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>]

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?
 
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.

> 
>> +
>> +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?


> C++ module streaming also needs to handle it.

Which file is for this C++ module streaming?

> 
>> 
>>  /* 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”? 
> 
> 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?
> 
>>  /* 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).
> 
>> +{
>> +  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.

> 
>> +    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.
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. 

> 
>> +	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. 

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)