Provide new GCC builtin __builtin_counted_by_ref [PR116016]

With the addition of the 'counted_by' attribute and its wide roll-out
within the Linux kernel, a use case has been found that would be very
nice to have for object allocators: being able to set the counted_by
counter variable without knowing its name.

For example, given:

  struct foo {
    ...
    int counter;
    ...
    struct bar array[] __attribute__((counted_by (counter)));
  } *p;

The existing Linux object allocators are roughly:

  #define MAX(A, B) (A > B) ? (A) : (B)
  #define alloc(P, FAM, COUNT) ({ \
    __auto_type __p = &(P); \
    size_t __size = MAX (sizeof(*P),
			 __builtin_offsetof (__typeof(*P), FAM)
			 + sizeof (*(P->FAM)) * COUNT); \
    *__p = kmalloc(__size); \
  })

Right now, any addition of a counted_by annotation must also
include an open-coded assignment of the counter variable after
the allocation:

  p = alloc(p, array, how_many);
  p->counter = how_many;

In order to avoid the tedious and error-prone work of manually adding
the open-coded counted-by intializations everywhere in the Linux
kernel, a new GCC builtin __builtin_counted_by_ref will be very useful
to be added to help the adoption of the counted-by attribute.

 -- Built-in Function: TYPE __builtin_counted_by_ref (PTR)
     The built-in function '__builtin_counted_by_ref' checks whether the
     array object pointed by the pointer PTR has another object
     associated with it that represents the number of elements in the
     array object through the 'counted_by' attribute (i.e.  the
     counted-by object).  If so, returns a pointer to the corresponding
     counted-by object.  If such counted-by object does not exist,
     returns a null pointer.

     This built-in function is only available in C for now.

     The argument PTR must be a pointer to an array.  The TYPE of the
     returned value is a pointer type pointing to the corresponding
     type of the counted-by object or a void pointer type in case of a
     null pointer being returned.

With this new builtin, the central allocator could be updated to:

  #define MAX(A, B) (A > B) ? (A) : (B)
  #define alloc(P, FAM, COUNT) ({ \
    __auto_type __p = &(P); \
    __auto_type __c = (COUNT); \
    size_t __size = MAX (sizeof (*(*__p)),\
			 __builtin_offsetof (__typeof(*(*__p)),FAM) \
			 + sizeof (*((*__p)->FAM)) * __c); \
    if ((*__p = kmalloc(__size))) { \
      __auto_type ret = __builtin_counted_by_ref((*__p)->FAM); \
      *_Generic(ret, void *: &(size_t){0}, default: ret) = __c; \
    } \
  })

And then structs can gain the counted_by attribute without needing
additional open-coded counter assignments for each struct, and
unannotated structs could still use the same allocator.

	PR c/116016

gcc/c-family/ChangeLog:

	* c-common.cc: Add new __builtin_counted_by_ref.
	* c-common.h (enum rid): Add RID_BUILTIN_COUNTED_BY_REF.

gcc/c/ChangeLog:

	* c-decl.cc (names_builtin_p): Add RID_BUILTIN_COUNTED_BY_REF.
	* c-parser.cc (has_counted_by_object): New routine.
	(get_counted_by_ref): New routine.
	(c_parser_postfix_expression): Handle New RID_BUILTIN_COUNTED_BY_REF.
	* c-tree.h: New routine handle_counted_by_for_component_ref.
	* c-typeck.cc (handle_counted_by_for_component_ref): New routine.
	(build_component_ref): Call the new routine.

gcc/ChangeLog:

	* doc/extend.texi: Add documentation for __builtin_counted_by_ref.

gcc/testsuite/ChangeLog:

	* gcc.dg/builtin-counted-by-ref-1.c: New test.
	* gcc.dg/builtin-counted-by-ref.c: New test.

1 files changed, 79 insertions, 0 deletions

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 0ffbdc7..0834f8c 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -10781,6 +10781,37 @@ c_parser_predefined_identifier (c_parser *parser)
   return expr;
 }
 
+/* Check whether the ARRAY_REF has an counted-by object associated with it
+   through the "counted_by" attribute.  */
+
+static bool
+has_counted_by_object (tree array_ref)
+{
+  /* Currently, only when the array_ref is an indirect_ref to a call to the
+     .ACCESS_WITH_SIZE, return true.
+     More cases can be included later when the counted_by attribute is
+     extended to other situations.  */
+  if (TREE_CODE (array_ref) == INDIRECT_REF
+      && is_access_with_size_p (TREE_OPERAND (array_ref, 0)))
+    return true;
+  return false;
+}
+
+/* Get the reference to the counted-by object associated with the ARRAY_REF.  */
+
+static tree
+get_counted_by_ref (tree array_ref)
+{
+  /* Currently, only when the array_ref is an indirect_ref to a call to the
+     .ACCESS_WITH_SIZE, get the corresponding counted_by ref.
+     More cases can be included later when the counted_by attribute is
+     extended to other situations.  */
+  if (TREE_CODE (array_ref) == INDIRECT_REF
+      && is_access_with_size_p (TREE_OPERAND (array_ref, 0)))
+    return CALL_EXPR_ARG (TREE_OPERAND (array_ref, 0), 1);
+  return NULL_TREE;
+}
+
 /* Parse a postfix expression (C90 6.3.1-6.3.2, C99 6.5.1-6.5.2,
    C11 6.5.1-6.5.2).  Compound literals aren't handled here; callers have to
    call c_parser_postfix_expression_after_paren_type on encountering them.
@@ -11875,6 +11906,54 @@ c_parser_postfix_expression (c_parser *parser)
 	    set_c_expr_source_range (&expr, loc, close_paren_loc);
 	    break;
 	  }
+	case RID_BUILTIN_COUNTED_BY_REF:
+	  {
+	    vec<c_expr_t, va_gc> *cexpr_list;
+	    c_expr_t *e_p;
+	    location_t close_paren_loc;
+
+	    c_parser_consume_token (parser);
+	    if (!c_parser_get_builtin_args (parser,
+					    "__builtin_counted_by_ref",
+					    &cexpr_list, false,
+					    &close_paren_loc))
+	      {
+		expr.set_error ();
+		break;
+	      }
+	    if (vec_safe_length (cexpr_list) != 1)
+	      {
+		error_at (loc, "wrong number of arguments to "
+			       "%<__builtin_counted_by_ref%>");
+		expr.set_error ();
+		break;
+	      }
+
+	    e_p = &(*cexpr_list)[0];
+	    tree ref = e_p->value;
+
+	    if (TREE_CODE (TREE_TYPE (ref)) != ARRAY_TYPE)
+	      {
+		error_at (loc, "the argument to %<__builtin_counted_by_ref%>"
+				" must be an array");
+		expr.set_error ();
+		break;
+	      }
+
+	    /* If the array ref is inside TYPEOF or ALIGNOF, the call to
+	       .ACCESS_WITH_SIZE was not generated by the routine
+	       build_component_ref by default, we should generate it here.  */
+	    if ((in_typeof || in_alignof) && TREE_CODE (ref) == COMPONENT_REF)
+	      ref = handle_counted_by_for_component_ref (loc, ref);
+
+	    if (has_counted_by_object (ref))
+	      expr.value = get_counted_by_ref (ref);
+	    else
+	      expr.value = null_pointer_node;
+
+	    set_c_expr_source_range (&expr, loc, close_paren_loc);
+	    break;
+	  }
 	case RID_BUILTIN_SHUFFLE:
 	  {
 	    vec<c_expr_t, va_gc> *cexpr_list;