Date: Tue, 14 Jun 2005 00:20:06 -0600 (MDT)
From: Roger Sayle <roger at eyesopen dot com>
Subject: [PATCH] PR 7776: Warn about if ("abc" < "xyz") ... (take 2) 

The following patch is a revised version of my proposed solution to
PR middle-end/7776, which addresses Daniel Jacobowitz's concerns about
the potential utility of testing string literals for equality/inequality
with NULL.

See http://gcc.gnu.org/ml/gcc-patches/2005-06/msg00369.html (and the
following thread) for details and motivation.

The following patch has been tested on i686-pc-linux-gnu with a full
"make bootstrap", all default languages, and regression tested with a
top-level "make -k check" with no new failures.

Ok for mainline?

2005-06-13  Roger Sayle  <roger@eyesopen.com>

	PR middle-end/7776
	* common.opt (Wstring-literal-comparison): New command line option.
	* c-opts.c (c_common_handle_option): Set it with -Wall.
	* c-typeck.c (parser_build_binary_op): Issue warning if either
	operand of a comparison operator is a string literal, except for
	testing equality or inequality against NULL.

	* gcc.dg/Wstring-literal-comparison-1.c: New test case.
	* gcc.dg/Wstring-literal-comparison-2.c: Likewise.
	* gcc.dg/Wstring-literal-comparison-3.c: Likewise.
	* gcc.dg/Wstring-literal-comparison-4.c: Likewise.


*** a/gcc/common.opt	4 Jun 2005 17:07:55 -0000	1.73
--- b/gcc/common.opt	13 Jun 2005 16:59:28 -0000
*************** Wstrict-aliasing=
*** 117,122 ****
--- 117,126 ----
  Common Joined UInteger
  Warn about code which might break strict aliasing rules

+ Wstring-literal-comparison
+ Common Var(warn_string_literal_comparison)
+ Warn about comparisons to constant string literals
+
  Wswitch
  Common Var(warn_switch)
  Warn about enumerated switches, with no default, missing a case
*** a/gcc/c-opts.c	25 May 2005 03:58:55 -0000	1.146
--- b/gcc/c-opts.c	13 Jun 2005 16:59:29 -0000
*************** c_common_handle_option (size_t scode, co
*** 370,375 ****
--- 370,376 ----
  	warn_sign_compare = value;
        warn_switch = value;
        warn_strict_aliasing = value;
+       warn_string_literal_comparison = value;

        /* Only warn about unknown pragmas that are not in system
  	 headers.  */
*** a/gcc/c-typeck.c	11 Jun 2005 19:47:01 -0000	1.453
--- b/gcc/c-typeck.c	13 Jun 2005 16:59:31 -0000
*************** parser_build_binary_op (enum tree_code c
*** 2412,2417 ****
--- 2412,2434 ----

      }

+   /* Warn about comparisons against string literals, with the exception
+      of testing for equality or inequality of a string literal with NULL.  */
+   if (code == EQ_EXPR || code == NE_EXPR)
+     {
+       if ((TREE_CODE (arg1.value) == STRING_CST
+ 	   && !integer_zerop (arg2.value))
+ 	  || (TREE_CODE (arg2.value) == STRING_CST
+ 	      && !integer_zerop (arg1.value)))
+ 	warning (OPT_Wstring_literal_comparison,
+ 		 "comparison with string literal");
+     }
+   else if (TREE_CODE_CLASS (code) == tcc_comparison
+ 	   && (TREE_CODE (arg1.value) == STRING_CST
+ 	       || TREE_CODE (arg2.value) == STRING_CST))
+     warning (OPT_Wstring_literal_comparison,
+ 	     "comparison with string literal");
+
    unsigned_conversion_warning (result.value, arg1.value);
    unsigned_conversion_warning (result.value, arg2.value);
    overflow_warning (result.value);