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 2, 2023, at 3:07 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Wed, 1 Feb 2023, Qing Zhao wrote:
> 
>> 
>> 
>>> On Feb 1, 2023, at 6:41 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Tue, 31 Jan 2023, Qing Zhao wrote:
>>> 
>>>> GCC extension accepts the case when a struct with a flexible array member
>>>> is embedded into another struct (possibly recursively).
>>>> __builtin_object_size should treat such struct as flexible size per
>>>> -fstrict-flex-arrays.
>>>> 
>>>> 	PR tree-optimization/101832
>>>> 
>>>> gcc/ChangeLog:
>>>> 
>>>> 	PR tree-optimization/101832
>>>> 	* tree-object-size.cc (flexible_size_type_p): New function.
>>>> 	(addr_object_size): Handle structure/union type when it has
>>>> 	flexible size.
>>>> 
>>>> gcc/testsuite/ChangeLog:
>>>> 
>>>> 	PR tree-optimization/101832
>>>> 	* gcc.dg/builtin-object-size-pr101832-2.c: New test.
>>>> 	* gcc.dg/builtin-object-size-pr101832-3.c: New test.
>>>> 	* gcc.dg/builtin-object-size-pr101832-4.c: New test.
>>>> 	* gcc.dg/builtin-object-size-pr101832.c: New test.
>>>> ---
>>>> .../gcc.dg/builtin-object-size-pr101832-2.c   | 135 ++++++++++++++++++
>>>> .../gcc.dg/builtin-object-size-pr101832-3.c   | 135 ++++++++++++++++++
>>>> .../gcc.dg/builtin-object-size-pr101832-4.c   | 135 ++++++++++++++++++
>>>> .../gcc.dg/builtin-object-size-pr101832.c     | 119 +++++++++++++++
>>>> gcc/tree-object-size.cc                       | 115 +++++++++++----
>>>> 5 files changed, 611 insertions(+), 28 deletions(-)
>>>> create mode 100644 gcc/testsuite/gcc.dg/builtin-object-size-pr101832-2.c
>>>> create mode 100644 gcc/testsuite/gcc.dg/builtin-object-size-pr101832-3.c
>>>> create mode 100644 gcc/testsuite/gcc.dg/builtin-object-size-pr101832-4.c
>>>> create mode 100644 gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
>>>> 
>>>> diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-2.c b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-2.c
>>>> new file mode 100644
>>>> index 00000000000..f38babc5415
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-2.c
>>>> @@ -0,0 +1,135 @@
>>>> +/* PR 101832: 
>>>> +   GCC extension accepts the case when a struct with a flexible array member
>>>> +   is embedded into another struct (possibly recursively).
>>>> +   __builtin_object_size will treat such struct as flexible size per
>>>> +   -fstrict-flex-arrays.  */ 
>>>> +/* { dg-do run } */
>>>> +/* { dg-options "-O2 -fstrict-flex-arrays=1" } */
>>>> +
>>>> +#include <stdio.h>
>>>> +
>>>> +unsigned n_fails = 0;
>>>> +
>>>> +#define expect(p, _v) do { \
>>>> +  size_t v = _v; \
>>>> +  if (p == v) \
>>>> +    printf("ok:  %s == %zd\n", #p, p); \
>>>> +  else {\
>>>> +    printf("WAT: %s == %zd (expected %zd)\n", #p, p, v); \
>>>> +    n_fails++; \
>>>> +  } \
>>>> +} while (0);
>>>> +
>>>> +struct A {
>>>> +  int n;
>>>> +  char data[];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B {
>>>> +  int m;
>>>> +  struct A a;
>>>> +};
>>>> +
>>>> +struct C {
>>>> +  int q;
>>>> +  struct B b;
>>>> +};
>>>> +
>>>> +struct A0 {
>>>> +  int n;
>>>> +  char data[0];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B0 {
>>>> +  int m;
>>>> +  struct A0 a;
>>>> +};
>>>> +
>>>> +struct C0 {
>>>> +  int q;
>>>> +  struct B0 b;
>>>> +};
>>>> +
>>>> +struct A1 {
>>>> +  int n;
>>>> +  char data[1];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B1 {
>>>> +  int m;
>>>> +  struct A1 a;
>>>> +};
>>>> +
>>>> +struct C1 {
>>>> +  int q;
>>>> +  struct B1 b;
>>>> +};
>>>> +
>>>> +struct An {
>>>> +  int n;
>>>> +  char data[8];/* Content following header */
>>>> +};
>>>> +
>>>> +struct Bn {
>>>> +  int m;
>>>> +  struct An a;
>>>> +};
>>>> +
>>>> +struct Cn {
>>>> +  int q;
>>>> +  struct Bn b;
>>>> +};
>>>> +
>>>> +volatile void *magic1, *magic2;
>>>> +
>>>> +int main(int argc, char *argv[])
>>>> +{
>>>> +    struct B *outer;
>>>> +    struct C *outest;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer = (void *)magic1;
>>>> +    outest = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b.a, 1), -1);
>>>> +
>>>> +    struct B0 *outer0;
>>>> +    struct C0 *outest0;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer0 = (void *)magic1;
>>>> +    outest0 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer0->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b.a, 1), -1);
>>>> +
>>>> +    struct B1 *outer1;
>>>> +    struct C1 *outest1;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer1 = (void *)magic1;
>>>> +    outest1 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer1->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest1->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest1->b.a, 1), -1);
>>>> +
>>>> +    struct Bn *outern;
>>>> +    struct Cn *outestn;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outern = (void *)magic1;
>>>> +    outestn = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outern->a, 1), sizeof(outern->a));
>>>> +    expect(__builtin_object_size(&outestn->b, 1), sizeof(outestn->b));
>>>> +    expect(__builtin_object_size(&outestn->b.a, 1), sizeof(outestn->b.a));
>>>> +
>>>> +    if (n_fails > 0)
>>>> +      __builtin_abort ();
>>>> +
>>>> +    return 0;
>>>> +}
>>>> diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-3.c b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-3.c
>>>> new file mode 100644
>>>> index 00000000000..aaae99b8d67
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-3.c
>>>> @@ -0,0 +1,135 @@
>>>> +/* PR 101832: 
>>>> +   GCC extension accepts the case when a struct with a flexible array member
>>>> +   is embedded into another struct (possibly recursively).
>>>> +   __builtin_object_size will treat such struct as flexible size per
>>>> +   -fstrict-flex-arrays.  */
>>>> +/* { dg-do run } */
>>>> +/* { dg-options "-O2 -fstrict-flex-arrays=2" } */
>>>> +
>>>> +#include <stdio.h>
>>>> +
>>>> +unsigned n_fails = 0;
>>>> +
>>>> +#define expect(p, _v) do { \
>>>> +  size_t v = _v; \
>>>> +  if (p == v) \
>>>> +    printf("ok:  %s == %zd\n", #p, p); \
>>>> +  else {\
>>>> +    printf("WAT: %s == %zd (expected %zd)\n", #p, p, v); \
>>>> +    n_fails++; \
>>>> +  } \
>>>> +} while (0);
>>>> +
>>>> +struct A {
>>>> +  int n;
>>>> +  char data[];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B {
>>>> +  int m;
>>>> +  struct A a;
>>>> +};
>>>> +
>>>> +struct C {
>>>> +  int q;
>>>> +  struct B b;
>>>> +};
>>>> +
>>>> +struct A0 {
>>>> +  int n;
>>>> +  char data[0];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B0 {
>>>> +  int m;
>>>> +  struct A0 a;
>>>> +};
>>>> +
>>>> +struct C0 {
>>>> +  int q;
>>>> +  struct B0 b;
>>>> +};
>>>> +
>>>> +struct A1 {
>>>> +  int n;
>>>> +  char data[1];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B1 {
>>>> +  int m;
>>>> +  struct A1 a;
>>>> +};
>>>> +
>>>> +struct C1 {
>>>> +  int q;
>>>> +  struct B1 b;
>>>> +};
>>>> +
>>>> +struct An {
>>>> +  int n;
>>>> +  char data[8];/* Content following header */
>>>> +};
>>>> +
>>>> +struct Bn {
>>>> +  int m;
>>>> +  struct An a;
>>>> +};
>>>> +
>>>> +struct Cn {
>>>> +  int q;
>>>> +  struct Bn b;
>>>> +};
>>>> +
>>>> +volatile void *magic1, *magic2;
>>>> +
>>>> +int main(int argc, char *argv[])
>>>> +{
>>>> +    struct B *outer;
>>>> +    struct C *outest;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer = (void *)magic1;
>>>> +    outest = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b.a, 1), -1);
>>>> +
>>>> +    struct B0 *outer0;
>>>> +    struct C0 *outest0;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer0 = (void *)magic1;
>>>> +    outest0 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer0->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b.a, 1), -1);
>>>> +
>>>> +    struct B1 *outer1;
>>>> +    struct C1 *outest1;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer1 = (void *)magic1;
>>>> +    outest1 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer1->a, 1), sizeof(outer1->a));
>>>> +    expect(__builtin_object_size(&outest1->b, 1), sizeof(outest1->b));
>>>> +    expect(__builtin_object_size(&outest1->b.a, 1), sizeof(outest1->b.a));
>>>> +
>>>> +    struct Bn *outern;
>>>> +    struct Cn *outestn;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outern = (void *)magic1;
>>>> +    outestn = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outern->a, 1), sizeof(outern->a));
>>>> +    expect(__builtin_object_size(&outestn->b, 1), sizeof(outestn->b));
>>>> +    expect(__builtin_object_size(&outestn->b.a, 1), sizeof(outestn->b.a));
>>>> +
>>>> +    if (n_fails > 0)
>>>> +      __builtin_abort ();
>>>> +
>>>> +    return 0;
>>>> +}
>>>> diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-4.c b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-4.c
>>>> new file mode 100644
>>>> index 00000000000..424264e2acd
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832-4.c
>>>> @@ -0,0 +1,135 @@
>>>> +/* PR 101832: 
>>>> +   GCC extension accepts the case when a struct with a flexible array member
>>>> +   is embedded into another struct (possibly recursively).
>>>> +   __builtin_object_size will treat such struct as flexible size per
>>>> +   -fstrict-flex-arrays.  */
>>>> +/* { dg-do run } */
>>>> +/* { dg-options "-O2 -fstrict-flex-arrays=3" } */
>>>> +
>>>> +#include <stdio.h>
>>>> +
>>>> +unsigned n_fails = 0;
>>>> +
>>>> +#define expect(p, _v) do { \
>>>> +  size_t v = _v; \
>>>> +  if (p == v) \
>>>> +    printf("ok:  %s == %zd\n", #p, p); \
>>>> +  else {\
>>>> +    printf("WAT: %s == %zd (expected %zd)\n", #p, p, v); \
>>>> +    n_fails++; \
>>>> +  } \
>>>> +} while (0);
>>>> +
>>>> +struct A {
>>>> +  int n;
>>>> +  char data[];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B {
>>>> +  int m;
>>>> +  struct A a;
>>>> +};
>>>> +
>>>> +struct C {
>>>> +  int q;
>>>> +  struct B b;
>>>> +};
>>>> +
>>>> +struct A0 {
>>>> +  int n;
>>>> +  char data[0];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B0 {
>>>> +  int m;
>>>> +  struct A0 a;
>>>> +};
>>>> +
>>>> +struct C0 {
>>>> +  int q;
>>>> +  struct B0 b;
>>>> +};
>>>> +
>>>> +struct A1 {
>>>> +  int n;
>>>> +  char data[1];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B1 {
>>>> +  int m;
>>>> +  struct A1 a;
>>>> +};
>>>> +
>>>> +struct C1 {
>>>> +  int q;
>>>> +  struct B1 b;
>>>> +};
>>>> +
>>>> +struct An {
>>>> +  int n;
>>>> +  char data[8];/* Content following header */
>>>> +};
>>>> +
>>>> +struct Bn {
>>>> +  int m;
>>>> +  struct An a;
>>>> +};
>>>> +
>>>> +struct Cn {
>>>> +  int q;
>>>> +  struct Bn b;
>>>> +};
>>>> +
>>>> +volatile void *magic1, *magic2;
>>>> +
>>>> +int main(int argc, char *argv[])
>>>> +{
>>>> +    struct B *outer;
>>>> +    struct C *outest;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer = (void *)magic1;
>>>> +    outest = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b.a, 1), -1);
>>>> +
>>>> +    struct B0 *outer0;
>>>> +    struct C0 *outest0;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer0 = (void *)magic1;
>>>> +    outest0 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer0->a, 1), sizeof(outer0->a));
>>>> +    expect(__builtin_object_size(&outest0->b, 1), sizeof(outest0->b));
>>>> +    expect(__builtin_object_size(&outest0->b.a, 1), sizeof(outest0->b.a));
>>>> +
>>>> +    struct B1 *outer1;
>>>> +    struct C1 *outest1;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outer1 = (void *)magic1;
>>>> +    outest1 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer1->a, 1), sizeof(outer1->a));
>>>> +    expect(__builtin_object_size(&outest1->b, 1), sizeof(outest1->b));
>>>> +    expect(__builtin_object_size(&outest1->b.a, 1), sizeof(outest1->b.a));
>>>> +
>>>> +    struct Bn *outern;
>>>> +    struct Cn *outestn;
>>>> +
>>>> +    /* Make sure optimization can't find some other object size. */
>>>> +    outern = (void *)magic1;
>>>> +    outestn = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outern->a, 1), sizeof(outern->a));
>>>> +    expect(__builtin_object_size(&outestn->b, 1), sizeof(outestn->b));
>>>> +    expect(__builtin_object_size(&outestn->b.a, 1), sizeof(outestn->b.a));
>>>> +
>>>> +    if (n_fails > 0)
>>>> +      __builtin_abort ();
>>>> +
>>>> +    return 0;
>>>> +}
>>>> diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
>>>> new file mode 100644
>>>> index 00000000000..8ed6980edf0
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
>>>> @@ -0,0 +1,119 @@
>>>> +/* PR 101832: 
>>>> +   GCC extension accepts the case when a struct with a flexible array member
>>>> +   is embedded into another struct (possibly recursively).
>>>> +   __builtin_object_size will treat such struct as flexible size per
>>>> +   -fstrict-flex-arrays.  */
>>>> +/* { dg-do run } */
>>>> +/* { dg-options "-O2" } */
>>>> +
>>>> +#include <stdio.h>
>>>> +
>>>> +unsigned n_fails = 0;
>>>> +
>>>> +#define expect(p, _v) do { \
>>>> +  size_t v = _v; \
>>>> +  if (p == v) \
>>>> +    printf("ok:  %s == %zd\n", #p, p); \
>>>> +  else {\
>>>> +    printf("WAT: %s == %zd (expected %zd)\n", #p, p, v); \
>>>> +    n_fails++; \
>>>> +  } \
>>>> +} while (0);
>>>> +
>>>> +struct A {
>>>> +  int n;
>>>> +  char data[];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B {
>>>> +  int m;
>>>> +  struct A a;
>>>> +};
>>>> +
>>>> +struct C {
>>>> +  int q;
>>>> +  struct B b;
>>>> +};
>>>> +
>>>> +struct A0 {
>>>> +  int n;
>>>> +  char data[0];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B0 {
>>>> +  int m;
>>>> +  struct A0 a;
>>>> +};
>>>> +
>>>> +struct C0 {
>>>> +  int q;
>>>> +  struct B0 b;
>>>> +};
>>>> +
>>>> +struct A1 {
>>>> +  int n;
>>>> +  char data[1];/* Content following header */
>>>> +};
>>>> +
>>>> +struct B1 {
>>>> +  int m;
>>>> +  struct A1 a;
>>>> +};
>>>> +
>>>> +struct C1 {
>>>> +  int q;
>>>> +  struct B1 b;
>>>> +};
>>>> +
>>>> +struct An {
>>>> +  int n;
>>>> +  char data[8];/* Content following header */
>>>> +};
>>>> +
>>>> +struct Bn {
>>>> +  int m;
>>>> +  struct An a;
>>>> +};
>>>> +
>>>> +struct Cn {
>>>> +  int q;
>>>> +  struct Bn b;
>>>> +};
>>>> +
>>>> +volatile void *magic1, *magic2;
>>>> +
>>>> +int main(int argc, char *argv[])
>>>> +{
>>>> +    struct B *outer = (void *)magic1;
>>>> +    struct C *outest = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest->b.a, 1), -1);
>>>> +
>>>> +    struct B0 *outer0 = (void *)magic1;
>>>> +    struct C0 *outest0 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer0->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest0->b.a, 1), -1);
>>>> +
>>>> +    struct B1 *outer1 = (void *)magic1;
>>>> +    struct C1 *outest1 = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outer1->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outest1->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outest1->b.a, 1), -1);
>>>> +
>>>> +    struct Bn *outern = (void *)magic1;
>>>> +    struct Cn *outestn = (void *)magic2;
>>>> +
>>>> +    expect(__builtin_object_size(&outern->a, 1), -1);
>>>> +    expect(__builtin_object_size(&outestn->b, 1), -1);
>>>> +    expect(__builtin_object_size(&outestn->b.a, 1), -1);
>>>> +
>>>> +    if (n_fails > 0)
>>>> +      __builtin_abort ();
>>>> +
>>>> +    return 0;
>>>> +}
>>>> diff --git a/gcc/tree-object-size.cc b/gcc/tree-object-size.cc
>>>> index 9a936a91983..56b78ca2a8c 100644
>>>> --- a/gcc/tree-object-size.cc
>>>> +++ b/gcc/tree-object-size.cc
>>>> @@ -500,6 +500,42 @@ decl_init_size (tree decl, bool min)
>>>>  return size;
>>>> }
>>>> 
>>>> +/* Determine whether TYPE is a structure with a flexible array member
>>>> +   per -fstrict-flex-array or a union containing such a structure
>>>> +   (possibly recursively).  */
>>>> +static bool
>>>> +flexible_size_type_p (const_tree type)
>>>> +{
>>>> +  tree x = NULL_TREE;
>>>> +  tree last = NULL_TREE;
>>>> +  switch (TREE_CODE (type))
>>>> +    {
>>>> +    case RECORD_TYPE:
>>>> +      for (x = TYPE_FIELDS (type); x != NULL_TREE; x = DECL_CHAIN (x))
>>>> +	if (TREE_CODE (x) == FIELD_DECL)
>>>> +	  last = x;
>>>> +      if (last == NULL_TREE)
>>>> +	return false;
>>>> +      if (TREE_CODE (TREE_TYPE (last)) == ARRAY_TYPE
>>>> +	  && !DECL_NOT_FLEXARRAY (last))
>>>> +	return true;
>>>> +      else if (TREE_CODE (TREE_TYPE (last)) == RECORD_TYPE
>>>> +	       || TREE_CODE (TREE_TYPE (last)) == UNION_TYPE)
>>>> +	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.

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)