)
>>>>>>>> +			&& (TREE_CODE (TREE_TYPE (v)) != RECORD_TYPE)
>>>>>>>> +			&& (TREE_CODE (TREE_TYPE (v)) != UNION_TYPE))
>>>>>>>> 		      {
>>>>>>>> 			v = NULL_TREE;
>>>>>>>> 			break;
>>>>>>>> 		      }
>>>>>>>> -		    is_flexible_array_mem_ref = array_ref_flexible_size_p (v);
>>>>>>>> -		    while (v != pt_var && TREE_CODE (v) == COMPONENT_REF)
>>>>>>>> -		      if (TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> -			  != UNION_TYPE
>>>>>>>> -			  && TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> -			  != QUAL_UNION_TYPE)
>>>>>>>> -			break;
>>>>>>>> -		      else
>>>>>>>> -			v = TREE_OPERAND (v, 0);
>>>>>>>> -		    if (TREE_CODE (v) == COMPONENT_REF
>>>>>>>> -			&& TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> -			   == RECORD_TYPE)
>>>>>>>> +		    /* if the record or union does not have flexible size
>>>>>>>> +		       compute the object size directly.  */
>>>>>>>> +		    if (TREE_CODE (TREE_TYPE (v)) == RECORD_TYPE
>>>>>>>> +			|| TREE_CODE (TREE_TYPE (v)) == UNION_TYPE)
>>>>>>>> 		      {
>>>>>>>> -			/* compute object size only if v is not a
>>>>>>>> -			   flexible array member.  */
>>>>>>>> -			if (!is_flexible_array_mem_ref)
>>>>>>>> +			if (!flexible_size_type_p (TREE_TYPE (v)))
>>>>>>>> 			  {
>>>>>>>> 			    v = NULL_TREE;
>>>>>>>> 			    break;
>>>>>>>> 			  }
>>>>>>>> -			v = TREE_OPERAND (v, 0);
>>>>>>>> +			else
>>>>>>>> +			  v = TREE_OPERAND (v, 0);
>>>>>>>> 		      }
>>>>>>>> -		    while (v != pt_var && TREE_CODE (v) == COMPONENT_REF)
>>>>>>>> -		      if (TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> -			  != UNION_TYPE
>>>>>>>> -			  && TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> -			  != QUAL_UNION_TYPE)
>>>>>>>> -			break;
>>>>>>>> -		      else
>>>>>>>> -			v = TREE_OPERAND (v, 0);
>>>>>>>> -		    if (v != pt_var)
>>>>>>>> -		      v = NULL_TREE;
>>>>>>>> 		    else
>>>>>>>> -		      v = pt_var;
>>>>>>>> +		      {
>>>>>>>> +			/* Now the ref is to an array type.  */
>>>>>>>> +			is_flexible_array_mem_ref
>>>>>>>> +			  = array_ref_flexible_size_p (v);
>>>>>>>> +			while (v != pt_var && TREE_CODE (v) == COMPONENT_REF)
>>>>>>>> +			if (TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> +			      != UNION_TYPE
>>>>>>>> +			    && TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> +				 != QUAL_UNION_TYPE)
>>>>>>>> +			  break;
>>>>>>>> +			else
>>>>>>>> +			  v = TREE_OPERAND (v, 0);
>>>>>>>> +			if (TREE_CODE (v) == COMPONENT_REF
>>>>>>>> +			    && TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> +				 == RECORD_TYPE)
>>>>>>>> +			  {
>>>>>>>> +			    /* compute object size only if v is not a
>>>>>>>> +			       flexible array member.  */
>>>>>>>> +			    if (!is_flexible_array_mem_ref)
>>>>>>>> +			      {
>>>>>>>> +				v = NULL_TREE;
>>>>>>>> +				break;
>>>>>>>> +			      }
>>>>>>>> +			    v = TREE_OPERAND (v, 0);
>>>>>>>> +			  }
>>>>>>>> +			while (v != pt_var && TREE_CODE (v) == COMPONENT_REF)
>>>>>>>> +			  if (TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> +				!= UNION_TYPE
>>>>>>>> +			      && TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>>>>>>>> +				   != QUAL_UNION_TYPE)
>>>>>>>> +			    break;
>>>>>>>> +			  else
>>>>>>>> +			    v = TREE_OPERAND (v, 0);
>>>>>>>> +			if (v != pt_var)
>>>>>>>> +			  v = NULL_TREE;
>>>>>>>> +			else
>>>>>>>> +			  v = pt_var;
>>>>>>>> +		      }
>>>>>>>> 		    break;
>>>>>>>> 		  default:
>>>>>>>> 		    v = pt_var;
>>>>>>>> 
>>>>>>> 
>>>>>>> -- 
>>>>>>> Richard Biener <rguenther@suse.de>
>>>>>>> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
>>>>>>> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
>>>>>>> HRB 36809 (AG Nuernberg)
>>>>>> 
>>>>>> 
>>>>> 
>>>>> -- 
>>>>> Richard Biener <rguenther@suse.de>
>>>>> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
>>>>> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
>>>>> HRB 36809 (AG Nuernberg)
>>>> 
>>>> 
>>> 
>>> -- 
>>> Richard Biener <rguenther@suse.de>
>>> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
>>> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
>>> HRB 36809 (AG Nuernberg)
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
> HRB 36809 (AG Nuernberg)