c, c++: Implement -Wsizeof-array-div [PR91741]

This patch implements a new warning, -Wsizeof-array-div.  It warns about
code like

  int arr[10];
  sizeof (arr) / sizeof (short);

where we have a division of two sizeof expressions, where the first
argument is an array, and the second sizeof does not equal the size
of the array element.  See e.g. <https://www.viva64.com/en/examples/v706/>.

Clang makes it possible to suppress the warning by parenthesizing the
second sizeof like this:

  sizeof (arr) / (sizeof (short));

so I followed suit.  In the C++ FE this was rather easy, because
finish_parenthesized_expr already set TREE_NO_WARNING.  In the C FE
I've added a new tree code, PAREN_SIZEOF_EXPR, to discern between the
non-() and () versions.

This warning is enabled by -Wall.  An example of the output:

x.c:5:23: warning: expression does not compute the number of elements in this array; element type is ‘int’, not ‘short int’ [-Wsizeof-array-div]
    5 |   return sizeof (arr) / sizeof (short);
      |          ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
x.c:5:25: note: add parentheses around ‘sizeof (short int)’ to silence this warning
    5 |   return sizeof (arr) / sizeof (short);
      |                         ^~~~~~~~~~~~~~
      |                         (             )
x.c:4:7: note: array ‘arr’ declared here
    4 |   int arr[10];
      |       ^~~

gcc/c-family/ChangeLog:

	PR c++/91741
	* c-common.c (verify_tree): Handle PAREN_SIZEOF_EXPR.
	(c_common_init_ts): Likewise.
	* c-common.def (PAREN_SIZEOF_EXPR): New tree code.
	* c-common.h (maybe_warn_sizeof_array_div): Declare.
	* c-warn.c (sizeof_pointer_memaccess_warning): Unwrap NOP_EXPRs.
	(maybe_warn_sizeof_array_div): New function.
	* c.opt (Wsizeof-array-div): New option.

gcc/c/ChangeLog:

	PR c++/91741
	* c-parser.c (c_parser_binary_expression): Implement -Wsizeof-array-div.
	(c_parser_postfix_expression): Set PAREN_SIZEOF_EXPR.
	(c_parser_expr_list): Handle PAREN_SIZEOF_EXPR like SIZEOF_EXPR.
	* c-tree.h (char_type_p): Declare.
	* c-typeck.c (char_type_p): No longer static.

gcc/cp/ChangeLog:

	PR c++/91741
	* typeck.c (cp_build_binary_op): Implement -Wsizeof-array-div.

gcc/ChangeLog:

	PR c++/91741
	* doc/invoke.texi: Document -Wsizeof-array-div.

gcc/testsuite/ChangeLog:

	PR c++/91741
	* c-c++-common/Wsizeof-pointer-div.c: Add dg-warning.
	* c-c++-common/Wsizeof-array-div1.c: New test.
	* g++.dg/warn/Wsizeof-array-div1.C: New test.
	* g++.dg/warn/Wsizeof-array-div2.C: New test.

1 files changed, 30 insertions, 18 deletions

diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 7d58356..b6a7ef4 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -7876,7 +7876,7 @@ c_parser_binary_expression (c_parser *parser, struct c_expr *after,
     enum tree_code op;
     /* The source location of this operation.  */
     location_t loc;
-    /* The sizeof argument if expr.original_code == SIZEOF_EXPR.  */
+    /* The sizeof argument if expr.original_code == {PAREN_,}SIZEOF_EXPR.  */
     tree sizeof_arg;
   } stack[NUM_PRECS];
   int sp;
@@ -7894,9 +7894,11 @@ c_parser_binary_expression (c_parser *parser, struct c_expr *after,
 	c_inhibit_evaluation_warnings -= (stack[sp - 1].expr.value	      \
 					  == truthvalue_true_node);	      \
 	break;								      \
-      case TRUNC_DIV_EXPR: 						      \
-	if (stack[sp - 1].expr.original_code == SIZEOF_EXPR		      \
-	    && stack[sp].expr.original_code == SIZEOF_EXPR)		      \
+      case TRUNC_DIV_EXPR:						      \
+	if ((stack[sp - 1].expr.original_code == SIZEOF_EXPR		      \
+	     || stack[sp - 1].expr.original_code == PAREN_SIZEOF_EXPR)	      \
+	    && (stack[sp].expr.original_code == SIZEOF_EXPR		      \
+		|| stack[sp].expr.original_code == PAREN_SIZEOF_EXPR))	      \
 	  {								      \
 	    tree type0 = stack[sp - 1].sizeof_arg;			      \
 	    tree type1 = stack[sp].sizeof_arg;				      \
@@ -7910,18 +7912,23 @@ c_parser_binary_expression (c_parser *parser, struct c_expr *after,
 		&& !(TREE_CODE (first_arg) == PARM_DECL			      \
 		     && C_ARRAY_PARAMETER (first_arg)			      \
 		     && warn_sizeof_array_argument))			      \
-	      {								\
-		auto_diagnostic_group d;					\
-		if (warning_at (stack[sp].loc, OPT_Wsizeof_pointer_div, \
-				  "division %<sizeof (%T) / sizeof (%T)%> " \
-				  "does not compute the number of array " \
-				  "elements",				\
-				  type0, type1))			\
-		  if (DECL_P (first_arg))				\
-		    inform (DECL_SOURCE_LOCATION (first_arg),		\
-			      "first %<sizeof%> operand was declared here"); \
-	      }								\
-	  }								\
+	      {								      \
+		auto_diagnostic_group d;				      \
+		if (warning_at (stack[sp].loc, OPT_Wsizeof_pointer_div,	      \
+				  "division %<sizeof (%T) / sizeof (%T)%> "   \
+				  "does not compute the number of array "     \
+				  "elements",				      \
+				  type0, type1))			      \
+		  if (DECL_P (first_arg))				      \
+		    inform (DECL_SOURCE_LOCATION (first_arg),		      \
+			      "first %<sizeof%> operand was declared here");  \
+	      }								      \
+	    else if (TREE_CODE (type0) == ARRAY_TYPE			      \
+		     && !char_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (type0)))  \
+		     && stack[sp].expr.original_code != PAREN_SIZEOF_EXPR)    \
+	      maybe_warn_sizeof_array_div (stack[sp].loc, first_arg, type0,   \
+					   stack[sp].sizeof_arg, type1);      \
+	  }								      \
 	break;								      \
       default:								      \
 	break;								      \
@@ -9177,6 +9184,9 @@ c_parser_postfix_expression (c_parser *parser)
 	  if (expr.original_code != C_MAYBE_CONST_EXPR
 	      && expr.original_code != SIZEOF_EXPR)
 	    expr.original_code = ERROR_MARK;
+	  /* Remember that we saw ( ) around the sizeof.  */
+	  if (expr.original_code == SIZEOF_EXPR)
+	    expr.original_code = PAREN_SIZEOF_EXPR;
 	  /* Don't change EXPR.ORIGINAL_TYPE.  */
 	  location_t loc_close_paren = c_parser_peek_token (parser)->location;
 	  set_c_expr_source_range (&expr, loc_open_paren, loc_close_paren);
@@ -10792,7 +10802,8 @@ c_parser_expr_list (c_parser *parser, bool convert_p, bool fold_p,
   if (locations)
     locations->safe_push (expr.get_location ());
   if (sizeof_arg != NULL
-      && expr.original_code == SIZEOF_EXPR)
+      && (expr.original_code == SIZEOF_EXPR
+	  || expr.original_code == PAREN_SIZEOF_EXPR))
     {
       sizeof_arg[0] = c_last_sizeof_arg;
       sizeof_arg_loc[0] = c_last_sizeof_loc;
@@ -10815,7 +10826,8 @@ c_parser_expr_list (c_parser *parser, bool convert_p, bool fold_p,
 	locations->safe_push (expr.get_location ());
       if (++idx < 3
 	  && sizeof_arg != NULL
-	  && expr.original_code == SIZEOF_EXPR)
+	  && (expr.original_code == SIZEOF_EXPR
+	      || expr.original_code == PAREN_SIZEOF_EXPR))
 	{
 	  sizeof_arg[idx] = c_last_sizeof_arg;
 	  sizeof_arg_loc[idx] = c_last_sizeof_loc;