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From: "siddhesh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107952] New: tree-object-size: inconsistent
 size for flexible arrays nested in structs
Date: Fri, 02 Dec 2022 14:04:50 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107952

            Bug ID: 107952
           Summary: tree-object-size: inconsistent size for flexible
                    arrays nested in structs
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: siddhesh at gcc dot gnu.org
  Target Milestone: ---

With -fstrict-flex-arrays, there is a clearer definition of what constitutes
flexible array members, controlled by the frontend.  tree-object-size howev=
er
doesn't seem to handle this well enough when the flex array is nested.  e.g=
.:

typedef struct {
  char pad;
  char data[8];
} F2;

typedef struct {
  unsigned pad;
  F2 flex;
} S2;

#define NULL (void *) 0

__SIZE_TYPE__
nested_flexarray (__SIZE_TYPE__ n)
{
  S2 *p =3D (S2 *) __builtin_malloc (n);

  return __builtin_dynamic_object_size (p->flex.data, 1);
}

__SIZE_TYPE__
nested_flexarray2 (S2 *p)
{
  return __builtin_dynamic_object_size (p->flex.data, 1);
}

The current behaviour is to treat data as a flex array and nested_flexarray
returns the size as if it were a flex array.  nested_flexarray2 however ret=
urns
the subscripted size, thinking of it as a fixed array, which should not be =
the
expected behaviour with -fstrict-flex-arrays=3D0.  Instead it should return=
 -1
for maximum size and perhaps 8 as minimum size.

If F2 is changed to:

typedef struct {
  char pad;
  char data[0];
} F2;

the current behaviour ends up returning 0 in both cases, which is incorrect=
.=20
Again, it should return the size as if it were a flex array in nested_flexa=
rray
and -1 for maximum size for nested_flexarray2.

I have a patch that does this, but I need to fix up tests that currently ex=
pect
older behaviour and wanted to get some consensus before I fixed them up.=