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

[Richard Biener <rguenther@suse.de>]

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.

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?

> > 
> >> +
> >> +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

> > 
> >> 
> >>  /* 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).

> > 
> > 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!?

So I'd rather move this function back to where it came from, ideally
to c-family/, removing the copy in objective C ...

> > 
> >> +    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.

> 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?  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

  /* 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;

any flexible_array_member_type_p that covers a different subset of
flex-array types would be hugely confusing.
 
> 
> > 
> >> +	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".  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);

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)