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

[Richard Biener <rguenther@suse.de>]

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.

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.

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

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)