Re: [V1][PATCH 2/3] Use the element_count atribute info in builtin object size [PR108896].

[Martin Uecker <uecker@tugraz.at>]

Thank you for working on this!


Here are a couple of comments:

How is the size for an object with FAM defined? 

There are at least three possible choices:

offset(..) + N * sizeof
sizeof(..) + N * sizeof
or the size of a struct with the replacement array.

Or is this not relevant here?


I would personally prefer an attribute which does
not use a string, but uses C expressions, so that
one could write something like this (although I would
limit it initially to the most simple case) 

struct {
  struct bar { int n; }* ptr;
  int buf[] [[element_count(.ptr->n + 3)]];
};

Of course, we could still support this later even
if we use a string now.

Martin




Am Donnerstag, dem 25.05.2023 um 16:14 +0000 schrieb Qing Zhao:
> 2023-05-17 Qing Zhao <qing.zhao@oracle.com>
> 
> gcc/ChangeLog:
> 
> 	PR C/108896
> 	* tree-object-size.cc (addr_object_size): Use the element_count
> 	attribute info.
> 	* tree.cc (component_ref_has_element_count_p): New function.
> 	(component_ref_get_element_count): New function.
> 	* tree.h (component_ref_has_element_count_p): New prototype.
> 	(component_ref_get_element_count): New prototype.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR C/108896
> 	* gcc.dg/flex-array-element-count-2.c: New test.
> ---
>  .../gcc.dg/flex-array-element-count-2.c       | 56 +++++++++++
>  gcc/tree-object-size.cc                       | 37 ++++++--
>  gcc/tree.cc                                   | 93 +++++++++++++++++++
>  gcc/tree.h                                    | 10 ++
>  4 files changed, 189 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/flex-array-element-count-2.c
> 
> diff --git a/gcc/testsuite/gcc.dg/flex-array-element-count-2.c b/gcc/testsuite/gcc.dg/flex-array-element-count-2.c
> new file mode 100644
> index 00000000000..5a280e8c731
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/flex-array-element-count-2.c
> @@ -0,0 +1,56 @@
> +/* test the attribute element_count and its usage in
> + * __builtin_dynamic_object_size.  */ 
> +/* { dg-do run } */
> +/* { dg-options "-O2" } */
> +
> +#include "builtin-object-size-common.h"
> +
> +#define expect(p, _v) do { \
> +    size_t v = _v; \
> +    if (p == v) \
> +	__builtin_printf ("ok:  %s == %zd\n", #p, p); \
> +    else \
> +	{  \
> +	  __builtin_printf ("WAT: %s == %zd (expected %zd)\n", #p, p, v); \
> +	  FAIL (); \
> +	} \
> +} while (0);
> +
> +struct flex {
> +  int b;
> +  int c[];
> +} *array_flex;
> +
> +struct annotated {
> +  int b;
> +  int c[] __attribute__ ((element_count ("b")));
> +} *array_annotated;
> +
> +void __attribute__((__noinline__)) setup (int normal_count, int attr_count)
> +{
> +  array_flex
> +    = (struct flex *)malloc (sizeof (struct flex)
> +				       + normal_count *  sizeof (int));
> +  array_flex->b = normal_count;
> +
> +  array_annotated
> +    = (struct annotated *)malloc (sizeof (struct annotated)
> +					    + attr_count *  sizeof (int));
> +  array_annotated->b = attr_count;
> +
> +  return;
> +}
> +
> +void __attribute__((__noinline__)) test ()
> +{
> +    expect(__builtin_dynamic_object_size(array_flex->c, 1), -1);
> +    expect(__builtin_dynamic_object_size(array_annotated->c, 1),
> +	   array_annotated->b * sizeof (int));
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +  setup (10,10);   
> +  test ();
> +  DONE ();
> +}
> diff --git a/gcc/tree-object-size.cc b/gcc/tree-object-size.cc
> index 9a936a91983..f9aadd59054 100644
> --- a/gcc/tree-object-size.cc
> +++ b/gcc/tree-object-size.cc
> @@ -585,6 +585,7 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>    if (pt_var != TREE_OPERAND (ptr, 0))
>      {
>        tree var;
> +      tree element_count_ref = NULL_TREE;
>  
> 
>        if (object_size_type & OST_SUBOBJECT)
>  	{
> @@ -600,11 +601,12 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>  	    var = TREE_OPERAND (var, 0);
>  	  if (var != pt_var && TREE_CODE (var) == ARRAY_REF)
>  	    var = TREE_OPERAND (var, 0);
> -	  if (! TYPE_SIZE_UNIT (TREE_TYPE (var))
> +	  if (! component_ref_has_element_count_p (var)
> +	     && ((! TYPE_SIZE_UNIT (TREE_TYPE (var))
>  	      || ! tree_fits_uhwi_p (TYPE_SIZE_UNIT (TREE_TYPE (var)))
>  	      || (pt_var_size && TREE_CODE (pt_var_size) == INTEGER_CST
>  		  && tree_int_cst_lt (pt_var_size,
> -				      TYPE_SIZE_UNIT (TREE_TYPE (var)))))
> +				      TYPE_SIZE_UNIT (TREE_TYPE (var)))))))
>  	    var = pt_var;
>  	  else if (var != pt_var && TREE_CODE (pt_var) == MEM_REF)
>  	    {
> @@ -612,6 +614,7 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>  	      /* For &X->fld, compute object size if fld isn't a flexible array
>  		 member.  */
>  	      bool is_flexible_array_mem_ref = false;
> +
>  	      while (v && v != pt_var)
>  		switch (TREE_CODE (v))
>  		  {
> @@ -639,6 +642,8 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>  			break;
>  		      }
>  		    is_flexible_array_mem_ref = array_ref_flexible_size_p (v);
> +		    element_count_ref = component_ref_get_element_count (v);
> +
>  		    while (v != pt_var && TREE_CODE (v) == COMPONENT_REF)
>  		      if (TREE_CODE (TREE_TYPE (TREE_OPERAND (v, 0)))
>  			  != UNION_TYPE
> @@ -652,8 +657,11 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>  			   == RECORD_TYPE)
>  		      {
>  			/* compute object size only if v is not a
> -			   flexible array member.  */
> -			if (!is_flexible_array_mem_ref)
> +			   flexible array member or the flexible array member
> +			   has a known element count indicated by the user
> +			   through attribute element_count.  */
> +			if (!is_flexible_array_mem_ref
> +			    || element_count_ref)
>  			  {
>  			    v = NULL_TREE;
>  			    break;
> @@ -686,9 +694,24 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
>  
> 
>        if (var != pt_var)
>  	{
> -	  var_size = TYPE_SIZE_UNIT (TREE_TYPE (var));
> -	  if (!TREE_CONSTANT (var_size))
> -	    var_size = get_or_create_ssa_default_def (cfun, var_size);
> +	  if (!element_count_ref)
> +	    {
> +	      var_size = TYPE_SIZE_UNIT (TREE_TYPE (var));
> +	      if (!TREE_CONSTANT (var_size))
> +		var_size = get_or_create_ssa_default_def (cfun, var_size);
> +	    }
> +	  else
> +	    {
> +	      gcc_assert (TREE_CODE (var) == COMPONENT_REF
> +			  && TREE_CODE (TREE_TYPE (var)) == ARRAY_TYPE);
> +	      tree element_size = TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (var)));
> +	      var_size
> +		= size_binop (MULT_EXPR,
> +			      fold_convert (sizetype, element_count_ref),
> +			      fold_convert (sizetype, element_size));
> +	      if (!todo)
> +		todo = TODO_update_ssa_only_virtuals;
> +	    }
>  	  if (!var_size)
>  	    return false;
>  	}
> diff --git a/gcc/tree.cc b/gcc/tree.cc
> index 7e6de288886..48753b94f2c 100644
> --- a/gcc/tree.cc
> +++ b/gcc/tree.cc
> @@ -12745,6 +12745,99 @@ array_ref_element_size (tree exp)
>      return SUBSTITUTE_PLACEHOLDER_IN_EXPR (TYPE_SIZE_UNIT (elmt_type), exp);
>  }
>  
> 
> +/*  For a component_ref that has an array type ARRAY_REF, return TRUE when
> +    an element_count attribute attached to the corresponding FIELD_DECL.
> +    return FALSE otherwise.  */
> +bool
> +component_ref_has_element_count_p (tree array_ref)
> +{
> +  if (TREE_CODE (array_ref) != COMPONENT_REF)
> +    return false;
> +
> +  if (TREE_CODE (TREE_TYPE (array_ref)) != ARRAY_TYPE)
> +    return false;
> +
> +  tree struct_object = TREE_OPERAND (array_ref, 0);
> +  tree struct_type = TREE_TYPE (struct_object);
> +
> +  if (!RECORD_OR_UNION_TYPE_P (struct_type))
> +    return false;
> +  tree field_decl = TREE_OPERAND (array_ref, 1);
> +  tree attr_element_count = lookup_attribute ("element_count",
> +					      DECL_ATTRIBUTES (field_decl));
> +
> +  if (!attr_element_count)
> +    return false;
> +  return true;
> +}
> +
> +
> +/* For a component_ref that has an array type ARRAY_REF, get the object that
> +   represents its element_count per the attribute element_count attached to
> +   the corresponding FIELD_DECL.  return NULL_TREE when cannot find such
> +   object.
> +   For example, if:
> +
> +    struct P {
> +      int k;
> +      int x[] __attribute__ ((element_count ("k")));
> +    } *p;
> +
> +    for the following reference:
> +
> +    p->x[b]
> +
> +    the object that represents its element count will be:
> +
> +    p->k
> +
> +    So, when component_ref_get_element_count (p->x[b]) is called, p->k should be
> +    returned.
> +*/
> +
> +tree
> +component_ref_get_element_count (tree array_ref)
> +{
> +  if (! component_ref_has_element_count_p (array_ref))
> +    return NULL_TREE;
> +
> +  tree struct_object = TREE_OPERAND (array_ref, 0);
> +  tree struct_type = TREE_TYPE (struct_object);
> +  tree field_decl = TREE_OPERAND (array_ref, 1);
> +  tree attr_element_count = lookup_attribute ("element_count",
> +					      DECL_ATTRIBUTES (field_decl));
> +  gcc_assert (attr_element_count);
> +
> +  /* If there is an element_count attribute attached to the field,
> +     get the field that maps to the element_count.  */
> +
> +  const char *fieldname
> +    = TREE_STRING_POINTER (TREE_VALUE (TREE_VALUE (attr_element_count)));
> +
> +  tree element_count_field = NULL_TREE;
> +  for (tree field = TYPE_FIELDS (struct_type); field;
> +       field = DECL_CHAIN (field))
> +    if (TREE_CODE (field) == FIELD_DECL
> +	&& DECL_NAME (field) != NULL
> +	&& strcmp (IDENTIFIER_POINTER (DECL_NAME (field)), fieldname) == 0)
> +      {
> +	element_count_field = field;
> +	break;
> +      }
> +
> +  gcc_assert (element_count_field);
> +
> +  /* generate the tree node that represent the element_count of this array
> +     ref.  This is a COMPONENT_REF to the element_count_field of the
> +     containing structure.  */
> +
> +  tree element_count_ref = build3 (COMPONENT_REF,
> +				   TREE_TYPE (element_count_field),
> +				   struct_object, element_count_field,
> +				   NULL_TREE);
> +  return element_count_ref;
> +}
> +
>  /* Return a tree representing the lower bound of the array mentioned in
>     EXP, an ARRAY_REF or an ARRAY_RANGE_REF.  */
>  
> 
> diff --git a/gcc/tree.h b/gcc/tree.h
> index 0b72663e6a1..8f36370a02d 100644
> --- a/gcc/tree.h
> +++ b/gcc/tree.h
> @@ -5582,6 +5582,16 @@ extern tree get_base_address (tree t);
>     of EXP, an ARRAY_REF or an ARRAY_RANGE_REF.  */
>  extern tree array_ref_element_size (tree);
>  
> 
> +/* Give a component_ref that has an array type, return true when an
> +   attribute element_count attached to the corresponding FIELD_DECL.  */
> +extern bool component_ref_has_element_count_p (tree);
> +
> +/* Give a component_ref that has an array type, return the object that
> +   represents its element_count per the attribute element_count attached to
> +   the corresponding FIELD_DECL.  return NULL_TREE when cannot find such
> +   object.  */
> +extern tree component_ref_get_element_count (tree);
> +
>  /* Return a typenode for the "standard" C type with a given name.  */
>  extern tree get_typenode_from_name (const char *);
>  
>