Re: [GCC13][Patch][V2][1/2]Add a new option -fstrict-flex-array[=n] and attribute strict_flex_array(n) and use it in PR101836

[Qing Zhao <qing.zhao@oracle.com>]

> On Aug 2, 2022, at 3:03 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Mon, 1 Aug 2022, Qing Zhao wrote:
> 
>> 
>> 
>>> On Aug 1, 2022, at 3:38 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Fri, 29 Jul 2022, Qing Zhao wrote:
>>> 
>>>> Hi, Richard,
>>>> 
>>>> Thanks a lot for your comments and suggestions. (And sorry for my late reply).
>>>> 
>>>>> On Jul 28, 2022, at 3:26 AM, Richard Biener <rguenther@suse.de> wrote:
>>>>> 
>>>>> On Tue, 19 Jul 2022, Qing Zhao wrote:
>>>>> 
>>>>>> From 3854004802b8e2f132ebf218fc35a632f5e80c6a Mon Sep 17 00:00:00 2001
>>>>>> From: Qing Zhao <qing.zhao@oracle.com>
>>>>>> Date: Mon, 18 Jul 2022 17:04:12 +0000
>>>>>> Subject: [PATCH 1/2] Add a new option -fstrict-flex-array[=n] and new
>>>>>> attribute strict_flex_array
>>>>>> 
>>>>>> Add the following new option -fstrict-flex-array[=n] and a corresponding
>>>>>> attribute strict_flex_array to GCC:
>>>>>> 
>>>>>> '-fstrict-flex-array'
>>>>>>  Treat the trailing array of a structure as a flexible array member
>>>>>>  in a stricter way.  The positive form is equivalent to
>>>>>>  '-fstrict-flex-array=3', which is the strictest.  A trailing array
>>>>>>  is treated as a flexible array member only when it is declared as a
>>>>>>  flexible array member per C99 standard onwards.  The negative form
>>>>>>  is equivalent to '-fstrict-flex-array=0', which is the least
>>>>>>  strict.  All trailing arrays of structures are treated as flexible
>>>>>>  array members.
>>>>>> 
>>>>>> '-fstrict-flex-array=LEVEL'
>>>>>>  Treat the trailing array of a structure as a flexible array member
>>>>>>  in a stricter way.  The value of LEVEL controls the level of
>>>>>>  strictness.
>>>>>> 
>>>>>>  The possible values of LEVEL are the same as for the
>>>>>>  'strict_flex_array' attribute (*note Variable Attributes::).
>>>>>> 
>>>>>>  You can control this behavior for a specific trailing array field
>>>>>>  of a structure by using the variable attribute 'strict_flex_array'
>>>>>>  attribute (*note Variable Attributes::).
>>>>>> 
>>>>>> 'strict_flex_array (LEVEL)'
>>>>>>  The 'strict_flex_array' attribute should be attached to the
>>>>>>  trailing array field of a structure.  It specifies the level of
>>>>>>  strictness of treating the trailing array field of a structure as a
>>>>>>  flexible array member.  LEVEL must be an integer betwen 0 to 3.
>>>>>> 
>>>>>>  LEVEL=0 is the least strict level, all trailing arrays of
>>>>>>  structures are treated as flexible array members.  LEVEL=3 is the
>>>>>>  strictest level, only when the trailing array is declared as a
>>>>>>  flexible array member per C99 standard onwards ([]), it is treated
>>>>>>  as a flexible array member.
>>>>>> 
>>>>>>  There are two more levels in between 0 and 3, which are provided to
>>>>>>  support older codes that use GCC zero-length array extension ([0])
>>>>>>  or one-size array as flexible array member ([1]): When LEVEL is 1,
>>>>>>  the trailing array is treated as a flexible array member when it is
>>>>>>  declared as either [], [0], or [1]; When LEVEL is 2, the trailing
>>>>>>  array is treated as a flexible array member when it is declared as
>>>>>>  either [], or [0].
>>>>>> 
>>>>>>  This attribute can be used with or without '-fstrict-flex-array'.
>>>>>>  When both the attribute and the option present at the same time,
>>>>>>  the level of the strictness for the specific trailing array field
>>>>>>  is determined by the attribute.
>>>>>> 
>>>>>> gcc/c-family/ChangeLog:
>>>>>> 
>>>>>> 	* c-attribs.cc (handle_strict_flex_array_attribute): New function.
>>>>>> 	(c_common_attribute_table): New item for strict_flex_array.
>>>>>> 	* c.opt (fstrict-flex-array): New option.
>>>>>> 	(fstrict-flex-array=): New option.
>>>>>> 
>>>>>> gcc/c/ChangeLog:
>>>>>> 
>>>>>> 	* c-decl.cc (add_flexible_array_elts_to_size): Call new utility
>>>>>> 	routine flexible_array_member_p.
>>>>>> 	(is_flexible_array_member_p): New function.
>>>>>> 	(finish_struct): Set the new DECL_NOT_FLEXARRAY flag.
>>>>>> 
>>>>>> gcc/ChangeLog:
>>>>>> 
>>>>>> 	* doc/extend.texi: Document strict_flex_array attribute.
>>>>>> 	* doc/invoke.texi: Document -fstrict-flex-array[=n] option.
>>>>>> 	* tree-core.h (struct tree_decl_common): New bit field
>>>>>> 	decl_not_flexarray.
>>>>>> 	* tree.cc (component_ref_size): Reorg by using new utility functions.
>>>>>> 	(flexible_array_member_p): New function.
>>>>>> 	(zero_length_array_p): Likewise.
>>>>>> 	(one_element_array_p): Likewise.
>>>>>> 	(flexible_array_type_p): Likewise.
>>>>>> 	* tree.h (DECL_NOT_FLEXARRAY): New flag.
>>>>>> 	(zero_length_array_p): New function prototype.
>>>>>> 	(one_element_array_p): Likewise.
>>>>>> 	(flexible_array_member_p): Likewise.
>>>>>> 
>>>>>> gcc/testsuite/ChangeLog:
>>>>>> 
>>>>>> 	* gcc.dg/strict-flex-array-1.c: New test.
>>>>>> ---
>>>>>> gcc/c-family/c-attribs.cc                  |  47 ++++++++
>>>>>> gcc/c-family/c.opt                         |   7 ++
>>>>>> gcc/c/c-decl.cc                            |  91 +++++++++++++--
>>>>>> gcc/doc/extend.texi                        |  25 ++++
>>>>>> gcc/doc/invoke.texi                        |  27 ++++-
>>>>>> gcc/testsuite/gcc.dg/strict-flex-array-1.c |  31 +++++
>>>>>> gcc/tree-core.h                            |   5 +-
>>>>>> gcc/tree.cc                                | 130 ++++++++++++++-------
>>>>>> gcc/tree.h                                 |  16 ++-
>>>>>> 9 files changed, 322 insertions(+), 57 deletions(-)
>>>>>> create mode 100644 gcc/testsuite/gcc.dg/strict-flex-array-1.c
>>>>>> 
>>>>>> diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
>>>>>> index c8d96723f4c..10d16532f0d 100644
>>>>>> --- a/gcc/c-family/c-attribs.cc
>>>>>> +++ b/gcc/c-family/c-attribs.cc
>>>>>> @@ -101,6 +101,8 @@ static tree handle_special_var_sec_attribute (tree *, tree, tree, int, bool *);
>>>>>> static tree handle_aligned_attribute (tree *, tree, tree, int, bool *);
>>>>>> static tree handle_warn_if_not_aligned_attribute (tree *, tree, tree,
>>>>>> 						  int, bool *);
>>>>>> +static tree handle_strict_flex_array_attribute (tree *, tree, tree,
>>>>>> +						int, bool *);
>>>>>> static tree handle_weak_attribute (tree *, tree, tree, int, bool *) ;
>>>>>> static tree handle_noplt_attribute (tree *, tree, tree, int, bool *) ;
>>>>>> static tree handle_alias_ifunc_attribute (bool, tree *, tree, tree, bool *);
>>>>>> @@ -367,6 +369,8 @@ const struct attribute_spec c_common_attribute_table[] =
>>>>>> 	                      attr_aligned_exclusions },
>>>>>> { "warn_if_not_aligned",    0, 1, false, false, false, false,
>>>>>> 			      handle_warn_if_not_aligned_attribute, NULL },
>>>>>> +  { "strict_flex_array",      1, 1, false, false, false, false,
>>>>>> +			      handle_strict_flex_array_attribute, NULL },
>>>>>> { "weak",                   0, 0, true,  false, false, false,
>>>>>> 			      handle_weak_attribute, NULL },
>>>>>> { "noplt",                   0, 0, true,  false, false, false,
>>>>>> @@ -2498,6 +2502,49 @@ handle_warn_if_not_aligned_attribute (tree *node, tree name,
>>>>>> 					  no_add_attrs, true);
>>>>>> }
>>>>>> 
>>>>>> +/* Handle a "strict_flex_array" attribute; arguments as in
>>>>>> +   struct attribute_spec.handler.  */
>>>>>> +
>>>>>> +static tree
>>>>>> +handle_strict_flex_array_attribute (tree *node, tree name,
>>>>>> +				    tree args, int ARG_UNUSED (flags),
>>>>>> +				    bool *no_add_attrs)
>>>>>> +{
>>>>>> +  tree decl = *node;
>>>>>> +  tree argval = TREE_VALUE (args);
>>>>>> +
>>>>>> +  /* This attribute only applies to field decls of a structure.  */
>>>>>> +  if (TREE_CODE (decl) != FIELD_DECL)
>>>>>> +    {
>>>>>> +      error_at (DECL_SOURCE_LOCATION (decl),
>>>>>> +		"%qE attribute may not be specified for %q+D", name, decl);
>>>>>> +      *no_add_attrs = true;
>>>>>> +    }
>>>>>> +  /* This attribute only applies to field with array type.  */
>>>>>> +  else if (TREE_CODE (TREE_TYPE (decl)) != ARRAY_TYPE)
>>>>>> +    {
>>>>>> +      error_at (DECL_SOURCE_LOCATION (decl),
>>>>>> +		"%qE attribute may not be specified for a non array field",
>>>>>> +		name);
>>>>>> +      *no_add_attrs = true;
>>>>>> +    }
>>>>>> +  else if (TREE_CODE (argval) != INTEGER_CST)
>>>>>> +    {
>>>>>> +      error_at (DECL_SOURCE_LOCATION (decl),
>>>>>> +		"%qE attribute argument not an integer", name);
>>>>>> +      *no_add_attrs = true;
>>>>>> +    }
>>>>>> +  else if (!tree_fits_uhwi_p (argval) || tree_to_uhwi (argval) > 3)
>>>>>> +    {
>>>>>> +      error_at (DECL_SOURCE_LOCATION (decl),
>>>>>> +		"%qE attribute argument %qE is not an integer constant"
>>>>>> +		" between 0 and 3", name, argval);
>>>>>> +      *no_add_attrs = true;
>>>>>> +    }
>>>>>> +
>>>>>> +  return NULL_TREE;
>>>>>> +}
>>>>>> +
>>>>>> /* Handle a "weak" attribute; arguments as in
>>>>>> struct attribute_spec.handler.  */
>>>>>> 
>>>>>> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
>>>>>> index 44e1a60ce24..864cd8df1d3 100644
>>>>>> --- a/gcc/c-family/c.opt
>>>>>> +++ b/gcc/c-family/c.opt
>>>>>> @@ -2060,6 +2060,13 @@ fsized-deallocation
>>>>>> C++ ObjC++ Var(flag_sized_deallocation) Init(-1)
>>>>>> Enable C++14 sized deallocation support.
>>>>>> 
>>>>>> +fstrict-flex-array
>>>>>> +C C++ Common Alias(fstrict-flex-array=,3,0)
>>>>>> +
>>>>>> +fstrict-flex-array=
>>>>>> +C C++ Common Joined RejectNegative UInteger Var(flag_strict_flex_array) Init(0) IntegerRange(0,3)
>>>>>> +-fstrict-flex-array=<level>    Treat the trailing array of a structure as a flexible array in a stricter way.  The default is treating all trailing arrays of structures as flexible arrays.
>>>>>> +
>>>>>> fsquangle
>>>>>> C++ ObjC++ WarnRemoved
>>>>>> 
>>>>>> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
>>>>>> index ae8990c138f..14defae9584 100644
>>>>>> --- a/gcc/c/c-decl.cc
>>>>>> +++ b/gcc/c/c-decl.cc
>>>>>> @@ -5013,10 +5013,7 @@ add_flexible_array_elts_to_size (tree decl, tree init)
>>>>>> 
>>>>>> elt = CONSTRUCTOR_ELTS (init)->last ().value;
>>>>>> type = TREE_TYPE (elt);
>>>>>> -  if (TREE_CODE (type) == ARRAY_TYPE
>>>>>> -      && TYPE_SIZE (type) == NULL_TREE
>>>>>> -      && TYPE_DOMAIN (type) != NULL_TREE
>>>>>> -      && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
>>>>>> +  if (flexible_array_member_p (type))
>>>>>>  {
>>>>>>    complete_array_type (&type, elt, false);
>>>>>>    DECL_SIZE (decl)
>>>>>> @@ -8720,6 +8717,80 @@ finish_incomplete_vars (tree incomplete_vars, bool toplevel)
>>>>>>  }
>>>>>> }
>>>>>> 
>>>>>> +/* Determine whether the FIELD_DECL X is a flexible array member according to
>>>>>> +   the following info:
>>>>>> +  A. whether the FIELD_DECL X is the last field of the DECL_CONTEXT;
>>>>>> +  B. whether the FIELD_DECL is an array that is declared as "[]", "[0]",
>>>>>> +     or "[1]";
>>>>>> +  C. flag_strict_flex_array;
>>>>>> +  D. the attribute strict_flex_array that is attached to the field
>>>>>> +     if presenting.
>>>>>> +  Return TRUE when it's a flexible array member, FALSE otherwise.  */
>>>>>> +
>>>>>> +static bool
>>>>>> +is_flexible_array_member_p (bool is_last_field,
>>>>>> +			    tree x)
>>>>>> +{
>>>>>> +  /* if not the last field, return false.  */
>>>>>> +  if (!is_last_field)
>>>>>> +    return false;
>>>>>> +
>>>>>> +  /* if not an array field, return false.  */
>>>>>> +  if (TREE_CODE (TREE_TYPE (x)) != ARRAY_TYPE)
>>>>>> +    return false;
>>>>>> +
>>>>>> +  bool is_zero_length_array = zero_length_array_p (TREE_TYPE (x));
>>>>>> +  bool is_one_element_array = one_element_array_p (TREE_TYPE (x));
>>>>>> +  bool is_flexible_array = flexible_array_member_p (TREE_TYPE (x));
>>>>>> +
>>>>>> +  unsigned int strict_flex_array_level = flag_strict_flex_array;
>>>>>> +
>>>>>> +  tree attr_strict_flex_array = lookup_attribute ("strict_flex_array",
>>>>>> +						  DECL_ATTRIBUTES (x));
>>>>>> +  /* if there is a strict_flex_array attribute attached to the field,
>>>>>> +     override the flag_strict_flex_array.  */
>>>>>> +  if (attr_strict_flex_array)
>>>>>> +    {
>>>>>> +      /* get the value of the level first from the attribute.  */
>>>>>> +      unsigned HOST_WIDE_INT attr_strict_flex_array_level = 0;
>>>>>> +      gcc_assert (TREE_VALUE (attr_strict_flex_array) != NULL_TREE);
>>>>>> +      attr_strict_flex_array = TREE_VALUE (attr_strict_flex_array);
>>>>>> +      gcc_assert (TREE_VALUE (attr_strict_flex_array) != NULL_TREE);
>>>>>> +      attr_strict_flex_array = TREE_VALUE (attr_strict_flex_array);
>>>>>> +      gcc_assert (tree_fits_uhwi_p (attr_strict_flex_array));
>>>>>> +      attr_strict_flex_array_level = tree_to_uhwi (attr_strict_flex_array);
>>>>>> +
>>>>>> +      /* the attribute has higher priority than flag_struct_flex_array.  */
>>>>>> +      strict_flex_array_level = attr_strict_flex_array_level;
>>>>>> +    }
>>>>>> +
>>>>>> +  switch (strict_flex_array_level)
>>>>>> +    {
>>>>>> +      case 0:
>>>>>> +	/* default, all trailing arrays are flexiable array members.  */
>>>>>> +	return true;
>>>>>> +      case 1:
>>>>>> +	/* Level 1: all "[1]", "[0]", and "[]" are flexiable array members.  */
>>>>>> +	if (is_one_element_array)
>>>>>> +	  return true;
>>>>>> +	/* FALLTHROUGH.  */
>>>>>> +      case 2:
>>>>>> +	/* Level 2: all "[0]", and "[]" are flexiable array members.  */
>>>>>> +	if (is_zero_length_array)
>>>>>> +	  return true;
>>>>>> +	/* FALLTHROUGH.  */
>>>>>> +      case 3:
>>>>>> +	/* Level 3: Only "[]" are flexible array members.  */
>>>>>> +	if (is_flexible_array)
>>>>>> +	  return true;
>>>>>> +	break;
>>>>>> +      default:
>>>>>> +	gcc_unreachable ();
>>>>>> +    }
>>>>>> +  return false;
>>>>>> +}
>>>>>> +
>>>>>> +
>>>>>> /* Fill in the fields of a RECORD_TYPE or UNION_TYPE node, T.
>>>>>> LOC is the location of the RECORD_TYPE or UNION_TYPE's definition.
>>>>>> FIELDLIST is a chain of FIELD_DECL nodes for the fields.
>>>>>> @@ -8781,6 +8852,8 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
>>>>>> bool saw_named_field = false;
>>>>>> for (x = fieldlist; x; x = DECL_CHAIN (x))
>>>>>>  {
>>>>>> +      bool is_last_field = (DECL_CHAIN (x) == NULL_TREE);
>>>>>> +
>>>>>>    if (TREE_TYPE (x) == error_mark_node)
>>>>>> 	continue;
>>>>>> 
>>>>>> @@ -8819,10 +8892,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
>>>>>> 	DECL_PACKED (x) = 1;
>>>>>> 
>>>>>>    /* Detect flexible array member in an invalid context.  */
>>>>>> -      if (TREE_CODE (TREE_TYPE (x)) == ARRAY_TYPE
>>>>>> -	  && TYPE_SIZE (TREE_TYPE (x)) == NULL_TREE
>>>>>> -	  && TYPE_DOMAIN (TREE_TYPE (x)) != NULL_TREE
>>>>>> -	  && TYPE_MAX_VALUE (TYPE_DOMAIN (TREE_TYPE (x))) == NULL_TREE)
>>>>>> +      if (flexible_array_member_p (TREE_TYPE (x)))
>>>>>> 	{
>>>>>> 	  if (TREE_CODE (t) == UNION_TYPE)
>>>>>> 	    {
>>>>>> @@ -8830,7 +8900,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
>>>>>> 			"flexible array member in union");
>>>>>> 	      TREE_TYPE (x) = error_mark_node;
>>>>>> 	    }
>>>>>> -	  else if (DECL_CHAIN (x) != NULL_TREE)
>>>>>> +	  else if (!is_last_field)
>>>>>> 	    {
>>>>>> 	      error_at (DECL_SOURCE_LOCATION (x),
>>>>>> 			"flexible array member not at end of struct");
>>>>>> @@ -8850,6 +8920,9 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
>>>>>> 	pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
>>>>>> 		 "invalid use of structure with flexible array member");
>>>>>> 
>>>>>> +      /* Set DECL_NOT_FLEXARRAY flag for FIELD_DECL x.  */
>>>>>> +      DECL_NOT_FLEXARRAY (x) = !is_flexible_array_member_p (is_last_field, x);
>>>>>> +
>>>>>>    if (DECL_NAME (x)
>>>>>> 	  || RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
>>>>>> 	saw_named_field = true;
>>>>>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>>>>>> index dfbe33ac652..7451410a011 100644
>>>>>> --- a/gcc/doc/extend.texi
>>>>>> +++ b/gcc/doc/extend.texi
>>>>>> @@ -7436,6 +7436,31 @@ This warning can be disabled by @option{-Wno-if-not-aligned}.
>>>>>> The @code{warn_if_not_aligned} attribute can also be used for types
>>>>>> (@pxref{Common Type Attributes}.)
>>>>>> 
>>>>>> +@cindex @code{strict_flex_array} variable attribute
>>>>>> +@item strict_flex_array (@var{level})
>>>>>> +The @code{strict_flex_array} attribute should be attached to the trailing
>>>>>> +array field of a structure.  It specifies the level of strictness of
>>>>>> +treating the trailing array field of a structure as a flexible array
>>>>>> +member. @var{level} must be an integer betwen 0 to 3.
>>>>>> +
>>>>>> +@var{level}=0 is the least strict level, all trailing arrays of structures
>>>>>> +are treated as flexible array members. @var{level}=3 is the strictest level,
>>>>>> +only when the trailing array is declared as a flexible array member per C99
>>>>>> +standard onwards ([]), it is treated as a flexible array member.
>>>>> 
>>>>> How is level 3 (thus -fstrict-flex-array) interpreted when you specify 
>>>>> -std=c89?  How for -std=gnu89?
>>>> 
>>>> 1. what’s the major difference between -std=c89 and -std=gnu89 on flexible array? (Checked online, cannot find a concrete answer on this).
>>>> 	** my understanding is:   -std=c89 will not support any flexible array (neither [], [0], [1]), but -std=gnu89 will support [0] and [1], but not [].
>>>>        Is this correct?
>>> 
>>> Yes.
>>> 
>>>> If my answer to the first question is correct, then:
>>>> 
>>>> 2. When -fstrict-flex-array=n and -std=c89 present at the same time, which one has the higher priority? 
>>>>    	** I think that -std=c89 should be honored over -fstrict-flex-array, therefore we should disable -fstrict-flex-array=n when n > 0  and issue warnings to the user.
>>>> 
>>>> 
>>>> 3. how about -fstrict-flex-array=n and -std=gnu89 present at the same time? 
>>>>    	** When -std=gnu89 present, [] is not supported. So, we need to issue an warning to disable -fstrict-flex-array=3; but level 1 and level 2 is Okay.
>>>> 
>>>> We also need to document the above.
>>>> 
>>>> Let me know if you have any more comment and suggestions here.
>>> 
>>> In my opinion -fstrict-flex-array only makes sense with C99 and above.  
>>> The extra sub-levels allowing [0] or [1] are of questionable benefit.
>> The multiple-levels control for -fstrict-flex-arrays was an agreement on the discussion of PR101836:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836
>> 
>> Please see comment#17 till #25 for the details.
>> 
>> A summary of the discussion is:  (Please comment on the following paragraph if you see any issue with it)
>> 
>> =====
>> Flexible Array Member (FAM) is a feature introduced in the C99 standard of the C language in May 2000.  It's used as the last element of a structure which is really a header for a variable-length object.
>> This feature was officially standardized in C99. However, compilers (GCC, Micorsoft compiler, etc.) supported this feature well before C99, with the following variants:
>>  A. trailing zero-length array of a structure was treated as FAM.  
>>     Before C99, a valid GCC extension, zero-length array was used to support flexible array member.  See more details at:  https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html.
>>  B. trailing one-element array of a structure was treated as FAM.
>>     In the absence of the zero-length array extension, in ISO C90, one-element arrays at the end of structures were used to represent FAMs as a convertion.
>>  C. any trailing array of a structure was treated as FAM.
>>     Even earlier time, in the absence of both zero-length array GCC extension and one-element array convention, all trailing arrays of structures could be used as a FAM.     
>> As a result, older codes (before C99) might include any of the above variants (A, B, and C) to represent FAMs. 
>> Another complication is C++, standard C++ doesn't allow either C99 flexible array members or zero-length array. A C++ programmer has to use the GCC zero-length array extension or one-element array convention for the purpose of FAM.
>> =====
>> 
>> As mentioned by Jakub in comment #25: 
>> 
>> "differentiating between only allowing [] as flex, or [] and [0],
>> or [], [0] and [1], or any trailing array is useful."
> 
> OK.
> 
>>> 
>>> I think that with -fstrict-flex-array we're missing a -Wstrict-flex-array
>>> that would diagnose accesses that are no longer treated as flex array
>>> access but would be without -fstrict-flex-array?  The goal of
>>> -fstrict-flex-array + -Wstrict-flex-array is to make code standard
>>> conformant, no?  Not to enable extra optimization for [1] arrays when
>>> we know [0] is used as flexarray?
>> 
>> Yes, I agree, and this will make the feature more complete and more
>> useful to help users to get rid of the non-standard-comforming styles. 
>> 
>> Please see comments #23, #35
>> 
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836#c23
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836#c35
>> 
>> A summary is:
>> 
>> ====
>> I think that -Wstrict-flex-arrays will need to be cooperated with -fstrict-flex-arrays=N, i.e, only when -fstrict-flex-arrays=N is presenting, -Wstrict-flex-arrays will be valid and report the warnings when see a [0], or [1], or any trailing array based on N:
>> 
>> when -fstrict-flex-arrays
>> =0, -Wstrict-flex-arrays will NOT issue any warnings;
>> =1, -Wstrict-flex-arrays will issue warnings when an array ref's index is larger than the upper bounds of a trailing array that is a regular trailing array;
>> =2, -Wstrict-flex-arrays will issue warnings when an array ref's index is larger than the upper bounds of a trailing array that is a regular trailing array or [1];
>> =3, -Wstrict-flex-arrays will issue warnings when an array ref's index is larger than the upper bounds of a trailing array that is a regular trailing array or [1], or [0].
>> 
>> let me know if you have any comment and suggestion on this.
>> 
>> ====
> 
> Ah, I see - good that this has been discussed.  -Wstrict-flex-array
> shouldn't be a requirement for -fstrict-flex-array to go in, but it
> would be nice to followup with an implementation for the above
> (it might be tricky to find a good place for it, but eventually it
> sounds like -Warray-bounds diagnosing places could eventually be
> adjusted)
Yes, this is on my to-do list, I will finish this part in GCC13.  The following is my plan:
=====

	1. Add a new IR flag “DECL_NOT_FLEXARRAY” to FIELD_DECL; 
	2. In C/C++ FE, set the new flag “DECL_NOT_FLEXARRAY” for a FIELD_DECL based on [0], [1], [ ] and the option -fstrict-flex-arrays, the attribute strict_flex_arrays and whether it’s the last field of the DECL_CONTEXT.
	3. In Middle end,  update all places that treat any trailing array as flexible array member with the new flag  DECL_NOT_FLEXARRAY + array_at_struct_end_p to control the behavior with multiple levels consistently.
	    3.1. update tree-object-size.cc to fix PR101836;
	    3.2. update other places to make GCC consistently handle flexible array member;
	4. In Middle end,  add -Wstrict-flex-array and update the warnings emitted by -Warray-bounds based on multiple levels;
	5. Add a new -Wzero-length-array warning; (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94428).
=====

Let me know if you have any comment or suggestion here.

> 
>> 
>>> 
>>>>> 
>>>>>> +
>>>>>> +There are two more levels in between 0 and 3, which are provided to support
>>>>>> +older codes that use GCC zero-length array extension ([0]) or one-size array
>>>>>> +as flexible array member ([1]):
>>>>>> +When @var{level} is 1, the trailing array is treated as a flexible array member
>>>>>> +when it is declared as either "[]", "[0]", or "[1]";
>>>>>> +When @var{level} is 2, the trailing array is treated as a flexible array member
>>>>>> +when it is declared as either "[]", or "[0]".
>>>>> 
>>>>> Given the above does adding level 2 make sense given that [0] is a GNU
>>>>> extension?
>>>> I think Kees already answered this question.
>>>>> 
>>>>>> +This attribute can be used with or without @option{-fstrict-flex-array}. When
>>>>>> +both the attribute and the option present at the same time, the level of the
>>>>>> +strictness for the specific trailing array field is determined by the attribute.
>>>>>> +
>>>>>> +
>>>>>> @item alloc_size (@var{position})
>>>>>> @itemx alloc_size (@var{position-1}, @var{position-2})
>>>>>> @cindex @code{alloc_size} variable attribute
>>>>>> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
>>>>>> index 94fe57aa4e2..9befe601817 100644
>>>>>> --- a/gcc/doc/invoke.texi
>>>>>> +++ b/gcc/doc/invoke.texi
>>>>>> @@ -207,7 +207,8 @@ in the following sections.
>>>>>> -fopenmp  -fopenmp-simd @gol
>>>>>> -fpermitted-flt-eval-methods=@var{standard} @gol
>>>>>> -fplan9-extensions  -fsigned-bitfields  -funsigned-bitfields @gol
>>>>>> --fsigned-char  -funsigned-char  -fsso-struct=@var{endianness}}
>>>>>> +-fsigned-char  -funsigned-char -fstrict-flex-array[=@var{n}] @gol
>>>>>> +-fsso-struct=@var{endianness}}
>>>>>> 
>>>>>> @item C++ Language Options
>>>>>> @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
>>>>>> @@ -2825,6 +2826,30 @@ The type @code{char} is always a distinct type from each of
>>>>>> @code{signed char} or @code{unsigned char}, even though its behavior
>>>>>> is always just like one of those two.
>>>>>> 
>>>>>> +@item -fstrict-flex-array
>>>>>> +@opindex fstrict-flex-array
>>>>>> +@opindex fno-strict-flex-array
>>>>>> +Treat the trailing array of a structure as a flexible array member in a
>>>>>> +stricter way.
>>>>>> +The positive form is equivalent to @option{-fstrict-flex-array=3}, which is the
>>>>>> +strictest.  A trailing array is treated as a flexible array member only when it
>>>>>> +is declared as a flexible array member per C99 standard onwards.
>>>>>> +The negative form is equivalent to @option{-fstrict-flex-array=0}, which is the
>>>>>> +least strict.  All trailing arrays of structures are treated as flexible array
>>>>>> +members.
>>>>>> +
>>>>>> +@item -fstrict-flex-array=@var{level}
>>>>>> +@opindex fstrict-flex-array=@var{level}
>>>>>> +Treat the trailing array of a structure as a flexible array member in a
>>>>>> +stricter way.  The value of @var{level} controls the level of strictness.
>>>>>> +
>>>>>> +The possible values of @var{level} are the same as for the
>>>>>> +@code{strict_flex_array} attribute (@pxref{Variable Attributes}).
>>>>>> +
>>>>>> +You can control this behavior for a specific trailing array field of a
>>>>>> +structure by using the variable attribute @code{strict_flex_array} attribute
>>>>>> +(@pxref{Variable Attributes}).
>>>>>> +
>>>>>> @item -fsso-struct=@var{endianness}
>>>>>> @opindex fsso-struct
>>>>>> Set the default scalar storage order of structures and unions to the
>>>>>> diff --git a/gcc/testsuite/gcc.dg/strict-flex-array-1.c b/gcc/testsuite/gcc.dg/strict-flex-array-1.c
>>>>>> new file mode 100644
>>>>>> index 00000000000..ec886c99b25
>>>>>> --- /dev/null
>>>>>> +++ b/gcc/testsuite/gcc.dg/strict-flex-array-1.c
>>>>>> @@ -0,0 +1,31 @@
>>>>>> +/* testing the correct usage of attribute strict_flex_array.  */   
>>>>>> +/* { dg-do compile } */
>>>>>> +/* { dg-options "-O2" } */
>>>>>> +
>>>>>> +
>>>>>> +int x __attribute__ ((strict_flex_array (1))); /* { dg-error "'strict_flex_array' attribute may not be specified for 'x'" } */
>>>>>> +
>>>>>> +struct trailing {
>>>>>> +    int a;
>>>>>> +    int c __attribute ((strict_flex_array)); /* { dg-error "wrong number of arguments specified for 'strict_flex_array' attribute" } */
>>>>>> +};
>>>>>> +
>>>>>> +struct trailing_1 {
>>>>>> +    int a;
>>>>>> +    int b;
>>>>>> +    int c __attribute ((strict_flex_array (2))); /* { dg-error "'strict_flex_array' attribute may not be specified for a non array field" } */
>>>>>> +};
>>>>>> +
>>>>>> +extern int d;
>>>>>> +
>>>>>> +struct trailing_array_2 {
>>>>>> +    int a;
>>>>>> +    int b;
>>>>>> +    int c[1] __attribute ((strict_flex_array (d))); /* { dg-error "'strict_flex_array' attribute argument not an integer" } */
>>>>>> +};
>>>>>> +
>>>>>> +struct trailing_array_3 {
>>>>>> +    int a;
>>>>>> +    int b;
>>>>>> +    int c[0] __attribute ((strict_flex_array (5))); /* { dg-error "'strict_flex_array' attribute argument '5' is not an integer constant between 0 and 3" } */
>>>>>> +};
>>>>>> diff --git a/gcc/tree-core.h b/gcc/tree-core.h
>>>>>> index ea9f281f1cc..458c6e6ceea 100644
>>>>>> --- a/gcc/tree-core.h
>>>>>> +++ b/gcc/tree-core.h
>>>>>> @@ -1813,7 +1813,10 @@ struct GTY(()) tree_decl_common {
>>>>>>   TYPE_WARN_IF_NOT_ALIGN.  */
>>>>>> unsigned int warn_if_not_align : 6;
>>>>>> 
>>>>>> -  /* 14 bits unused.  */
>>>>>> +  /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY.  */
>>>>>> +  unsigned int decl_not_flexarray : 1;
>>>>>> +
>>>>>> +  /* 13 bits unused.  */
>>>>> 
>>>>> I've not seen it so you are probably missing it - the bit has to be
>>>>> streamed in tree-streamer-{in,out}.cc to be usable from LTO.
>>>> 
>>>> You mean add it to the routine “unpack_ts_decl_common_value_fields” of tree-streamer-in.cc
>>>> And “pack_ts_decl_common_value_fields” of tree-streamer-out.cc?
>>> 
>>> Yes.
>>> 
>>>> 
>>>>> C++ module streaming also needs to handle it.
>>>> 
>>>> Which file is for this C++ module streaming?
>>> 
>>> Not sure, maybe cp/module.cc
>> 
>> Just checked this file, looks like it’s “Experimental”:
>> /* C++ modules.  Experimental!
>> 
>> Is it used right now? To whom I can consult with to get more detail on C++?
> 
> There's testsuite in g++.dg/modules/, Nathan Sidwell should know since
> he implemented it.
Thanks, will check on this and confirm with Nathan.
> 
>>> 
>>>>> 
>>>>>> 
>>>>>> /* UID for points-to sets, stable over copying from inlining.  */
>>>>>> unsigned int pt_uid;
>>>>>> diff --git a/gcc/tree.cc b/gcc/tree.cc
>>>>>> index 84000dd8b69..02e274699fb 100644
>>>>>> --- a/gcc/tree.cc
>>>>>> +++ b/gcc/tree.cc
>>>>>> @@ -12862,7 +12862,7 @@ get_initializer_for (tree init, tree decl)
>>>>>> /* Determines the size of the member referenced by the COMPONENT_REF
>>>>>> REF, using its initializer expression if necessary in order to
>>>>>> determine the size of an initialized flexible array member.
>>>>>> -   If non-null, set *ARK when REF refers to an interior zero-length
>>>>>> +   If non-null, set *SAM when REF refers to an interior zero-length
>>>>>> array or a trailing one-element array.
>>>>>> Returns the size as sizetype (which might be zero for an object
>>>>>> with an uninitialized flexible array member) or null if the size
>>>>>> @@ -12878,16 +12878,32 @@ component_ref_size (tree ref, special_array_member *sam /* = NULL */)
>>>>>>  sam = &sambuf;
>>>>>> *sam = special_array_member::none;
>>>>>> 
>>>>>> +  /* Whether this ref is an array at the end of a structure.  */
>>>>>> +  bool trailing = array_at_struct_end_p (ref);
>>>>>> +
>>>>> 
>>>>> actually array_at_struct_end_p returns whether ref possibly refers to
>>>>> a trailing array.  In particular it may return false for arrays at
>>>>> struct end with a known length as in a.b[i] for the reference to global 
>>>>> 'a':
>>>>> 
>>>>> struct { int b[1]; } a;
>>>> 
>>>> Yes, I noticed this behavior during my recent debugging of this routine. I thought it’s a bug and planned to file another bug against it.
>>>> Looks like that this is an expected behavior.
>>>> 
>>>> Then I really feel the name and the comments of the routine is very confusing…
>>>> Shall we change the name of this routine to a more descriptive one? For example, “flexible_array_member_p”? 
>>> 
>>> Note the routine is about classifying a concrete reference, so a better
>>> name could be flex_array_ref_p ().  The name change should be done
>>> separately (if at all).
>> 
>> Okay. 
>> 
>>> 
>>>>> 
>>>>> so in the end it should be array_at_struct_end_p also honoring
>>>>> DECL_NOT_FLEXARRAY.
>>>> 
>>>> Then it’s make more sense to check DECL_NOT_FLEXARRAY inside this utility routine?
>>> 
>>> Yes.
>>> 
>>>>> 
>>>>>> /* The object/argument referenced by the COMPONENT_REF and its type.  */
>>>>>> tree arg = TREE_OPERAND (ref, 0);
>>>>>> tree argtype = TREE_TYPE (arg);
>>>>>> -  /* The referenced member.  */
>>>>>> -  tree member = TREE_OPERAND (ref, 1);
>>>>>> 
>>>>>> +  /* The referenced field member.  */
>>>>>> +  tree member = TREE_OPERAND (ref, 1);
>>>>>> +  tree memtype = TREE_TYPE (member);
>>>>>> tree memsize = DECL_SIZE_UNIT (member);
>>>>>> +
>>>>>> +  bool is_zero_length_array_ref = zero_length_array_p (memtype);
>>>>>> +  bool is_constant_length_array_ref = false;
>>>>>> +  bool is_one_element_array_ref
>>>>>> +	= one_element_array_p (memtype, &is_constant_length_array_ref);
>>>>>> +
>>>>>> +  /* Determine the type of the special array member.  */
>>>>>> +  if (is_zero_length_array_ref)
>>>>>> +    *sam = trailing ? special_array_member::trail_0
>>>>>> +	   : special_array_member::int_0;
>>>>>> +  else if (is_one_element_array_ref && trailing)
>>>>>> +    *sam = special_array_member::trail_1;
>>>>>> +
>>>>>> if (memsize)
>>>>>>  {
>>>>>> -      tree memtype = TREE_TYPE (member);
>>>>>>    if (TREE_CODE (memtype) != ARRAY_TYPE)
>>>>>> 	/* DECL_SIZE may be less than TYPE_SIZE in C++ when referring
>>>>>> 	   to the type of a class with a virtual base which doesn't
>>>>>> @@ -12897,50 +12913,30 @@ component_ref_size (tree ref, special_array_member *sam /* = NULL */)
>>>>>> 	return (tree_int_cst_equal (memsize, TYPE_SIZE_UNIT (memtype))
>>>>>> 		? memsize : NULL_TREE);
>>>>>> 
>>>>>> -      bool trailing = array_at_struct_end_p (ref);
>>>>>> -      bool zero_length = integer_zerop (memsize);
>>>>>> -      if (!trailing && !zero_length)
>>>>>> +      if (!trailing && !is_zero_length_array_ref)
>>>>>> 	/* MEMBER is either an interior array or is an array with
>>>>>> 	   more than one element.  */
>>>>>> 	return memsize;
>>>>>> 
>>>>>> -      if (zero_length)
>>>>>> -	{
>>>>>> -	  if (trailing)
>>>>>> -	    *sam = special_array_member::trail_0;
>>>>>> -	  else
>>>>>> -	    {
>>>>>> -	      *sam = special_array_member::int_0;
>>>>>> -	      memsize = NULL_TREE;
>>>>>> -	    }
>>>>>> -	}
>>>>>> +      if (*sam != special_array_member::trail_1
>>>>>> +	  && is_constant_length_array_ref)
>>>>>> +	/* MEMBER is a constant length array which is not a one-element
>>>>>> +	   trailing array.  */
>>>>>> +	return memsize;
>>>>>> 
>>>>>> -      if (!zero_length)
>>>>>> -	if (tree dom = TYPE_DOMAIN (memtype))
>>>>>> -	  if (tree min = TYPE_MIN_VALUE (dom))
>>>>>> -	    if (tree max = TYPE_MAX_VALUE (dom))
>>>>>> -	      if (TREE_CODE (min) == INTEGER_CST
>>>>>> -		  && TREE_CODE (max) == INTEGER_CST)
>>>>>> -		{
>>>>>> -		  offset_int minidx = wi::to_offset (min);
>>>>>> -		  offset_int maxidx = wi::to_offset (max);
>>>>>> -		  offset_int neltsm1 = maxidx - minidx;
>>>>>> -		  if (neltsm1 > 0)
>>>>>> -		    /* MEMBER is an array with more than one element.  */
>>>>>> -		    return memsize;
>>>>>> -
>>>>>> -		  if (neltsm1 == 0)
>>>>>> -		    *sam = special_array_member::trail_1;
>>>>>> -		}
>>>>>> +      if (*sam == special_array_member::int_0)
>>>>>> +	memsize = NULL_TREE;
>>>>>> 
>>>>>> -      /* For a reference to a zero- or one-element array member of a union
>>>>>> -	 use the size of the union instead of the size of the member.  */
>>>>>> +      /* For a reference to a flexible array member, an interior zero length
>>>>>> +	 array, or an array with variable length of a union, use the size of
>>>>>> +	 the union instead of the size of the member.  */
>>>>>>    if (TREE_CODE (argtype) == UNION_TYPE)
>>>>>> 	memsize = TYPE_SIZE_UNIT (argtype);
>>>>>>  }
>>>>>> 
>>>>>> -  /* MEMBER is either a bona fide flexible array member, or a zero-length
>>>>>> -     array member, or an array of length one treated as such.  */
>>>>>> +  /* MEMBER now is a flexible array member, an interior zero length array, or
>>>>>> +     an array with variable length.  We need to decide its size from its
>>>>>> +     initializer.  */
>>>>>> 
>>>>>> /* If the reference is to a declared object and the member a true
>>>>>>   flexible array, try to determine its size from its initializer.  */
>>>>>> @@ -14351,6 +14347,59 @@ default_is_empty_record (const_tree type)
>>>>>> return is_empty_type (TYPE_MAIN_VARIANT (type));
>>>>>> }
>>>>>> 
>>>>>> +/* Determine whether TYPE is a ISO flexible array memeber type "[]".  */
>>>>> 
>>>>> ISO C99
>>>> 
>>>> Okay, will add this.
>>>>> 
>>>>>> +bool
>>>>>> +flexible_array_member_p (const_tree type)
>>>>> 
>>>>> since you pass in a type a better name would be
>>>>> flexible_array_type_p?
>>>> Okay, how about “flexible_array_member_type_p”? (Since “flexible array member” is a complete concept).
>>> 
>>> works for me
>>> 
>>>>> 
>>>>>> +{
>>>>>> +  if (TREE_CODE (type) == ARRAY_TYPE
>>>>>> +      && TYPE_SIZE (type) == NULL_TREE
>>>>>> +      && TYPE_DOMAIN (type) != NULL_TREE
>>>>> 
>>>>> why require a specified TYPE_DOMAIN?
>>>> 
>>>> There are multiple places in the current GCC used the following sequence:
>>>> 
>>>> -      if (TREE_CODE (type)) == ARRAY_TYPE
>>>> -         && TYPE_SIZE (type) == NULL_TREE
>>>> -         && TYPE_DOMAIN (type) != NULL_TREE
>>>> -         && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
>>>> 
>>>> To check whether the type is a flexible_array_member type.  
>>>> (For example, the routine “add_flexible_array_elts_to_size” in c/c-decl.cc
>>>> 			the routine “finish_struct” in c/c-decl.cc
>>>> 			the routine “flexible_array_type_p” in tree.cc)
>>>> 
>>>> That’s the reason I come up with this common routine to replace all these sequences.
>>>> 
>>>> 
>>>>>> +      && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
>>>>> 
>>>>> and why a NULL TYPE_MAX_VALUE?  Isn't an unknown TYPE_SIZE enough?
>>>> 
>>>> Does the current FE generate such IR for a []?  An array type, without TYPE_SIZE, with a TYPE_DOMAIN, but the MAX_VALUE of the TYPE_DOMAIN is NULL?
>>>> 
>>>>> 
>>>>> That said, I'm not sure providing this abstraction is a good idea
>>>>> given the use I see is in frontend code.
>>>> 
>>>> This one is also used in middle-end, for example “flexible_array_type_p” in tree.cc.
>>> 
>>> That was originally in c-decl.c, seems some target wanted to use it.  The
>>> issue with this is that it tests the C frontend way of declaring a flex
>>> array (and it is used by the frontend), but there are many other possible
>>> representations the middle-end should classify as flex array.
>>> 
>>> The Objective C frontend also has a function named this way and it seems
>>> to be a 1:1 copy :/
>>> 
>>> The nios2 use of the function doesn't make much sense to me - externals
>>> should not end up in _any_ section!?
>> 
>> What’s you mean by “nios2” ?  Not quite understand this sentence. -:) could you explain a little bit?
> 
> nios2 is one of GCCs target architectures, it was a use in the nios2
> backend this function was moved to generic code for but the actual use
> doesn't really make sense to me ... (it's not your fault of course)

Okay, I see.  Thanks for the detailed explanation. 

And now I understand your point, GCC has multiple FEs, so the FE specific concept
 (for example, this flexible array member concept is a C/C++ specific concept) should 
not be put into middle end. 

I will keep this in mind in the next version.


> 
>>> 
>>> So I'd rather move this function back to where it came from, ideally
>>> to c-family/, removing the copy in objective C ...
>> 
>> Okay, I will try  this in the next version.
>>> 
>>>>> 
>>>>>> +    return true;
>>>>>> +
>>>>>> +  return false;
>>>>>> +}
>>>>>> +
>>>>>> +/* Determine whether TYPE is a zero-length array type "[0]".  */
>>>>>> +bool
>>>>>> +zero_length_array_p (const_tree type)
>>>>>> +{
>>>>>> +  if (TREE_CODE (type) == ARRAY_TYPE)
>>>>>> +    if (tree type_size = TYPE_SIZE_UNIT (type))
>>>>>> +      if ((integer_zerop (type_size))
>>>>>> +	   && TYPE_DOMAIN (type) != NULL_TREE
>>>>>> +	   && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
>>>>> 
>>>>> that again seems very C(?) frontend specific, please drop it.
>>>> Currently, the middle-end utility routine “component_ref_size” need to check zero_length_array, one_element_array, etc, 
>>>> So, this is not only a FE specific routine.
>>> 
>>> Sigh, it's nother strange function that was introduced just for 
>>> diagnostics (so it's not "correct"), it shouldn't have ended up in
>>> tree.cc IMHO.
>> 
>> The codes in GCC to handle flexible array are really messy, I hope that we can make it clean and consistent through this work. 
>> Of course gradually…
> 
> As said, it's unfortunate to have many APIs doing similar things and
> I'd rather avoid introducing new APIs (with to me, questionable
> implementation or general usefulness) when possible.

Okay, will keep the APIs as simple as possible.
> 
>>> 
>>>> And I think that for making the -fstrict-flex-array work clear, we might want to emphasize  and distinguish the concepts of 
>>>> 
>>>> []          flexible_array_member_type_p
>>>> [0]        zero_length_array_type_p
>>>> [1]        one_element_array_type_p
>>>> 
>>>> Across GCC.
>>> 
>>> Why?
>> 
>> Because the whole confusion of flexible array handlings in our compiler started from the confusion of these concepts.
>> During my study of this whole problem, I found that these concepts were really confusion, and after I was finally clear on
>> These concepts, I started to know how to resolve the whole issue. 
>> 
>> In our GCC doc, https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>> 
>> We already have such concepts, but not very clear.  
>> I think making these concepts clear is very important to resolve such messy situation in GCC.
>> 
>>> The only important thing to the middle-end is whether the
>>> size of the array is known (aka array_at_struct_end_p).  This
>>> routine classifies it with
>> 
>> But whether the size of the array is known depends on whether the array is a
>> 
>> [0],
>> [1],
>> []
>> 
>> +  -fstrict-flex-arrays=n
> 
> But only array_at_struct_end_p should care and for it we will have
> DECL_NOT_FLEXARRAY.  And frontends may have different ideas how they
> represent [0], [1] or [] (and in fact they do).  The rest of the
> middle-end really should _not_ care.

Agreed now.  -:)
> 
>> 
>>> 
>>> /* The array now is at struct end.  Treat flexible arrays as
>>>    always subject to extend, even into just padding constrained by
>>>    an underlying decl.  */
>>> if (! TYPE_SIZE (atype)
>>>     || ! TYPE_DOMAIN (atype)
>>>     || ! TYPE_MAX_VALUE (TYPE_DOMAIN (atype)))
>>>   return true;
>> 
>> This sequence is only for identifying []. Right?
>> 
>> The [0] and [1] is not handled by the above.
> 
> Yes.  But also int a[1:] would be identified this way, other frontends
> like Ada or Fortran have means to more elaborately specify array
> dimensions.

Okay. I see.
> 
>>> 
>>> any flexible_array_member_type_p that covers a different subset of
>>> flex-array types would be hugely confusing.
>> 
>> Don’t quite understand this sentence...
> 
> I mean if array_at_struct_end_p decides sth is a flex array reference
> but flexible_array_member_type_p on the type decides it is not, that
> would be confusing.  Thus if adding such new API then it should be
> possible to use it from array_at_struct_end_p, removing then some
> of the redundant checks there.

Okay, I see now.

Thanks. Will work on the next version.

Qing
> 
>>> 
>>>> 
>>>>> 
>>>>>> +	return true;
>>>>>> +  return false;
>>>>>> +}
>>>>>> +
>>>>>> +/* Determine whether TYPE is a one-element array type "[1]".
>>>>>> +   Set IS_CONSTANT_LENGTH to true if the length is constant,
>>>>>> +   otherwise, IS_CONSTANT_LENGTH is set to false.  */
>>>>>> +bool
>>>>>> +one_element_array_p (const_tree type, bool *is_constant_length /* = NULL */)
>>>>>> +{
>>>>>> +  if (is_constant_length)
>>>>>> +    *is_constant_length = false;
>>>>>> +
>>>>>> +  if (TREE_CODE (type) == ARRAY_TYPE)
>>>>>> +    if (tree dom = TYPE_DOMAIN (type))
>>>>>> +      if (tree min = TYPE_MIN_VALUE (dom))
>>>>>> +	if (tree max = TYPE_MAX_VALUE (dom))
>>>>>> +	  if (TREE_CODE (min) == INTEGER_CST
>>>>>> +	      && TREE_CODE (max) == INTEGER_CST)
>>>>>> +	    {
>>>>>> +	      offset_int minidx = wi::to_offset (min);
>>>>>> +	      offset_int maxidx = wi::to_offset (max);
>>>>>> +	      offset_int neltsm1 = maxidx - minidx;
>>>>>> +	      if (is_constant_length)
>>>>>> +		*is_constant_length = true;
>>>>>> +	      if (neltsm1 == 0)
>>>>>> +		return true;
>>>>>> +	    }
>>>>> 
>>>>> I'd say likewise.  Maybe move them to c-family/c-common.{cc,h} instead
>>>>> in a more specialized way for the single use you have?
>>>> 
>>>> It’s not single use, it’s also used in “component_ref_size” routine. 
>>> 
>>> Don't copy from a routine that's not designed to be "correct”.
>> 
>> Okay….
>> 
>> 
>>> In
>>> fact the middle-end already has array_type_nelts, so one_element_array_p
>>> can be written as integer_zerop (array_type_nelts (type)).  It also
>>> has the comment
>>> 
>>> /* TYPE_MAX_VALUE may not be set if the array has unknown length.  */
>>> if (!max)
>>>   {
>>>     /* zero sized arrays are represented from C FE as complete types 
>>> with
>>>        NULL TYPE_MAX_VALUE and zero TYPE_SIZE, while C++ FE represents
>>>        them as min 0, max -1.  */
>>>     if (COMPLETE_TYPE_P (type)
>>>         && integer_zerop (TYPE_SIZE (type))
>>>         && integer_zerop (min))
>>>       return build_int_cst (TREE_TYPE (min), -1);
>> 
>> Thanks for the info, I will check on the above, and rewrite “one_element_array_p” with “array_type_nelts”.
>> 
>> Qing
>>> 
>>> Richard.
>>> 
>>>> Thanks a lot for your comments.
>>>> 
>>>> Qing
>>>>> 
>>>>>> +  return false;
>>>>>> +}
>>>>>> +
>>>>>> /* Determine whether TYPE is a structure with a flexible array member,
>>>>>> or a union containing such a structure (possibly recursively).  */
>>>>>> 
>>>>>> @@ -14367,10 +14416,7 @@ flexible_array_type_p (const_tree type)
>>>>>> 	  last = x;
>>>>>>    if (last == NULL_TREE)
>>>>>> 	return false;
>>>>>> -      if (TREE_CODE (TREE_TYPE (last)) == ARRAY_TYPE
>>>>>> -	  && TYPE_SIZE (TREE_TYPE (last)) == NULL_TREE
>>>>>> -	  && TYPE_DOMAIN (TREE_TYPE (last)) != NULL_TREE
>>>>>> -	  && TYPE_MAX_VALUE (TYPE_DOMAIN (TREE_TYPE (last))) == NULL_TREE)
>>>>>> +      if (flexible_array_member_p (TREE_TYPE (last)))
>>>>>> 	return true;
>>>>>>    return false;
>>>>>>  case UNION_TYPE:
>>>>>> diff --git a/gcc/tree.h b/gcc/tree.h
>>>>>> index e6564aaccb7..3107de5b499 100644
>>>>>> --- a/gcc/tree.h
>>>>>> +++ b/gcc/tree.h
>>>>>> @@ -2993,6 +2993,11 @@ extern void decl_value_expr_insert (tree, tree);
>>>>>> #define DECL_PADDING_P(NODE) \
>>>>>> (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_3)
>>>>>> 
>>>>>> +/* Used in a FIELD_DECL to indicate whether this field is not a flexible
>>>>>> +   array member.  */
>>>>>> +#define DECL_NOT_FLEXARRAY(NODE) \
>>>>>> +  (FIELD_DECL_CHECK (NODE)->decl_common.decl_not_flexarray)
>>>>>> +
>>>>>> /* A numeric unique identifier for a LABEL_DECL.  The UID allocation is
>>>>>> dense, unique within any one function, and may be used to index arrays.
>>>>>> If the value is -1, then no UID has been assigned.  */
>>>>>> @@ -5531,10 +5536,10 @@ extern tree component_ref_field_offset (tree);
>>>>>> returns null.  */
>>>>>> enum struct special_array_member
>>>>>> {
>>>>>> -   none,      /* Not a special array member.  */
>>>>>> -   int_0,     /* Interior array member with size zero.  */
>>>>>> -   trail_0,   /* Trailing array member with size zero.  */
>>>>>> -   trail_1    /* Trailing array member with one element.  */
>>>>>> +    none,	/* Not a special array member.  */
>>>>>> +    int_0,	/* Interior array member with size zero.  */
>>>>>> +    trail_0,	/* Trailing array member with size zero.  */
>>>>>> +    trail_1	/* Trailing array member with one element.  */
>>>>>> };
>>>>>> 
>>>>>> /* Return the size of the member referenced by the COMPONENT_REF, using
>>>>>> @@ -6489,6 +6494,9 @@ extern void gt_pch_nx (tree &, gt_pointer_operator, void *);
>>>>>> extern bool nonnull_arg_p (const_tree);
>>>>>> extern bool is_empty_type (const_tree);
>>>>>> extern bool default_is_empty_record (const_tree);
>>>>>> +extern bool zero_length_array_p (const_tree);
>>>>>> +extern bool one_element_array_p (const_tree, bool * = NULL);
>>>>>> +extern bool flexible_array_member_p (const_tree);
>>>>>> extern bool flexible_array_type_p (const_tree);
>>>>>> extern HOST_WIDE_INT arg_int_size_in_bytes (const_tree);
>>>>>> extern tree arg_size_in_bytes (const_tree);
>>>>>> 
>>>>> 
>>>>> -- 
>>>>> Richard Biener <rguenther@suse.de>
>>>>> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
>>>>> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
>>>>> HRB 36809 (AG Nuernberg)
>>>> 
>>>> 
>>> 
>>> -- 
>>> Richard Biener <rguenther@suse.de>
>>> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
>>> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
>>> HRB 36809 (AG Nuernberg)
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
> HRB 36809 (AG Nuernberg)