On Thu, 2 Feb 2023, Qing Zhao wrote:
>
>
> > On Feb 2, 2023, at 8:54 AM, Richard Biener <rguent...@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 <rguent...@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 <rguent...@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 <rguent...@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 <rguent...@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)
