[PATCH v3 0/4]Allow flexible array members in unions and alone in structures [PR53548]

[PATCH v3 0/4]Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

Hi,

This is the 3rd version for 
Allow flexible array members in unions and alone in structures [PR53548]

(for your reference, the 1st version is at:
https://gcc.gnu.org/pipermail/gcc-patches/2024-April/649737.html
The 2nd version is at:
https://gcc.gnu.org/pipermail/gcc-patches/2024-April/650019.html)

compared to the 2nd version, the major difference are:

A. Add two testing cases for _bos per Sid's comments;
B. One update to the C FE routine "add_flexible_array_elts_to_size"
   to handle the cases when the declaration is union.
C. One update the the C++ FE routine "layout_var_decl" to handle
   the cases when the declaratin is a union with a flexible array
   member initializer.

All the above new change is included in the new 4th patch.

Review needed:

   C++ FE changes (in Patch 2 and new Patch 4);
   Middle-end changes (in Patch 2);
   New C FE change (in new Patch 4);
   New testing cases for _bos (in new Patch 4);

Approval status:

   All other changes (C FE and documentation and related testing cases)
   have been approved with minor updates.

The patch set includes:
 1. Documentation change.
    Allow flexible array members in unions and alone in structures
    [PR53548]
 2. C and C++ FE changes to support flexible array members in unions and
   alone in structures.
    Adjust testcases for flexible array member in union and alone in
   structure extension.
 3. Add testing cases for flexible array members in unions and alone in
   structures.
 4. Update the C FE routine "add_flexible_array_elts_to_size" C++ FE
   routine "layout_var_decl"  to handle the cases when the DECL is
   union.
    Add testing cvases to test the _bos for FAM in union and alone in
   structures.

bootstrapped and regression tested on both x86 and aarch64, no issue.

Okay for GCC15?

thanks.

Qing

[PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

The request for GCC to accept that the C99 flexible array member can be
in a union or alone in a structure has been made a long time ago around 2012
for supporting several practical cases including glibc.

A GCC PR has been opened for such request at that time:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548

However, this PR was closed as WONTFIX around 2015 due to the following reason:

"there is an existing extension that makes the requested functionality possible"
i.e GCC fully supported that the zero-length array can be in a union or alone
in a structure for a long time. (though I didn't see any official documentation
on such extension)

It's reasonable to close PR53548 at that time since zero-length array extension
can be used for such purpose.

However, since GCC13, in order to improve the C/C++ security, we introduced
-fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
usages from C/C++ source code. As a result, zero-length arrays eventually
will be replaced by C99 flexiable array member completely.

Therefore, GCC needs to explicitly allow such extensions directly for C99
flexible arrays, since flexable array member in unions or alone in structs
are common code patterns in active use by the Linux kernel (and other projects).

For example, these do not error by default with GCC:

union one {
  int a;
  int b[0];
};

union two {
  int a;
  struct {
    struct { } __empty;
    int b[];
  };
};

But these do:

union three {
  int a;
  int b[];
};

struct four {
  int b[];
}

Clang has supported such extensions since March, 2024
https://github.com/llvm/llvm-project/pull/84428

GCC should also support such extensions. This will allow for
a seamless transition for code bases away from zero-length arrays without
losing existing code patterns.

gcc/ChangeLog:

	* doc/extend.texi: Add documentation for Flexible Array Members in
	Unions and Flexible Array Members alone in Structures.
---
 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7b54a241a7bf..cba98c8aadd7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible Array Members.
+* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 
+@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If all the members of a union are flexible array member, the size of
+such union is zero.
+
+@node Flexible Array Members alone in Structures
+@section Structures with only Flexible Array Members
+@cindex structures with only flexible array members
+@cindex structures with only FAMs
+
+GCC permits a C99 flexible array member (FAM) to be alone in a structure:
+
+@smallexample
+struct only_fam @{
+  int b[];
+@};
+@end smallexample
+
+The size of such structure gives the size zero.
+
 @node Variable Length
 @section Arrays of Variable Length
 @cindex variable-length arrays
-- 
2.31.1

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Jason Merrill]

On 4/30/24 07:58, Qing Zhao wrote:
> The request for GCC to accept that the C99 flexible array member can be
> in a union or alone in a structure has been made a long time ago around 2012
> for supporting several practical cases including glibc.
> 
> A GCC PR has been opened for such request at that time:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
> 
> However, this PR was closed as WONTFIX around 2015 due to the following reason:
> 
> "there is an existing extension that makes the requested functionality possible"
> i.e GCC fully supported that the zero-length array can be in a union or alone
> in a structure for a long time. (though I didn't see any official documentation
> on such extension)
> 
> It's reasonable to close PR53548 at that time since zero-length array extension
> can be used for such purpose.
> 
> However, since GCC13, in order to improve the C/C++ security, we introduced
> -fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
> usages from C/C++ source code. As a result, zero-length arrays eventually
> will be replaced by C99 flexiable array member completely.
> 
> Therefore, GCC needs to explicitly allow such extensions directly for C99
> flexible arrays, since flexable array member in unions or alone in structs
> are common code patterns in active use by the Linux kernel (and other projects).
> 
> For example, these do not error by default with GCC:
> 
> union one {
>    int a;
>    int b[0];
> };
> 
> union two {
>    int a;
>    struct {
>      struct { } __empty;
>      int b[];
>    };
> };
> 
> But these do:
> 
> union three {
>    int a;
>    int b[];
> };
> 
> struct four {
>    int b[];
> }
> 
> Clang has supported such extensions since March, 2024
> https://github.com/llvm/llvm-project/pull/84428
> 
> GCC should also support such extensions. This will allow for
> a seamless transition for code bases away from zero-length arrays without
> losing existing code patterns.
> 
> gcc/ChangeLog:
> 
> 	* doc/extend.texi: Add documentation for Flexible Array Members in
> 	Unions and Flexible Array Members alone in Structures.
> ---
>   gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
>   1 file changed, 34 insertions(+)
> 
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 7b54a241a7bf..cba98c8aadd7 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
>   * Named Address Spaces::Named address spaces.
>   * Zero Length::         Zero-length arrays.
>   * Empty Structures::    Structures with no members.
> +* Flexible Array Members in Unions::  Unions with Flexible Array Members.
> +* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
>   * Variable Length::     Arrays whose length is computed at run time.
>   * Variadic Macros::     Macros with a variable number of arguments.
>   * Escaped Newlines::    Slightly looser rules for escaped newlines.
> @@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
>   of the language.  G++ treats empty structures as if they had a single
>   member of type @code{char}.
>   
> +@node Flexible Array Members in Unions
> +@section Unions with Flexible Array Members
> +@cindex unions with flexible array members
> +@cindex unions with FAMs
> +
> +GCC permits a C99 flexible array member (FAM) to be in a union:
> +
> +@smallexample
> +union with_fam @{
> +  int a;
> +  int b[];
> +@};
> +@end smallexample
> +
> +If all the members of a union are flexible array member, the size of

"If the only member of a union is a flexible array member"

> +such union is zero.

"such a union"

> +
> +@node Flexible Array Members alone in Structures
> +@section Structures with only Flexible Array Members
> +@cindex structures with only flexible array members
> +@cindex structures with only FAMs
> +
> +GCC permits a C99 flexible array member (FAM) to be alone in a structure:
> +
> +@smallexample
> +struct only_fam @{
> +  int b[];
> +@};
> +@end smallexample
> +
> +The size of such structure gives the size zero.

"The size of such a structure is zero"

> +
>   @node Variable Length
>   @section Arrays of Variable Length
>   @cindex variable-length arrays

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

[-- Attachment #1: Type: text/plain, Size: 4590 bytes --]



On Apr 30, 2024, at 15:27, Jason Merrill <jason@redhat.com> wrote:

On 4/30/24 07:58, Qing Zhao wrote:
The request for GCC to accept that the C99 flexible array member can be
in a union or alone in a structure has been made a long time ago around 2012
for supporting several practical cases including glibc.
A GCC PR has been opened for such request at that time:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
However, this PR was closed as WONTFIX around 2015 due to the following reason:
"there is an existing extension that makes the requested functionality possible"
i.e GCC fully supported that the zero-length array can be in a union or alone
in a structure for a long time. (though I didn't see any official documentation
on such extension)
It's reasonable to close PR53548 at that time since zero-length array extension
can be used for such purpose.
However, since GCC13, in order to improve the C/C++ security, we introduced
-fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
usages from C/C++ source code. As a result, zero-length arrays eventually
will be replaced by C99 flexiable array member completely.
Therefore, GCC needs to explicitly allow such extensions directly for C99
flexible arrays, since flexable array member in unions or alone in structs
are common code patterns in active use by the Linux kernel (and other projects).
For example, these do not error by default with GCC:
union one {
  int a;
  int b[0];
};
union two {
  int a;
  struct {
    struct { } __empty;
    int b[];
  };
};
But these do:
union three {
  int a;
  int b[];
};
struct four {
  int b[];
}
Clang has supported such extensions since March, 2024
https://github.com/llvm/llvm-project/pull/84428
GCC should also support such extensions. This will allow for
a seamless transition for code bases away from zero-length arrays without
losing existing code patterns.
gcc/ChangeLog:
* doc/extend.texi: Add documentation for Flexible Array Members in
Unions and Flexible Array Members alone in Structures.
---
 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7b54a241a7bf..cba98c8aadd7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible Array Members.
+* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 +@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If all the members of a union are flexible array member, the size of

It’s for the following case:

union with_fam_3 {
  char a[];
  int b[];
}

And also include:  (the only member of a union is a flexible array member as you mentioned below)

union with_fam_1 {
  char a[];
}

So, I think the original sentence:

“If all the members of a union are flexible array member, the size of”

Should be better than the below:

"If the only member of a union is a flexible array member”


+such union is zero.

"such a union"

Okay.


+
+@node Flexible Array Members alone in Structures
+@section Structures with only Flexible Array Members
+@cindex structures with only flexible array members
+@cindex structures with only FAMs
+
+GCC permits a C99 flexible array member (FAM) to be alone in a structure:
+
+@smallexample
+struct only_fam @{
+  int b[];
+@};
+@end smallexample
+
+The size of such structure gives the size zero.

"The size of such a structure is zero"

Okay.

thanks.

Qing

+
 @node Variable Length
 @section Arrays of Variable Length
 @cindex variable-length arrays

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

[-- Attachment #1: Type: text/plain, Size: 2702 bytes --]



On Apr 30, 2024, at 15:45, Qing Zhao <qing.zhao@oracle.com> wrote:



 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7b54a241a7bf..cba98c8aadd7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible Array Members.
+* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 +@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If all the members of a union are flexible array member, the size of

It’s for the following case:

union with_fam_3 {
  char a[];
  int b[];
}

And also include:  (the only member of a union is a flexible array member as you mentioned below)

union with_fam_1 {
  char a[];
}

So, I think the original sentence:

“If all the members of a union are flexible array member, the size of”

Should be better than the below:

"If the only member of a union is a flexible array member”

How about the following wording?

"If every member of a union is flexible array member, the size of”

Qing


+such union is zero.

"such a union"

Okay.


+
+@node Flexible Array Members alone in Structures
+@section Structures with only Flexible Array Members
+@cindex structures with only flexible array members
+@cindex structures with only FAMs
+
+GCC permits a C99 flexible array member (FAM) to be alone in a structure:
+
+@smallexample
+struct only_fam @{
+  int b[];
+@};
+@end smallexample
+
+The size of such structure gives the size zero.

"The size of such a structure is zero"

Okay.

thanks.

Qing

+
 @node Variable Length
 @section Arrays of Variable Length
 @cindex variable-length arrays

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Jason Merrill]

On 4/30/24 14:49, Qing Zhao wrote:
> 
> 
>> On Apr 30, 2024, at 15:45, Qing Zhao <qing.zhao@oracle.com> wrote:
>>
>>
>>
>>>>  gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 34 insertions(+)
>>>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>>>> index 7b54a241a7bf..cba98c8aadd7 100644
>>>> --- a/gcc/doc/extend.texi
>>>> +++ b/gcc/doc/extend.texi
>>>> @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
>>>>  * Named Address Spaces::Named address spaces.
>>>>  * Zero Length::         Zero-length arrays.
>>>>  * Empty Structures::    Structures with no members.
>>>> +* Flexible Array Members in Unions::  Unions with Flexible Array 
>>>> Members.
>>>> +* Flexible Array Members alone in Structures::  Structures with 
>>>> only Flexible Array Members.
>>>>  * Variable Length::     Arrays whose length is computed at run time.
>>>>  * Variadic Macros::     Macros with a variable number of arguments.
>>>>  * Escaped Newlines::    Slightly looser rules for escaped newlines.
>>>> @@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty 
>>>> structures are part
>>>>  of the language.  G++ treats empty structures as if they had a single
>>>>  member of type @code{char}.
>>>>  +@node Flexible Array Members in Unions
>>>> +@section Unions with Flexible Array Members
>>>> +@cindex unions with flexible array members
>>>> +@cindex unions with FAMs
>>>> +
>>>> +GCC permits a C99 flexible array member (FAM) to be in a union:
>>>> +
>>>> +@smallexample
>>>> +union with_fam @{
>>>> +  int a;
>>>> +  int b[];
>>>> +@};
>>>> +@end smallexample
>>>> +
>>>> +If all the members of a union are flexible array member, the size of
>>
>> It’s for the following case:
>>
>> union with_fam_3 {
>>   char a[];
>>   int b[];
>> }
>>
>> And also include:  (the only member of a union is a flexible array 
>> member as you mentioned below)
>>
>> union with_fam_1 {
>>   char a[];
>> }
>>
>> So, I think the original sentence:
>>
>> “If all the members of a union are flexible array member, the size of”
>>
>> Should be better than the below:
>>>
>>> "If the only member of a union is a flexible array member”
> 
> How about the following wording?
> 
> "If every member of a union is flexible array member, the size of”

"is a flexible array member", sure.

Jason

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

> On Apr 30, 2024, at 15:52, Jason Merrill <jason@redhat.com> wrote:
> 
> On 4/30/24 14:49, Qing Zhao wrote:
>>> On Apr 30, 2024, at 15:45, Qing Zhao <qing.zhao@oracle.com> wrote:
>>> 
>>> 
>>> 
>>>>>  gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
>>>>>  1 file changed, 34 insertions(+)
>>>>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>>>>> index 7b54a241a7bf..cba98c8aadd7 100644
>>>>> --- a/gcc/doc/extend.texi
>>>>> +++ b/gcc/doc/extend.texi
>>>>> @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
>>>>>  * Named Address Spaces::Named address spaces.
>>>>>  * Zero Length::         Zero-length arrays.
>>>>>  * Empty Structures::    Structures with no members.
>>>>> +* Flexible Array Members in Unions::  Unions with Flexible Array Members.
>>>>> +* Flexible Array Members alone in Structures::  Structures with only Flexible Array Members.
>>>>>  * Variable Length::     Arrays whose length is computed at run time.
>>>>>  * Variadic Macros::     Macros with a variable number of arguments.
>>>>>  * Escaped Newlines::    Slightly looser rules for escaped newlines.
>>>>> @@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty structures are part
>>>>>  of the language.  G++ treats empty structures as if they had a single
>>>>>  member of type @code{char}.
>>>>>  +@node Flexible Array Members in Unions
>>>>> +@section Unions with Flexible Array Members
>>>>> +@cindex unions with flexible array members
>>>>> +@cindex unions with FAMs
>>>>> +
>>>>> +GCC permits a C99 flexible array member (FAM) to be in a union:
>>>>> +
>>>>> +@smallexample
>>>>> +union with_fam @{
>>>>> +  int a;
>>>>> +  int b[];
>>>>> +@};
>>>>> +@end smallexample
>>>>> +
>>>>> +If all the members of a union are flexible array member, the size of
>>> 
>>> It’s for the following case:
>>> 
>>> union with_fam_3 {
>>>   char a[];
>>>   int b[];
>>> }
>>> 
>>> And also include:  (the only member of a union is a flexible array member as you mentioned below)
>>> 
>>> union with_fam_1 {
>>>   char a[];
>>> }
>>> 
>>> So, I think the original sentence:
>>> 
>>> “If all the members of a union are flexible array member, the size of”
>>> 
>>> Should be better than the below:
>>>> 
>>>> "If the only member of a union is a flexible array member”
>> How about the following wording?
>> "If every member of a union is flexible array member, the size of”
> 
> "is a flexible array member", sure.

Okay, will update the doc.

Thanks a lot.

Qing
> 
> Jason
> 

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Jason Merrill]

On 4/30/24 14:45, Qing Zhao wrote:
> 
> 
>> On Apr 30, 2024, at 15:27, Jason Merrill <jason@redhat.com> wrote:
>>
>> On 4/30/24 07:58, Qing Zhao wrote:
>>> The request for GCC to accept that the C99 flexible array member can be
>>> in a union or alone in a structure has been made a long time ago 
>>> around 2012
>>> for supporting several practical cases including glibc.
>>> A GCC PR has been opened for such request at that time:
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
>>> However, this PR was closed as WONTFIX around 2015 due to the 
>>> following reason:
>>> "there is an existing extension that makes the requested 
>>> functionality possible"
>>> i.e GCC fully supported that the zero-length array can be in a union 
>>> or alone
>>> in a structure for a long time. (though I didn't see any official 
>>> documentation
>>> on such extension)
>>> It's reasonable to close PR53548 at that time since zero-length array 
>>> extension
>>> can be used for such purpose.
>>> However, since GCC13, in order to improve the C/C++ security, we 
>>> introduced
>>> -fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
>>> usages from C/C++ source code. As a result, zero-length arrays eventually
>>> will be replaced by C99 flexiable array member completely.
>>> Therefore, GCC needs to explicitly allow such extensions directly for C99
>>> flexible arrays, since flexable array member in unions or alone in 
>>> structs
>>> are common code patterns in active use by the Linux kernel (and other 
>>> projects).
>>> For example, these do not error by default with GCC:
>>> union one {
>>>   int a;
>>>   int b[0];
>>> };
>>> union two {
>>>   int a;
>>>   struct {
>>>     struct { } __empty;
>>>     int b[];
>>>   };
>>> };
>>> But these do:
>>> union three {
>>>   int a;
>>>   int b[];
>>> };
>>> struct four {
>>>   int b[];
>>> }
>>> Clang has supported such extensions since March, 2024
>>> https://github.com/llvm/llvm-project/pull/84428
>>> GCC should also support such extensions. This will allow for
>>> a seamless transition for code bases away from zero-length arrays without
>>> losing existing code patterns.
>>> gcc/ChangeLog:
>>> * doc/extend.texi: Add documentation for Flexible Array Members in
>>> Unions and Flexible Array Members alone in Structures.
>>> ---
>>>  gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
>>>  1 file changed, 34 insertions(+)
>>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>>> index 7b54a241a7bf..cba98c8aadd7 100644
>>> --- a/gcc/doc/extend.texi
>>> +++ b/gcc/doc/extend.texi
>>> @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
>>>  * Named Address Spaces::Named address spaces.
>>>  * Zero Length::         Zero-length arrays.
>>>  * Empty Structures::    Structures with no members.
>>> +* Flexible Array Members in Unions::  Unions with Flexible Array 
>>> Members.
>>> +* Flexible Array Members alone in Structures::  Structures with only 
>>> Flexible Array Members.
>>>  * Variable Length::     Arrays whose length is computed at run time.
>>>  * Variadic Macros::     Macros with a variable number of arguments.
>>>  * Escaped Newlines::    Slightly looser rules for escaped newlines.
>>> @@ -1873,6 +1875,38 @@ The structure has size zero.  In C++, empty 
>>> structures are part
>>>  of the language.  G++ treats empty structures as if they had a single
>>>  member of type @code{char}.
>>>  +@node Flexible Array Members in Unions
>>> +@section Unions with Flexible Array Members
>>> +@cindex unions with flexible array members
>>> +@cindex unions with FAMs
>>> +
>>> +GCC permits a C99 flexible array member (FAM) to be in a union:
>>> +
>>> +@smallexample
>>> +union with_fam @{
>>> +  int a;
>>> +  int b[];
>>> +@};
>>> +@end smallexample
>>> +
>>> +If all the members of a union are flexible array member, the size of
> 
> It’s for the following case:
> 
> union with_fam_3 {
>    char a[];
>    int b[];
> }
> 
> And also include:  (the only member of a union is a flexible array 
> member as you mentioned below)
> 
> union with_fam_1 {
>    char a[];
> }
> 
> So, I think the original sentence:
> 
> “If all the members of a union are flexible array member, the size of”
> 
> Should be better than the below:
>>
>> "If the only member of a union is a flexible array member”

Makes sense, but then it should be "members" both times rather than 
"members" and then "member".

Jason

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Kees Cook]

On Tue, Apr 30, 2024 at 05:51:20PM -0400, Jason Merrill wrote:
> On 4/30/24 14:45, Qing Zhao wrote:
> > 
> > 
> > > On Apr 30, 2024, at 15:27, Jason Merrill <jason@redhat.com> wrote:
> > > 
> > > On 4/30/24 07:58, Qing Zhao wrote:
> > > > The request for GCC to accept that the C99 flexible array member can be
> > > > in a union or alone in a structure has been made a long time ago
> > > > around 2012
> > > > for supporting several practical cases including glibc.
> > > > A GCC PR has been opened for such request at that time:
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
> > > > However, this PR was closed as WONTFIX around 2015 due to the
> > > > following reason:
> > > > "there is an existing extension that makes the requested
> > > > functionality possible"
> > > > i.e GCC fully supported that the zero-length array can be in a
> > > > union or alone
> > > > in a structure for a long time. (though I didn't see any
> > > > official documentation
> > > > on such extension)
> > > > It's reasonable to close PR53548 at that time since zero-length
> > > > array extension
> > > > can be used for such purpose.
> > > > However, since GCC13, in order to improve the C/C++ security, we
> > > > introduced
> > > > -fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
> > > > usages from C/C++ source code. As a result, zero-length arrays eventually
> > > > will be replaced by C99 flexiable array member completely.
> > > > Therefore, GCC needs to explicitly allow such extensions directly for C99
> > > > flexible arrays, since flexable array member in unions or alone
> > > > in structs
> > > > are common code patterns in active use by the Linux kernel (and
> > > > other projects).
> > > > For example, these do not error by default with GCC:
> > > > union one {
> > > >   int a;
> > > >   int b[0];
> > > > };
> > > > union two {
> > > >   int a;
> > > >   struct {
> > > >     struct { } __empty;
> > > >     int b[];
> > > >   };
> > > > };
> > > > But these do:
> > > > union three {
> > > >   int a;
> > > >   int b[];
> > > > };
> > > > struct four {
> > > >   int b[];
> > > > }
> > > > Clang has supported such extensions since March, 2024
> > > > https://github.com/llvm/llvm-project/pull/84428
> > > > GCC should also support such extensions. This will allow for
> > > > a seamless transition for code bases away from zero-length arrays without
> > > > losing existing code patterns.
> > > > gcc/ChangeLog:
> > > > * doc/extend.texi: Add documentation for Flexible Array Members in
> > > > Unions and Flexible Array Members alone in Structures.
> > > > ---
> > > >  gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 34 insertions(+)
> > > > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > > > index 7b54a241a7bf..cba98c8aadd7 100644
> > > > --- a/gcc/doc/extend.texi
> > > > +++ b/gcc/doc/extend.texi
> > > > @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
> > > >  * Named Address Spaces::Named address spaces.
> > > >  * Zero Length::         Zero-length arrays.
> > > >  * Empty Structures::    Structures with no members.
> > > > +* Flexible Array Members in Unions::  Unions with Flexible
> > > > Array Members.
> > > > +* Flexible Array Members alone in Structures::  Structures with
> > > > only Flexible Array Members.
> > > >  * Variable Length::     Arrays whose length is computed at run time.
> > > >  * Variadic Macros::     Macros with a variable number of arguments.
> > > >  * Escaped Newlines::    Slightly looser rules for escaped newlines.
> > > > @@ -1873,6 +1875,38 @@ The structure has size zero.  In C++,
> > > > empty structures are part
> > > >  of the language.  G++ treats empty structures as if they had a single
> > > >  member of type @code{char}.
> > > >  +@node Flexible Array Members in Unions
> > > > +@section Unions with Flexible Array Members
> > > > +@cindex unions with flexible array members
> > > > +@cindex unions with FAMs
> > > > +
> > > > +GCC permits a C99 flexible array member (FAM) to be in a union:
> > > > +
> > > > +@smallexample
> > > > +union with_fam @{
> > > > +  int a;
> > > > +  int b[];
> > > > +@};
> > > > +@end smallexample
> > > > +
> > > > +If all the members of a union are flexible array member, the size of
> > 
> > It’s for the following case:
> > 
> > union with_fam_3 {
> >    char a[];
> >    int b[];
> > }
> > 
> > And also include:  (the only member of a union is a flexible array
> > member as you mentioned below)
> > 
> > union with_fam_1 {
> >    char a[];
> > }
> > 
> > So, I think the original sentence:
> > 
> > “If all the members of a union are flexible array member, the size of”
> > 
> > Should be better than the below:
> > > 
> > > "If the only member of a union is a flexible array member”
> 
> Makes sense, but then it should be "members" both times rather than
> "members" and then "member".

"If every member of a union is a flexible array, the size ..." ?

-- 
Kees Cook

Re: [PATCH v3 1/4] Allow flexible array members in unions and alone in structures [PR53548]

[Qing Zhao]

[-- Attachment #1: Type: text/plain, Size: 4381 bytes --]



On Apr 30, 2024, at 17:55, Kees Cook <keescook@chromium.org> wrote:

On Tue, Apr 30, 2024 at 05:51:20PM -0400, Jason Merrill wrote:
On 4/30/24 14:45, Qing Zhao wrote:


On Apr 30, 2024, at 15:27, Jason Merrill <jason@redhat.com> wrote:

On 4/30/24 07:58, Qing Zhao wrote:
The request for GCC to accept that the C99 flexible array member can be
in a union or alone in a structure has been made a long time ago
around 2012
for supporting several practical cases including glibc.
A GCC PR has been opened for such request at that time:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
However, this PR was closed as WONTFIX around 2015 due to the
following reason:
"there is an existing extension that makes the requested
functionality possible"
i.e GCC fully supported that the zero-length array can be in a
union or alone
in a structure for a long time. (though I didn't see any
official documentation
on such extension)
It's reasonable to close PR53548 at that time since zero-length
array extension
can be used for such purpose.
However, since GCC13, in order to improve the C/C++ security, we
introduced
-fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
usages from C/C++ source code. As a result, zero-length arrays eventually
will be replaced by C99 flexiable array member completely.
Therefore, GCC needs to explicitly allow such extensions directly for C99
flexible arrays, since flexable array member in unions or alone
in structs
are common code patterns in active use by the Linux kernel (and
other projects).
For example, these do not error by default with GCC:
union one {
  int a;
  int b[0];
};
union two {
  int a;
  struct {
    struct { } __empty;
    int b[];
  };
};
But these do:
union three {
  int a;
  int b[];
};
struct four {
  int b[];
}
Clang has supported such extensions since March, 2024
https://github.com/llvm/llvm-project/pull/84428
GCC should also support such extensions. This will allow for
a seamless transition for code bases away from zero-length arrays without
losing existing code patterns.
gcc/ChangeLog:
* doc/extend.texi: Add documentation for Flexible Array Members in
Unions and Flexible Array Members alone in Structures.
---
 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7b54a241a7bf..cba98c8aadd7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible
Array Members.
+* Flexible Array Members alone in Structures::  Structures with
only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++,
empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 +@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If all the members of a union are flexible array member, the size of

It’s for the following case:

union with_fam_3 {
  char a[];
  int b[];
}

And also include:  (the only member of a union is a flexible array
member as you mentioned below)

union with_fam_1 {
  char a[];
}

So, I think the original sentence:

“If all the members of a union are flexible array member, the size of”

Should be better than the below:

"If the only member of a union is a flexible array member”

Makes sense, but then it should be "members" both times rather than
"members" and then "member".

"If every member of a union is a flexible array, the size ..." ?

Yes, I have updated the doc as this. -:).

Qing

--
Kees Cook

[PATCH v3 2/4] C and C++ FE changes

[Qing Zhao]

 to support flexible array members
 in unions and alone in structures. Adjust testcases for flexible array member
 in union and alone in structure extension.

gcc/c/ChangeLog:

	* c-decl.cc (finish_struct): Change errors to pedwarns for the cases
	flexible array members in union or alone in structures.

gcc/cp/ChangeLog:

	* class.cc (diagnose_flexarrays): Change error to pdewarn for the case
	flexible array members alone in structures.
	* decl.cc (grokdeclarator): Change error to pdewarn for the case
	flexible array members in unions.

gcc/ChangeLog:

	* stor-layout.cc (place_union_field): Use zero sizes for flexible array
	member fields.

gcc/testsuite/ChangeLog:

	* c-c++-common/builtin-clear-padding-3.c: Adjust testcase.
	* g++.dg/ext/flexary12.C: Likewise.
	* g++.dg/ext/flexary19.C: Likewise.
	* g++.dg/ext/flexary2.C: Likewise.
	* g++.dg/ext/flexary3.C: Likewise.
	* g++.dg/ext/flexary36.C: Likewise.
	* g++.dg/ext/flexary4.C: Likewise.
	* g++.dg/ext/flexary5.C: Likewise.
	* g++.dg/ext/flexary8.C: Likewise.
	* g++.dg/torture/pr64280.C: Likewise.
	* gcc.dg/20050620-1.c: Likewise.
	* gcc.dg/940510-1.c: Likewise.
---
 gcc/c/c-decl.cc                               | 16 ++----
 gcc/cp/class.cc                               | 11 ++--
 gcc/cp/decl.cc                                |  7 ++-
 gcc/stor-layout.cc                            |  9 +++-
 .../c-c++-common/builtin-clear-padding-3.c    | 10 ++--
 gcc/testsuite/g++.dg/ext/flexary12.C          |  6 +--
 gcc/testsuite/g++.dg/ext/flexary19.C          | 42 +++++++--------
 gcc/testsuite/g++.dg/ext/flexary2.C           |  2 +-
 gcc/testsuite/g++.dg/ext/flexary3.C           |  2 +-
 gcc/testsuite/g++.dg/ext/flexary36.C          |  2 +-
 gcc/testsuite/g++.dg/ext/flexary4.C           | 54 +++++++++----------
 gcc/testsuite/g++.dg/ext/flexary5.C           |  4 +-
 gcc/testsuite/g++.dg/ext/flexary8.C           |  8 +--
 gcc/testsuite/g++.dg/torture/pr64280.C        |  2 +-
 gcc/testsuite/gcc.dg/20050620-1.c             |  2 +-
 gcc/testsuite/gcc.dg/940510-1.c               |  4 +-
 16 files changed, 91 insertions(+), 90 deletions(-)

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 345090dae38b..947f3cd589eb 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9471,11 +9471,8 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       if (flexible_array_member_type_p (TREE_TYPE (x)))
 	{
 	  if (TREE_CODE (t) == UNION_TYPE)
-	    {
-	      error_at (DECL_SOURCE_LOCATION (x),
-			"flexible array member in union");
-	      TREE_TYPE (x) = error_mark_node;
-	    }
+	    pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
+		     "flexible array member in union is a GCC extension");
 	  else if (!is_last_field)
 	    {
 	      error_at (DECL_SOURCE_LOCATION (x),
@@ -9483,12 +9480,9 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 	      TREE_TYPE (x) = error_mark_node;
 	    }
 	  else if (!saw_named_field)
-	    {
-	      error_at (DECL_SOURCE_LOCATION (x),
-			"flexible array member in a struct with no named "
-			"members");
-	      TREE_TYPE (x) = error_mark_node;
-	    }
+	    pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
+		     "flexible array member in a struct with no named "
+		     "members is a GCC extension");
 	}
 
       if (pedantic && TREE_CODE (t) == RECORD_TYPE
diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 5f258729940b..0c8afb72550f 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -7624,6 +7624,7 @@ diagnose_flexarrays (tree t, const flexmems_t *fmem)
   bool diagd = false;
 
   const char *msg = 0;
+  const char *msg_fam = 0;
 
   if (TYPE_DOMAIN (TREE_TYPE (fmem->array)))
     {
@@ -7649,15 +7650,19 @@ diagnose_flexarrays (tree t, const flexmems_t *fmem)
       if (fmem->after[0])
 	msg = G_("flexible array member %qD not at end of %q#T");
       else if (!fmem->first)
-	msg = G_("flexible array member %qD in an otherwise empty %q#T");
+	msg_fam = G_("flexible array member %qD in an otherwise"
+		     " empty %q#T is a GCC extension");
 
-      if (msg)
+      if (msg || msg_fam)
 	{
 	  location_t loc = DECL_SOURCE_LOCATION (fmem->array);
 	  diagd = true;
 
 	  auto_diagnostic_group d;
-	  error_at (loc, msg, fmem->array, t);
+	  if (msg)
+	    error_at (loc, msg, fmem->array, t);
+	  else
+	    pedwarn (loc, OPT_Wpedantic, msg_fam, fmem->array, t);
 
 	  /* In the unlikely event that the member following the flexible
 	     array member is declared in a different class, or the member
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 65ab64885ff8..9a91c6f80da1 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -14566,10 +14566,9 @@ grokdeclarator (const cp_declarator *declarator,
 	    if (ctype
 		&& (TREE_CODE (ctype) == UNION_TYPE
 		    || TREE_CODE (ctype) == QUAL_UNION_TYPE))
-	      {
-		error_at (id_loc, "flexible array member in union");
-		type = error_mark_node;
-	      }
+	      pedwarn (id_loc, OPT_Wpedantic,
+		       "flexible array member in union is a GCC extension");
+
 	    else
 	      {
 		/* Array is a flexible member.  */
diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index e34be19689c0..10c0809914cd 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1245,13 +1245,18 @@ place_union_field (record_layout_info rli, tree field)
       && TYPE_TYPELESS_STORAGE (TREE_TYPE (field)))
     TYPE_TYPELESS_STORAGE (rli->t) = 1;
 
+  /* We might see a flexible array member field (with no DECL_SIZE_UNIT), use
+     zero size for such field.  */
+  tree field_size_unit = DECL_SIZE_UNIT (field)
+			 ? DECL_SIZE_UNIT (field)
+			 : build_int_cst (sizetype, 0);
   /* We assume the union's size will be a multiple of a byte so we don't
      bother with BITPOS.  */
   if (TREE_CODE (rli->t) == UNION_TYPE)
-    rli->offset = size_binop (MAX_EXPR, rli->offset, DECL_SIZE_UNIT (field));
+    rli->offset = size_binop (MAX_EXPR, rli->offset, field_size_unit);
   else if (TREE_CODE (rli->t) == QUAL_UNION_TYPE)
     rli->offset = fold_build3 (COND_EXPR, sizetype, DECL_QUALIFIER (field),
-			       DECL_SIZE_UNIT (field), rli->offset);
+			       field_size_unit, rli->offset);
 }
 
 /* A bitfield of SIZE with a required access alignment of ALIGN is allocated
diff --git a/gcc/testsuite/c-c++-common/builtin-clear-padding-3.c b/gcc/testsuite/c-c++-common/builtin-clear-padding-3.c
index d16cc6aad05f..a4f49f26db14 100644
--- a/gcc/testsuite/c-c++-common/builtin-clear-padding-3.c
+++ b/gcc/testsuite/c-c++-common/builtin-clear-padding-3.c
@@ -2,14 +2,12 @@
 /* { dg-do compile } */
 /* { dg-options "" } */
 
-union U { int a; char b[] __attribute__((aligned (2 * sizeof (int)))); };	/* { dg-error "flexible array member in union" } */
+union U { int a; char b[] __attribute__((aligned (2 * sizeof (int)))); };
 struct V { int a; union U b; };
-struct W { int a; union U b; int c; };
 
 void
-foo (union U *u, struct V *v, struct W *w)
+foo (union U *u, struct V *v)
 {
-  __builtin_clear_padding (u);
-  __builtin_clear_padding (v);
-  __builtin_clear_padding (w);
+  __builtin_clear_padding (u); /* { dg-error "flexible array member" "does not have well defined padding bits" } */
+  __builtin_clear_padding (v); /* { dg-error "flexible array member" "does not have well defined padding bits" } */
 }
diff --git a/gcc/testsuite/g++.dg/ext/flexary12.C b/gcc/testsuite/g++.dg/ext/flexary12.C
index b0964948731d..6ba4b6417135 100644
--- a/gcc/testsuite/g++.dg/ext/flexary12.C
+++ b/gcc/testsuite/g++.dg/ext/flexary12.C
@@ -6,7 +6,7 @@
 // { dg-options "-Wno-pedantic" }
 
 struct A {
-  int a [];  // { dg-error "flexible array member .A::a. in an otherwise empty .struct A." }
+  int a [];
 };
 
 void f1 ()
@@ -40,7 +40,7 @@ void f2 ()
 }
 
 struct D {
-  int a [];  // { dg-error "flexible array member .D::a. in an otherwise empty .struct D." }
+  int a [];
   D ();
 };
 
@@ -52,7 +52,7 @@ D::D ():    // { dg-error "initializer for flexible array member" }
 
 template <class T>
 struct C {
-  T a [];  // { dg-error "flexible array member" }
+  T a [];
 };
 
 void f3 ()
diff --git a/gcc/testsuite/g++.dg/ext/flexary19.C b/gcc/testsuite/g++.dg/ext/flexary19.C
index abfbc43028af..9a06f9ca758f 100644
--- a/gcc/testsuite/g++.dg/ext/flexary19.C
+++ b/gcc/testsuite/g++.dg/ext/flexary19.C
@@ -12,7 +12,7 @@ struct S1
   // The following declares a named data member of an unnamed struct
   // (i.e., it is not an anonymous struct).
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s;
 };
 
@@ -21,7 +21,7 @@ struct S2
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s[1];
 };
 
@@ -30,7 +30,7 @@ struct S3
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s[];
 };
 
@@ -39,7 +39,7 @@ struct S4
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s[2];
 };
 
@@ -48,7 +48,7 @@ struct S5
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s[1][2];
 };
 
@@ -57,7 +57,7 @@ struct S6
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s[][2];
 };
 
@@ -66,7 +66,7 @@ struct S7
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } *s;
 };
 
@@ -75,7 +75,7 @@ struct S8
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } **s;
 };
 
@@ -84,7 +84,7 @@ struct S9
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } *s[1];
 };
 
@@ -93,7 +93,7 @@ struct S10
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } *s[];
 };
 
@@ -102,7 +102,7 @@ struct S11
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } **s[1];
 };
 
@@ -111,7 +111,7 @@ struct S12
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } **s[];
 };
 
@@ -120,7 +120,7 @@ struct S13
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } **s[2];
 };
 
@@ -129,7 +129,7 @@ struct S14
   int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } &s;
 };
 
@@ -138,7 +138,7 @@ struct S15
   int i;
 
   typedef struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } T15;
 };
 
@@ -159,8 +159,8 @@ struct S17
 {
   int i;
 
-  union {           // anonymous union
-    int a[];        // { dg-error "flexible array member in union" }
+  union {           // { dg-warning "invalid use" } 
+    int a[];        // { dg-warning "flexible array member in union" }
   };
 };
 
@@ -209,7 +209,7 @@ struct S22
   struct S22S {
     static int i;
 
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s;
 };
 
@@ -218,7 +218,7 @@ struct S23
   struct {          // { dg-warning "10:ISO C\\+\\+ prohibits anonymous struct" }
     static int i;   // { dg-error "static data member" }
 
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   };
 };
 
@@ -227,7 +227,7 @@ struct S24
   static int i;
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s;
 };
 
@@ -252,7 +252,7 @@ struct S26
   };
 
   struct {
-    int a[];        // { dg-error "in an otherwise empty" }
+    int a[];        // { dg-warning "in an otherwise empty" }
   } s;
 };
 
diff --git a/gcc/testsuite/g++.dg/ext/flexary2.C b/gcc/testsuite/g++.dg/ext/flexary2.C
index c0253777a1e9..7095a3b0fb2d 100644
--- a/gcc/testsuite/g++.dg/ext/flexary2.C
+++ b/gcc/testsuite/g++.dg/ext/flexary2.C
@@ -13,7 +13,7 @@ struct A {
 
 struct B {
   B() {}
-  A a[];   // { dg-error "extension|flexible array .* in an otherwise empty" }
+  A a[];
 };
 
 struct C {
diff --git a/gcc/testsuite/g++.dg/ext/flexary3.C b/gcc/testsuite/g++.dg/ext/flexary3.C
index 8344b42dd163..7785c9b190a4 100644
--- a/gcc/testsuite/g++.dg/ext/flexary3.C
+++ b/gcc/testsuite/g++.dg/ext/flexary3.C
@@ -11,7 +11,7 @@
 // { dg-options "" }
 
 struct s {
-    char c[];   // { dg-error "flexible array member .* in an otherwise empty" }
+    char c[];
 };
 
 int main()
diff --git a/gcc/testsuite/g++.dg/ext/flexary36.C b/gcc/testsuite/g++.dg/ext/flexary36.C
index 5bb827cfd0e0..87d6fb092100 100644
--- a/gcc/testsuite/g++.dg/ext/flexary36.C
+++ b/gcc/testsuite/g++.dg/ext/flexary36.C
@@ -25,7 +25,7 @@ struct {
 
 
 union {
-  int a[];          // { dg-error "flexible array member in union" }
+  int a[];
   int b;
 } du = { 1 };
 
diff --git a/gcc/testsuite/g++.dg/ext/flexary4.C b/gcc/testsuite/g++.dg/ext/flexary4.C
index bd28cf55de2b..a87b7e71edb6 100644
--- a/gcc/testsuite/g++.dg/ext/flexary4.C
+++ b/gcc/testsuite/g++.dg/ext/flexary4.C
@@ -11,79 +11,79 @@
 #include "flexary.h"
 
 struct Sx {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 // Verify that non-data members or static data members either before
 // or after a flexible array member in an otherwise empty struct don't
 // suppress the diagnostic.
 struct Sx2 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   typedef int I;
 };
 
 struct Sx3 {
   typedef int I;
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx4 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   enum E { e };
 };
 
 struct Sx5 {
   enum E { e };
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx6 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   static int i;
 };
 
 struct Sx7 {
   static int i;
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx8 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   Sx8 () { }
 };
 
 struct Sx9 {
   Sx9 () { }
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx10 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   virtual ~Sx10 () { }
 };
 
 struct Sx11 {
   virtual ~Sx11 () { }
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx12 {
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
   virtual void foo () = 0;
 };
 
 struct Sx13 {
   virtual void foo () = 0;
-  int a[];                  // { dg-error "in an otherwise empty" }
+  int a[];
 };
 
 struct Sx14 {
-  int a[][1];               // { dg-error "in an otherwise empty" }
+  int a[][1];
 };
 
 struct Sx15 {
   typedef int A[];
-  A a;                      // { dg-error "in an otherwise empty" }
+  A a;
 };
 
 // Verify also that a zero-size array doesn't suppress the diagnostic.
@@ -91,7 +91,7 @@ struct Sx16 {
   // a_0 below is diagnosed with -Wpedantic only and emits
   // warning: ISO C++ forbids zero-size arrays
   int a_0 [0];
-  int a_x [];               // { dg-error "in an otherwise empty" }
+  int a_x [];
 };
 
 struct Sx17 {
@@ -123,7 +123,7 @@ struct Sx19 {
 // so doesn't contribute its member to that of the enclosing struct.
 struct Sx20 {
   struct S { int i; };
-  int a_x [];               // { dg-error "in an otherwise empty" }
+  int a_x [];
 };
 
 struct Sx21 {
@@ -148,12 +148,12 @@ struct Sx24 {
 
 struct Sx25 {
   struct S { };
-  S a_x [];                 // { dg-error "flexible array member" }
+  S a_x [];
 };
 
 struct Sx26 {
   struct { }
-    a_x [];                   // { dg-error "flexible array member" }
+    a_x [];
 };
 
 struct Sx27 {
@@ -193,13 +193,13 @@ struct Sx32 {
 ASSERT_AT_END (Sx32, a);
 
 struct Sx33 {
-  int a [];                 // { dg-error "otherwise empty" }
+  int a [];
   friend int foo ();
 };
 
 struct Sx34 {
   friend int foo ();
-  int a [];                 // { dg-error "otherwise empty" }
+  int a [];
 };
 
 // Verify that intervening non-field declarations of members other
@@ -277,7 +277,7 @@ ASSERT_AT_END (Sx44, a);
 struct S_S_S_x {
   struct A {
     struct B {
-      int a[];              // { dg-error "flexible array member" }
+      int a[];
     } b;
   } a;
 };
@@ -300,7 +300,7 @@ struct NotAnon1 {
   // The following is not an anonymous struct -- the type is unnamed
   // but the object has a name.
   struct {
-    int bad[];              // { dg-error "otherwise empty" }
+    int bad[];
   } name;
 };
 
@@ -328,7 +328,7 @@ ASSERT_AT_END (Anon3, good);
 
 struct Anon4 {
   struct {
-    int in_empty_struct[];  // { dg-error "in an otherwise empty" }
+    int in_empty_struct[];
   };
 };
 
@@ -366,7 +366,7 @@ struct Six {
 ASSERT_AT_END (Six, a);
 
 class Cx {
-  int a[];                  // { dg-error "flexible array member" }
+  int a[];
 };
 
 class Cix {
@@ -390,7 +390,7 @@ struct S0i {
 
 struct S_a0_ax {
   int a0[0];
-  int ax[];                 // { dg-error "flexible array member" }
+  int ax[];
 };
 
 struct S_a0_i_ax {
@@ -417,7 +417,7 @@ struct Si_ax_a0 {
 
 struct S_u0_ax {
   union { } u[0];
-  int ax[];                 // { dg-error "flexible array member" }
+  int ax[];
 };
 
 struct S_a1_s2 {
diff --git a/gcc/testsuite/g++.dg/ext/flexary5.C b/gcc/testsuite/g++.dg/ext/flexary5.C
index d5ec13204966..3fb88c0e8a2c 100644
--- a/gcc/testsuite/g++.dg/ext/flexary5.C
+++ b/gcc/testsuite/g++.dg/ext/flexary5.C
@@ -13,7 +13,7 @@ struct S_no_diag: T {
 
 template <class T>
 struct STx_1: T {
-  char a[];   // { dg-error "flexible array member" }
+  char a[];
 };
 
 template <class T, int I>
@@ -37,7 +37,7 @@ struct E1: E<0>, E<1> { };
 struct E2: E<2>, E<3> { };
 struct D1: E1, E2
 {
-    char a[];   // { dg-error "flexible array member" }
+    char a[];
 };
 
 struct NE { size_t i; };
diff --git a/gcc/testsuite/g++.dg/ext/flexary8.C b/gcc/testsuite/g++.dg/ext/flexary8.C
index 7a1811deaff2..a0477f0fa410 100644
--- a/gcc/testsuite/g++.dg/ext/flexary8.C
+++ b/gcc/testsuite/g++.dg/ext/flexary8.C
@@ -4,26 +4,26 @@
 
 union U_i_ax {
     int i;
-    int a[];                  // { dg-error "flexible array member in union" }
+    int a[];
 };
 
 struct SU1 {
   union {
-    int a[];                  // { dg-error "flexible array member in union" }
+    int a[];
   };
 };
 
 struct SU2 {
   int n;
   union {
-    int a[];                  // { dg-error "flexible array member in union" }
+    int a[];
   };
 };
 
 struct SU3 {
   union {
     int n;
-    int a[];                  // { dg-error "flexible array member in union" }
+    int a[];
   };
 };
 
diff --git a/gcc/testsuite/g++.dg/torture/pr64280.C b/gcc/testsuite/g++.dg/torture/pr64280.C
index 5c569e864b4c..1ea70c4e766e 100644
--- a/gcc/testsuite/g++.dg/torture/pr64280.C
+++ b/gcc/testsuite/g++.dg/torture/pr64280.C
@@ -15,7 +15,7 @@ public:
 typedef int jmp_buf[];
 struct C
 {
-  jmp_buf cond_;   // { dg-error "flexible array member" }
+  jmp_buf cond_;
 };
 class F
 {
diff --git a/gcc/testsuite/gcc.dg/20050620-1.c b/gcc/testsuite/gcc.dg/20050620-1.c
index befdd9636500..0a9e1d478454 100644
--- a/gcc/testsuite/gcc.dg/20050620-1.c
+++ b/gcc/testsuite/gcc.dg/20050620-1.c
@@ -5,7 +5,7 @@
 void
 foo (void)
 {
-  struct { int i[]; } u;	/* { dg-error "flexible array member" } */
+  struct { int i[]; } u;
 }
 
 void
diff --git a/gcc/testsuite/gcc.dg/940510-1.c b/gcc/testsuite/gcc.dg/940510-1.c
index 46183831d096..9bcd7881f715 100644
--- a/gcc/testsuite/gcc.dg/940510-1.c
+++ b/gcc/testsuite/gcc.dg/940510-1.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
 /* { dg-options "-std=c89 -pedantic" } */
 struct { int a[]; } x = { 0 };	/* { dg-warning "ISO C90 does not support flexible array members" } */
-/* { dg-error "flexible array member in a struct with no named members"  "" { target *-*-* }  .-1 } */
-
+/* { dg-warning "flexible array member in a struct with no named members is a GCC extension"  "" { target *-*-* }  .-1 } */
+/* { dg-warning "initialization of a flexible array member"  "" { target *-*-* }  .-2 } */
-- 
2.31.1

[PATCH v3 3/4] Add testing cases for flexible array members in unions and alone in structures.

[Qing Zhao]

gcc/testsuite/ChangeLog:

	* c-c++-common/fam-in-union-alone-in-struct-1.c: New testcase.
	* c-c++-common/fam-in-union-alone-in-struct-2.c: New testcase.
	* c-c++-common/fam-in-union-alone-in-struct-3.c: New testcase.
---
 .../fam-in-union-alone-in-struct-1.c          | 52 +++++++++++++++++++
 .../fam-in-union-alone-in-struct-2.c          | 51 ++++++++++++++++++
 .../fam-in-union-alone-in-struct-3.c          | 36 +++++++++++++
 3 files changed, 139 insertions(+)
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-1.c
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-2.c
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-3.c

diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-1.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-1.c
new file mode 100644
index 000000000000..7d4721aa95ac
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-1.c
@@ -0,0 +1,52 @@
+/* testing the correct usage of flexible array members in unions 
+   and alone in structures.  */
+/* { dg-do run} */
+/* { dg-options "-Wpedantic" } */
+
+union with_fam_1 {
+  int a;
+  int b[];  /* { dg-warning "flexible array member in union is a GCC extension" } */
+};
+
+union with_fam_2 {
+  char a;
+  int b[];  /* { dg-warning "flexible array member in union is a GCC extension" } */
+};
+
+union with_fam_3 {
+  char a[];  /* { dg-warning "flexible array member in union is a GCC extension" } */
+  /* { dg-warning "in an otherwise empty" "" { target c++ } .-1 } */
+  int b[];  /* { dg-warning "flexible array member in union is a GCC extension" } */
+};
+
+struct only_fam {
+  int b[];
+  /* { dg-warning "in a struct with no named members" "" { target c } .-1 } */
+  /* { dg-warning "in an otherwise empty" "" { target c++ } .-2 } */
+  /* { dg-warning "forbids flexible array member" "" { target c++ } .-3 } */
+};
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[];
+  /* { dg-warning "in a struct with no named members" "" { target c } .-1 } */
+  /* { dg-warning "in an otherwise empty" "" { target c++ } .-2 } */
+  /* { dg-warning "forbids flexible array member" "" { target c++ } .-3 } */
+};
+
+int main ()
+{
+  if (sizeof (union with_fam_1) != sizeof (int))
+    __builtin_abort ();
+  if (sizeof (union with_fam_2) != __alignof__ (int))
+    __builtin_abort ();
+  if (sizeof (union with_fam_3) != 0)
+    __builtin_abort ();
+  if (sizeof (struct only_fam) != 0)
+    __builtin_abort ();
+  if (sizeof (struct only_fam_2) != sizeof (int))
+    __builtin_abort ();
+  return 0;
+}
+
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-2.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-2.c
new file mode 100644
index 000000000000..3743f9e7dac5
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-2.c
@@ -0,0 +1,51 @@
+/* testing the correct usage of flexible array members in unions 
+   and alone in structures: initialization  */
+/* { dg-do run} */
+/* { dg-options "-O2" } */
+
+union with_fam_1 {
+  int a;
+  int b[]; 
+} with_fam_1_v = {.b = {1, 2, 3, 4}};
+
+union with_fam_2 {
+  int a;
+  char b[];  
+} with_fam_2_v = {.a = 0x1f2f3f4f};
+
+union with_fam_3 {
+  char a[];  
+  int b[];  
+} with_fam_3_v = {.b = {0x1f2f3f4f, 0x5f6f7f7f}};
+
+struct only_fam {
+  int b[]; 
+} only_fam_v = {{7, 11}};
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[]; 
+} only_fam_2_v = {{7, 11}};
+
+int main ()
+{
+  if (with_fam_1_v.b[3] != 4
+      || with_fam_1_v.b[0] != 1)
+    __builtin_abort ();
+  if (with_fam_2_v.b[3] != 0x1f
+      || with_fam_2_v.b[0] != 0x4f)
+    __builtin_abort ();
+  if (with_fam_3_v.a[0] != 0x4f
+      || with_fam_3_v.a[7] != 0x5f)
+    __builtin_abort ();
+  if (only_fam_v.b[0] != 7
+      || only_fam_v.b[1] != 11)
+    __builtin_abort ();
+  if (only_fam_2_v.b[0] != 7
+      || only_fam_2_v.b[1] != 11)
+    __builtin_abort ();
+
+  return 0;
+}
+
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-3.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-3.c
new file mode 100644
index 000000000000..dd36fa01306d
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-3.c
@@ -0,0 +1,36 @@
+/* testing the correct usage of flexible array members in unions 
+   and alone in structures.  */
+/* { dg-do compile } */
+/* { dg-options "-pedantic-errors" } */
+
+union with_fam_1 {
+  int a;
+  int b[];  /* { dg-error "flexible array member in union is a GCC extension" } */
+};
+
+union with_fam_2 {
+  char a;
+  int b[];  /* { dg-error "flexible array member in union is a GCC extension" } */
+};
+
+union with_fam_3 {
+  char a[];  /* { dg-error "flexible array member in union is a GCC extension" } */
+  /* { dg-error "in an otherwise empty" "" { target c++ } .-1 } */
+  int b[];  /* { dg-error "flexible array member in union is a GCC extension" } */
+};
+
+struct only_fam {
+  int b[];
+  /* { dg-error "in a struct with no named members" "" { target c } .-1 } */
+  /* { dg-error "in an otherwise empty" "" { target c++ } .-2 } */
+  /* { dg-error "forbids flexible array member" "" { target c++ } .-3 } */
+};
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[];
+  /* { dg-error "in a struct with no named members" "" { target c } .-1 } */
+  /* { dg-error "in an otherwise empty" "" { target c++ } .-2 } */
+  /* { dg-error "forbids flexible array member" "" { target c++ } .-3 } */
+};
-- 
2.31.1

[PATCH v3 4/4] Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_decl" to handle the cases when the DECL is union.

[Qing Zhao]

Add testing cases to test the _bos for flexible array members in unions
or alone in structures.

gcc/c/ChangeLog:

	* c-decl.cc (add_flexible_array_elts_to_size): Handle the cases
	when the DECL is union.

gcc/cp/ChangeLog:

	* decl.cc (layout_var_decl): Handle the cases when the DECL is
	union with a flexible array member initializer.

gcc/testsuite/ChangeLog:

	* c-c++-common/fam-in-union-alone-in-struct-bos-1.c: New test.
	* c-c++-common/fam-in-union-alone-in-struct-bos.c: New test.
---
 gcc/c/c-decl.cc                               | 32 +++++++--
 gcc/cp/decl.cc                                | 33 ++++++++--
 .../fam-in-union-alone-in-struct-bos-1.c      | 66 +++++++++++++++++++
 .../fam-in-union-alone-in-struct-bos.c        | 45 +++++++++++++
 4 files changed, 162 insertions(+), 14 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 947f3cd589eb..59302c5cfb1f 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -5337,9 +5337,9 @@ zero_length_array_type_p (const_tree type)
 }
 
 /* INIT is a constructor that forms DECL's initializer.  If the final
-   element initializes a flexible array field, add the size of that
-   initializer to DECL's size.  */
-
+   element initializes a flexible array field, adjust the size of the
+   DECL with the initializer based on whether the DECL is a union or
+   a structure.  */
 static void
 add_flexible_array_elts_to_size (tree decl, tree init)
 {
@@ -5348,15 +5348,33 @@ add_flexible_array_elts_to_size (tree decl, tree init)
   if (vec_safe_is_empty (CONSTRUCTOR_ELTS (init)))
     return;
 
+  bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);
+
   elt = CONSTRUCTOR_ELTS (init)->last ().value;
   type = TREE_TYPE (elt);
   if (flexible_array_member_type_p (type))
     {
       complete_array_type (&type, elt, false);
-      DECL_SIZE (decl)
-	= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
-      DECL_SIZE_UNIT (decl)
-	= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl), TYPE_SIZE_UNIT (type));
+      /* For a structure, add the size of the initializer to the DECL's
+	 size.  */
+      if (!is_decl_type_union)
+	{
+	  DECL_SIZE (decl)
+	    = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+	  DECL_SIZE_UNIT (decl)
+	    = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+			  TYPE_SIZE_UNIT (type));
+	}
+      /* For a union, the DECL's size is the maximum of the current size
+	 and the size of the initializer.  */
+      else
+	{
+	  DECL_SIZE (decl)
+	    = size_binop (MAX_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+	  DECL_SIZE_UNIT (decl)
+	    = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
+			  TYPE_SIZE_UNIT (type));
+	}
     }
 }
 \f
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 9a91c6f80da1..0ea3ef165b66 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -6555,8 +6555,9 @@ layout_var_decl (tree decl)
 	}
     }
 
-  /* If the final element initializes a flexible array field, add the size of
-     that initializer to DECL's size.  */
+  /* If the final element initializes a flexible array field, adjust
+     the size of the DECL with the initializer based on whether the
+     DECL is a union or a structure.  */
   if (type != error_mark_node
       && DECL_INITIAL (decl)
       && TREE_CODE (DECL_INITIAL (decl)) == CONSTRUCTOR
@@ -6568,6 +6569,7 @@ layout_var_decl (tree decl)
       && tree_int_cst_equal (DECL_SIZE (decl), TYPE_SIZE (type)))
     {
       constructor_elt &elt = CONSTRUCTOR_ELTS (DECL_INITIAL (decl))->last ();
+      bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);
       if (elt.index)
 	{
 	  tree itype = TREE_TYPE (elt.index);
@@ -6577,11 +6579,28 @@ layout_var_decl (tree decl)
 	      && TREE_CODE (vtype) == ARRAY_TYPE
 	      && COMPLETE_TYPE_P (vtype))
 	    {
-	      DECL_SIZE (decl)
-		= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (vtype));
-	      DECL_SIZE_UNIT (decl)
-		= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
-			      TYPE_SIZE_UNIT (vtype));
+	      /* For a structure, add the size of the initializer to the DECL's
+		 size.  */
+	      if (!is_decl_type_union)
+		{
+		  DECL_SIZE (decl)
+		    = size_binop (PLUS_EXPR, DECL_SIZE (decl),
+				  TYPE_SIZE (vtype));
+		  DECL_SIZE_UNIT (decl)
+		    = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+				  TYPE_SIZE_UNIT (vtype));
+		}
+	      /* a union, the DECL's size is the maximum of the current size
+		 and the size of the initializer.  */
+	      else
+		{
+		  DECL_SIZE (decl)
+		    = size_binop (MAX_EXPR, DECL_SIZE (decl),
+				  TYPE_SIZE (vtype));
+		  DECL_SIZE_UNIT (decl)
+		    = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
+				  TYPE_SIZE_UNIT (vtype));
+		}
 	    }
 	}
     }
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
new file mode 100644
index 000000000000..aae9cf39c8c7
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
@@ -0,0 +1,66 @@
+/* testing flexible array members in unions and alone in structures:
+   __bos/__bdos  */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+union with_fam_1 {
+  char a;
+  int b[]; 
+} *with_fam_1_v;
+
+union with_fam_2 {
+  int a;
+  char b[];  
+} *with_fam_2_v;
+
+union with_fam_3 {
+  char a[];  
+  int b[];  
+} *with_fam_3_v;
+
+struct only_fam {
+  int b[]; 
+} *only_fam_v;
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[]; 
+} *only_fam_2_v;
+
+void __attribute__((__noinline__))
+setup (int n1, int n2, int n3, int n4, int n5)
+{
+  with_fam_1_v = (union with_fam_1 *) __builtin_malloc (n1 * sizeof (int));
+  with_fam_2_v = (union with_fam_2 *) __builtin_malloc (n2 * sizeof (char));
+  with_fam_3_v = (union with_fam_3 *) __builtin_malloc (n3 * sizeof (int));
+  only_fam_v = (struct only_fam *) __builtin_malloc (n4 * sizeof (int));
+  only_fam_2_v = (struct only_fam_2 *) __builtin_malloc (n5 * sizeof (int));
+  return;
+}
+
+void __attribute__((__noinline__)) stuff(
+    union with_fam_1 *with_fam_1_v,
+    union with_fam_2 *with_fam_2_v,
+    union with_fam_3 *with_fam_3_v,
+    struct only_fam *only_fam_v,
+    struct only_fam_2 *only_fam_2_v)
+{
+  if (__builtin_object_size(with_fam_1_v->b, 1) != -1)
+    __builtin_abort (); 
+  if (__builtin_object_size(with_fam_2_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_3_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_2_v->b, 1) != -1)
+    __builtin_abort ();
+}
+
+int main (int argc, char *argv[])
+{
+  setup (2, 3, 4, 5, 6);
+  stuff (with_fam_1_v, with_fam_2_v, with_fam_3_v, only_fam_v, only_fam_2_v);
+  return 0;
+}
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
new file mode 100644
index 000000000000..21badc57982b
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
@@ -0,0 +1,45 @@
+/* testing flexible array members in unions and alone in structures:
+   __bos/__bdos  */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+union with_fam_1 {
+  char a;
+  int b[]; 
+} with_fam_1_v = {.b = {1, 2, 3, 4, 5}};
+
+union with_fam_2 {
+  int a;
+  char b[];  
+} with_fam_2_v = {.a = 0x1f2f3f4f};
+
+union with_fam_3 {
+  char a[];  
+  int b[];  
+} with_fam_3_v = {.b = {0x1f2f3f4f, 0x5f6f7f7f}};
+
+struct only_fam {
+  int b[]; 
+} only_fam_v = {{7, 11}};
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[]; 
+} only_fam_2_v = {{7, 11}};
+
+int main ()
+{
+  if (__builtin_object_size(with_fam_1_v.b, 1) != 20)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_2_v.b, 1) != 4)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_3_v.b, 1) != 8)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_v.b, 1) != 8)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_2_v.b, 1) != 8)
+    __builtin_abort ();
+
+  return 0;
+}
-- 
2.31.1

Re: [PATCH v3 4/4] Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_decl" to handle the cases when the DECL is union.

[Jason Merrill]

On 4/30/24 07:58, Qing Zhao wrote:
> Add testing cases to test the _bos for flexible array members in unions
> or alone in structures.
> 
> gcc/c/ChangeLog:
> 
> 	* c-decl.cc (add_flexible_array_elts_to_size): Handle the cases
> 	when the DECL is union.
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (layout_var_decl): Handle the cases when the DECL is
> 	union with a flexible array member initializer.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* c-c++-common/fam-in-union-alone-in-struct-bos-1.c: New test.
> 	* c-c++-common/fam-in-union-alone-in-struct-bos.c: New test.
> ---
>   gcc/c/c-decl.cc                               | 32 +++++++--
>   gcc/cp/decl.cc                                | 33 ++++++++--
>   .../fam-in-union-alone-in-struct-bos-1.c      | 66 +++++++++++++++++++
>   .../fam-in-union-alone-in-struct-bos.c        | 45 +++++++++++++
>   4 files changed, 162 insertions(+), 14 deletions(-)
>   create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
>   create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
> 
> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
> index 947f3cd589eb..59302c5cfb1f 100644
> --- a/gcc/c/c-decl.cc
> +++ b/gcc/c/c-decl.cc
> @@ -5337,9 +5337,9 @@ zero_length_array_type_p (const_tree type)
>   }
>   
>   /* INIT is a constructor that forms DECL's initializer.  If the final
> -   element initializes a flexible array field, add the size of that
> -   initializer to DECL's size.  */
> -
> +   element initializes a flexible array field, adjust the size of the
> +   DECL with the initializer based on whether the DECL is a union or
> +   a structure.  */
>   static void
>   add_flexible_array_elts_to_size (tree decl, tree init)
>   {
> @@ -5348,15 +5348,33 @@ add_flexible_array_elts_to_size (tree decl, tree init)
>     if (vec_safe_is_empty (CONSTRUCTOR_ELTS (init)))
>       return;
>   
> +  bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);

In both C and C++ front-ends, I don't see any reason for this variable 
to be declared outside the if, since it's only used inside it.

It's not clear to me that it's needed at all since it's only used once, 
but if you prefer to have it, please move it inside the if.

> +
>     elt = CONSTRUCTOR_ELTS (init)->last ().value;
>     type = TREE_TYPE (elt);
>     if (flexible_array_member_type_p (type))
>       {
>         complete_array_type (&type, elt, false);
> -      DECL_SIZE (decl)
> -	= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
> -      DECL_SIZE_UNIT (decl)
> -	= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl), TYPE_SIZE_UNIT (type));
> +      /* For a structure, add the size of the initializer to the DECL's
> +	 size.  */
> +      if (!is_decl_type_union)
> +	{
> +	  DECL_SIZE (decl)
> +	    = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
> +	  DECL_SIZE_UNIT (decl)
> +	    = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
> +			  TYPE_SIZE_UNIT (type));
> +	}
> +      /* For a union, the DECL's size is the maximum of the current size
> +	 and the size of the initializer.  */
> +      else
> +	{
> +	  DECL_SIZE (decl)
> +	    = size_binop (MAX_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
> +	  DECL_SIZE_UNIT (decl)
> +	    = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
> +			  TYPE_SIZE_UNIT (type));
> +	}
>       }
>   }
>   \f
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 9a91c6f80da1..0ea3ef165b66 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -6555,8 +6555,9 @@ layout_var_decl (tree decl)
>   	}
>       }
>   
> -  /* If the final element initializes a flexible array field, add the size of
> -     that initializer to DECL's size.  */
> +  /* If the final element initializes a flexible array field, adjust
> +     the size of the DECL with the initializer based on whether the
> +     DECL is a union or a structure.  */
>     if (type != error_mark_node
>         && DECL_INITIAL (decl)
>         && TREE_CODE (DECL_INITIAL (decl)) == CONSTRUCTOR
> @@ -6568,6 +6569,7 @@ layout_var_decl (tree decl)
>         && tree_int_cst_equal (DECL_SIZE (decl), TYPE_SIZE (type)))
>       {
>         constructor_elt &elt = CONSTRUCTOR_ELTS (DECL_INITIAL (decl))->last ();
> +      bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);
>         if (elt.index)
>   	{
>   	  tree itype = TREE_TYPE (elt.index);
> @@ -6577,11 +6579,28 @@ layout_var_decl (tree decl)
>   	      && TREE_CODE (vtype) == ARRAY_TYPE
>   	      && COMPLETE_TYPE_P (vtype))
>   	    {
> -	      DECL_SIZE (decl)
> -		= size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (vtype));
> -	      DECL_SIZE_UNIT (decl)
> -		= size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
> -			      TYPE_SIZE_UNIT (vtype));
> +	      /* For a structure, add the size of the initializer to the DECL's
> +		 size.  */
> +	      if (!is_decl_type_union)
> +		{
> +		  DECL_SIZE (decl)
> +		    = size_binop (PLUS_EXPR, DECL_SIZE (decl),
> +				  TYPE_SIZE (vtype));
> +		  DECL_SIZE_UNIT (decl)
> +		    = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
> +				  TYPE_SIZE_UNIT (vtype));
> +		}
> +	      /* a union, the DECL's size is the maximum of the current size
> +		 and the size of the initializer.  */
> +	      else
> +		{
> +		  DECL_SIZE (decl)
> +		    = size_binop (MAX_EXPR, DECL_SIZE (decl),
> +				  TYPE_SIZE (vtype));
> +		  DECL_SIZE_UNIT (decl)
> +		    = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
> +				  TYPE_SIZE_UNIT (vtype));
> +		}
>   	    }
>   	}
>       }
> diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
> new file mode 100644
> index 000000000000..aae9cf39c8c7
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
> @@ -0,0 +1,66 @@
> +/* testing flexible array members in unions and alone in structures:
> +   __bos/__bdos  */
> +/* { dg-do run } */
> +/* { dg-options "-O2" } */
> +
> +union with_fam_1 {
> +  char a;
> +  int b[];
> +} *with_fam_1_v;
> +
> +union with_fam_2 {
> +  int a;
> +  char b[];
> +} *with_fam_2_v;
> +
> +union with_fam_3 {
> +  char a[];
> +  int b[];
> +} *with_fam_3_v;
> +
> +struct only_fam {
> +  int b[];
> +} *only_fam_v;
> +
> +struct only_fam_2 {
> +  unsigned int : 2;
> +  unsigned int : 3;
> +  int b[];
> +} *only_fam_2_v;
> +
> +void __attribute__((__noinline__))
> +setup (int n1, int n2, int n3, int n4, int n5)
> +{
> +  with_fam_1_v = (union with_fam_1 *) __builtin_malloc (n1 * sizeof (int));
> +  with_fam_2_v = (union with_fam_2 *) __builtin_malloc (n2 * sizeof (char));
> +  with_fam_3_v = (union with_fam_3 *) __builtin_malloc (n3 * sizeof (int));
> +  only_fam_v = (struct only_fam *) __builtin_malloc (n4 * sizeof (int));
> +  only_fam_2_v = (struct only_fam_2 *) __builtin_malloc (n5 * sizeof (int));
> +  return;
> +}
> +
> +void __attribute__((__noinline__)) stuff(
> +    union with_fam_1 *with_fam_1_v,
> +    union with_fam_2 *with_fam_2_v,
> +    union with_fam_3 *with_fam_3_v,
> +    struct only_fam *only_fam_v,
> +    struct only_fam_2 *only_fam_2_v)
> +{
> +  if (__builtin_object_size(with_fam_1_v->b, 1) != -1)
> +    __builtin_abort ();
> +  if (__builtin_object_size(with_fam_2_v->b, 1) != -1)
> +    __builtin_abort ();
> +  if (__builtin_object_size(with_fam_3_v->b, 1) != -1)
> +    __builtin_abort ();
> +  if (__builtin_object_size(only_fam_v->b, 1) != -1)
> +    __builtin_abort ();
> +  if (__builtin_object_size(only_fam_2_v->b, 1) != -1)
> +    __builtin_abort ();
> +}
> +
> +int main (int argc, char *argv[])
> +{
> +  setup (2, 3, 4, 5, 6);
> +  stuff (with_fam_1_v, with_fam_2_v, with_fam_3_v, only_fam_v, only_fam_2_v);
> +  return 0;
> +}
> diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
> new file mode 100644
> index 000000000000..21badc57982b
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
> @@ -0,0 +1,45 @@
> +/* testing flexible array members in unions and alone in structures:
> +   __bos/__bdos  */
> +/* { dg-do run } */
> +/* { dg-options "-O2" } */
> +
> +union with_fam_1 {
> +  char a;
> +  int b[];
> +} with_fam_1_v = {.b = {1, 2, 3, 4, 5}};
> +
> +union with_fam_2 {
> +  int a;
> +  char b[];
> +} with_fam_2_v = {.a = 0x1f2f3f4f};
> +
> +union with_fam_3 {
> +  char a[];
> +  int b[];
> +} with_fam_3_v = {.b = {0x1f2f3f4f, 0x5f6f7f7f}};
> +
> +struct only_fam {
> +  int b[];
> +} only_fam_v = {{7, 11}};
> +
> +struct only_fam_2 {
> +  unsigned int : 2;
> +  unsigned int : 3;
> +  int b[];
> +} only_fam_2_v = {{7, 11}};
> +
> +int main ()
> +{
> +  if (__builtin_object_size(with_fam_1_v.b, 1) != 20)
> +    __builtin_abort ();
> +  if (__builtin_object_size(with_fam_2_v.b, 1) != 4)
> +    __builtin_abort ();
> +  if (__builtin_object_size(with_fam_3_v.b, 1) != 8)
> +    __builtin_abort ();
> +  if (__builtin_object_size(only_fam_v.b, 1) != 8)
> +    __builtin_abort ();
> +  if (__builtin_object_size(only_fam_2_v.b, 1) != 8)
> +    __builtin_abort ();
> +
> +  return 0;
> +}

Re: [PATCH v3 4/4] Update the C FE routine "add_flexible_array_elts_to_size" C++ FE routine "layout_var_decl" to handle the cases when the DECL is union.

[Qing Zhao]

[-- Attachment #1: Type: text/plain, Size: 8845 bytes --]



On Apr 30, 2024, at 15:29, Jason Merrill <jason@redhat.com> wrote:

On 4/30/24 07:58, Qing Zhao wrote:
Add testing cases to test the _bos for flexible array members in unions
or alone in structures.
gcc/c/ChangeLog:
* c-decl.cc (add_flexible_array_elts_to_size): Handle the cases
when the DECL is union.
gcc/cp/ChangeLog:
* decl.cc (layout_var_decl): Handle the cases when the DECL is
union with a flexible array member initializer.
gcc/testsuite/ChangeLog:
* c-c++-common/fam-in-union-alone-in-struct-bos-1.c: New test.
* c-c++-common/fam-in-union-alone-in-struct-bos.c: New test.
---
 gcc/c/c-decl.cc                               | 32 +++++++--
 gcc/cp/decl.cc                                | 33 ++++++++--
 .../fam-in-union-alone-in-struct-bos-1.c      | 66 +++++++++++++++++++
 .../fam-in-union-alone-in-struct-bos.c        | 45 +++++++++++++
 4 files changed, 162 insertions(+), 14 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
 create mode 100644 gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 947f3cd589eb..59302c5cfb1f 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -5337,9 +5337,9 @@ zero_length_array_type_p (const_tree type)
 }
   /* INIT is a constructor that forms DECL's initializer.  If the final
-   element initializes a flexible array field, add the size of that
-   initializer to DECL's size.  */
-
+   element initializes a flexible array field, adjust the size of the
+   DECL with the initializer based on whether the DECL is a union or
+   a structure.  */
 static void
 add_flexible_array_elts_to_size (tree decl, tree init)
 {
@@ -5348,15 +5348,33 @@ add_flexible_array_elts_to_size (tree decl, tree init)
   if (vec_safe_is_empty (CONSTRUCTOR_ELTS (init)))
     return;
 +  bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);

In both C and C++ front-ends, I don't see any reason for this variable to be declared outside the if, since it's only used inside it.

It's not clear to me that it's needed at all since it's only used once, but if you prefer to have it, please move it inside the if.

Okay, will do that.

thanks.

Qing

+
   elt = CONSTRUCTOR_ELTS (init)->last ().value;
   type = TREE_TYPE (elt);
   if (flexible_array_member_type_p (type))
     {
       complete_array_type (&type, elt, false);
-      DECL_SIZE (decl)
- = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
-      DECL_SIZE_UNIT (decl)
- = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl), TYPE_SIZE_UNIT (type));
+      /* For a structure, add the size of the initializer to the DECL's
+  size.  */
+      if (!is_decl_type_union)
+ {
+   DECL_SIZE (decl)
+     = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+   DECL_SIZE_UNIT (decl)
+     = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+   TYPE_SIZE_UNIT (type));
+ }
+      /* For a union, the DECL's size is the maximum of the current size
+  and the size of the initializer.  */
+      else
+ {
+   DECL_SIZE (decl)
+     = size_binop (MAX_EXPR, DECL_SIZE (decl), TYPE_SIZE (type));
+   DECL_SIZE_UNIT (decl)
+     = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
+   TYPE_SIZE_UNIT (type));
+ }
     }
 }

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 9a91c6f80da1..0ea3ef165b66 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -6555,8 +6555,9 @@ layout_var_decl (tree decl)
  }
     }
 -  /* If the final element initializes a flexible array field, add the size of
-     that initializer to DECL's size.  */
+  /* If the final element initializes a flexible array field, adjust
+     the size of the DECL with the initializer based on whether the
+     DECL is a union or a structure.  */
   if (type != error_mark_node
       && DECL_INITIAL (decl)
       && TREE_CODE (DECL_INITIAL (decl)) == CONSTRUCTOR
@@ -6568,6 +6569,7 @@ layout_var_decl (tree decl)
       && tree_int_cst_equal (DECL_SIZE (decl), TYPE_SIZE (type)))
     {
       constructor_elt &elt = CONSTRUCTOR_ELTS (DECL_INITIAL (decl))->last ();
+      bool is_decl_type_union = (TREE_CODE (TREE_TYPE (decl)) == UNION_TYPE);
       if (elt.index)
  {
    tree itype = TREE_TYPE (elt.index);
@@ -6577,11 +6579,28 @@ layout_var_decl (tree decl)
        && TREE_CODE (vtype) == ARRAY_TYPE
        && COMPLETE_TYPE_P (vtype))
      {
-       DECL_SIZE (decl)
- = size_binop (PLUS_EXPR, DECL_SIZE (decl), TYPE_SIZE (vtype));
-       DECL_SIZE_UNIT (decl)
- = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
-       TYPE_SIZE_UNIT (vtype));
+       /* For a structure, add the size of the initializer to the DECL's
+  size.  */
+       if (!is_decl_type_union)
+ {
+   DECL_SIZE (decl)
+     = size_binop (PLUS_EXPR, DECL_SIZE (decl),
+   TYPE_SIZE (vtype));
+   DECL_SIZE_UNIT (decl)
+     = size_binop (PLUS_EXPR, DECL_SIZE_UNIT (decl),
+   TYPE_SIZE_UNIT (vtype));
+ }
+       /* a union, the DECL's size is the maximum of the current size
+  and the size of the initializer.  */
+       else
+ {
+   DECL_SIZE (decl)
+     = size_binop (MAX_EXPR, DECL_SIZE (decl),
+   TYPE_SIZE (vtype));
+   DECL_SIZE_UNIT (decl)
+     = size_binop (MAX_EXPR, DECL_SIZE_UNIT (decl),
+   TYPE_SIZE_UNIT (vtype));
+ }
      }
  }
     }
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
new file mode 100644
index 000000000000..aae9cf39c8c7
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos-1.c
@@ -0,0 +1,66 @@
+/* testing flexible array members in unions and alone in structures:
+   __bos/__bdos  */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+union with_fam_1 {
+  char a;
+  int b[];
+} *with_fam_1_v;
+
+union with_fam_2 {
+  int a;
+  char b[];
+} *with_fam_2_v;
+
+union with_fam_3 {
+  char a[];
+  int b[];
+} *with_fam_3_v;
+
+struct only_fam {
+  int b[];
+} *only_fam_v;
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[];
+} *only_fam_2_v;
+
+void __attribute__((__noinline__))
+setup (int n1, int n2, int n3, int n4, int n5)
+{
+  with_fam_1_v = (union with_fam_1 *) __builtin_malloc (n1 * sizeof (int));
+  with_fam_2_v = (union with_fam_2 *) __builtin_malloc (n2 * sizeof (char));
+  with_fam_3_v = (union with_fam_3 *) __builtin_malloc (n3 * sizeof (int));
+  only_fam_v = (struct only_fam *) __builtin_malloc (n4 * sizeof (int));
+  only_fam_2_v = (struct only_fam_2 *) __builtin_malloc (n5 * sizeof (int));
+  return;
+}
+
+void __attribute__((__noinline__)) stuff(
+    union with_fam_1 *with_fam_1_v,
+    union with_fam_2 *with_fam_2_v,
+    union with_fam_3 *with_fam_3_v,
+    struct only_fam *only_fam_v,
+    struct only_fam_2 *only_fam_2_v)
+{
+  if (__builtin_object_size(with_fam_1_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_2_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_3_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_v->b, 1) != -1)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_2_v->b, 1) != -1)
+    __builtin_abort ();
+}
+
+int main (int argc, char *argv[])
+{
+  setup (2, 3, 4, 5, 6);
+  stuff (with_fam_1_v, with_fam_2_v, with_fam_3_v, only_fam_v, only_fam_2_v);
+  return 0;
+}
diff --git a/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
new file mode 100644
index 000000000000..21badc57982b
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/fam-in-union-alone-in-struct-bos.c
@@ -0,0 +1,45 @@
+/* testing flexible array members in unions and alone in structures:
+   __bos/__bdos  */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+union with_fam_1 {
+  char a;
+  int b[];
+} with_fam_1_v = {.b = {1, 2, 3, 4, 5}};
+
+union with_fam_2 {
+  int a;
+  char b[];
+} with_fam_2_v = {.a = 0x1f2f3f4f};
+
+union with_fam_3 {
+  char a[];
+  int b[];
+} with_fam_3_v = {.b = {0x1f2f3f4f, 0x5f6f7f7f}};
+
+struct only_fam {
+  int b[];
+} only_fam_v = {{7, 11}};
+
+struct only_fam_2 {
+  unsigned int : 2;
+  unsigned int : 3;
+  int b[];
+} only_fam_2_v = {{7, 11}};
+
+int main ()
+{
+  if (__builtin_object_size(with_fam_1_v.b, 1) != 20)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_2_v.b, 1) != 4)
+    __builtin_abort ();
+  if (__builtin_object_size(with_fam_3_v.b, 1) != 8)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_v.b, 1) != 8)
+    __builtin_abort ();
+  if (__builtin_object_size(only_fam_2_v.b, 1) != 8)
+    __builtin_abort ();
+
+  return 0;
+}