Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 226719)
+++ gcc/doc/invoke.texi	(working copy)
@@ -3720,10 +3720,19 @@ Warn about passing a null pointer for ar
 requiring a non-null value by the @code{nonnull} function attribute.
 
 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}.  It
 can be disabled with the @option{-Wno-nonnull} option.
 
+@item -Wnull-dereference
+@opindex Wnull-dereference
+@opindex Wno-null-dereference
+Warn if the compiler detects paths that trigger erroneous or
+undefined behavior due to dereferencing a null pointer.  This option
+is only active when @option{-fdelete-null-pointer-checks} is active,
+which is enabled by optimizations in most targets.  The precision of
+the warnings depends on the optimization options used.
+
 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
 @opindex Winit-self
 @opindex Wno-init-self
 Warn about uninitialized variables that are initialized with themselves.
 Note this option can only be used with the @option{-Wuninitialized} option.
@@ -4156,20 +4165,10 @@ All the above @option{-Wunused} options 
 
 In order to get a warning about an unused function parameter, you must
 either specify @option{-Wextra -Wunused} (note that @option{-Wall} implies
 @option{-Wunused}), or separately specify @option{-Wunused-parameter}.
 
-@item -Wnull-dereference
-@opindex Wnull-dereference
-@opindex Wno-null-dereference
-Warn if the compiler detects paths that trigger erroneous or
-undefined behavior due to dereferencing a null pointer.  This option
-is only active when @option{-fdelete-null-pointer-checks} is active,
-which is enabled by optimizations in most targets.  The precision of
-the warnings depends on the optimization options used.  This option is
-enabled by @option{-Wall}.
-
 @item -Wuninitialized
 @opindex Wuninitialized
 @opindex Wno-uninitialized
 Warn if an automatic variable is used without first being initialized
 or if a variable may be clobbered by a @code{setjmp} call. In C++,
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt	(revision 226719)
+++ gcc/c-family/c.opt	(working copy)
@@ -333,11 +333,11 @@ Warn about casts which discard qualifier
 Wchar-subscripts
 C ObjC C++ ObjC++ Var(warn_char_subscripts) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about subscripts whose type is \"char\"
 
 Wchkp
-C ObjC C++ ObjC++ Var(warn_chkp) Warning EnabledBy(Wall)
+C ObjC C++ ObjC++ Var(warn_chkp) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about memory access errors found by Pointer Bounds Checker
 
 Wclobbered
 C ObjC C++ ObjC++ Var(warn_clobbered) Warning EnabledBy(Wextra)
 Warn about variables that might be changed by \"longjmp\" or \"vfork\"
Index: gcc/optc-gen.awk
===================================================================
--- gcc/optc-gen.awk	(revision 226719)
+++ gcc/optc-gen.awk	(working copy)
@@ -28,11 +28,25 @@
 #            [-v header_name=header.h] < inputfile > options.c
 
 # Dump that array of options into a C file.
 END {
 
-# Record first EnabledBy and LangEnabledBy uses.
+
+# Combine the flags of identical switches.  Switches
+# appear many times if they are handled by many front
+# ends, for example.
+for (i = 0; i < n_opts; i++) {
+    merged_flags[i] = flags[i]
+}
+for (i = 0; i < n_opts; i++) {
+    while(i + 1 != n_opts && opts[i] == opts[i + 1] ) {
+	merged_flags[i + 1] = merged_flags[i] " " merged_flags[i + 1];
+	i++;
+    }
+}
+
+# Record EnabledBy and LangEnabledBy uses.
 n_enabledby = 0;
 for (i = 0; i < n_langs; i++) {
     n_enabledby_lang[i] = 0;
 }
 for (i = 0; i < n_opts; i++) {
@@ -46,19 +60,23 @@ for (i = 0; i < n_opts; i++) {
             # EnabledBy(arg) or EnabledBy(arg1 || arg2 || arg3)
             split_sep = " \\|\\| ";
         }
         n_enabledby_names = split(enabledby_arg, enabledby_names, split_sep);
         if (logical_and != 0 && n_enabledby_names > 2) {
-            print "#error EnabledBy (Wfoo && Wbar && Wbaz) not currently supported"
+            print "#error " opts[i] " EnabledBy(Wfoo && Wbar && Wbaz) currently not supported"
         }
         for (j = 1; j <= n_enabledby_names; j++) {
             enabledby_name = enabledby_names[j];
             enabledby_index = opt_numbers[enabledby_name];
             if (enabledby_index == "") {
-                print "#error Enabledby: " enabledby_name 
-            } else {
-                condition = "";
+                print "#error " opts[i] " Enabledby(" enabledby_name "), unknown option '" enabledby_name "'"
+            } else if (!flag_set_p("Common", merged_flags[enabledby_index])) {
+		print "#error " opts[i] " Enabledby(" enabledby_name "), '" \
+		    enabledby_name "' must have flag 'Common'"		\
+		    " to use Enabledby(), otherwise use LangEnabledBy()"
+	    } else {
+		condition = "";
                 if (logical_and != 0) {
                     opt_var_name_1 = search_var_name(enabledby_names[1], opt_numbers, opts, flags, n_opts);
                     opt_var_name_2 = search_var_name(enabledby_names[2], opt_numbers, opts, flags, n_opts);
                     if (opt_var_name_1 == "") {
                         print "#error " enabledby_names[1] " does not have a Var() flag"
Index: gcc/common.opt
===================================================================
--- gcc/common.opt	(revision 226719)
+++ gcc/common.opt	(working copy)
@@ -591,11 +591,11 @@ Common RejectNegative Joined Warning Und
 Wlarger-than=
 Common RejectNegative Joined UInteger Warning
 -Wlarger-than=<number>	Warn if an object is larger than <number> bytes
 
 Wnull-dereference
-Common Var(warn_null_dereference) Warning EnabledBy(Wall)
+Common Var(warn_null_dereference) Warning
 Warn if dereferencing a NULL pointer may lead to erroneous or undefined behavior
 
 Wunsafe-loop-optimizations
 Common Var(warn_unsafe_loop_optimizations) Warning
 Warn if the loop cannot be optimized due to nontrivial assumptions.
Index: gcc/opt-functions.awk
===================================================================
--- gcc/opt-functions.awk	(revision 226719)
+++ gcc/opt-functions.awk	(working copy)
@@ -322,21 +322,21 @@ function lang_enabled_by(enabledby_langs
     if (enabledby_posarg != "" && enabledby_negarg != "") {
         with_args = "," enabledby_posarg "," enabledby_negarg
     } else if (enabledby_posarg == "" && enabledby_negarg == "") {
         with_args = ""
     } else {
-        print "#error LangEnabledBy("enabledby_langs","enabledby_name", " \
+        print "#error " opts[i] " LangEnabledBy("enabledby_langs","enabledby_name", " \
             enabledby_posarg", " enabledby_negargs                  \
             ") with three arguments, it should have either 2 or 4"
     }
 
     n_enabledby_array = split(enabledby_name, enabledby_array, " \\|\\| ");
     for (k = 1; k <= n_enabledby_array; k++) {
         enabledby_index = opt_numbers[enabledby_array[k]];
         if (enabledby_index == "") {
-             print "#error LangEnabledBy("enabledby_langs","enabledby_name", " \
-                 enabledby_posarg", " enabledby_negargs") has invalid ENABLEDBY_NAME"
+             print "#error " opts[i] " LangEnabledBy("enabledby_langs","enabledby_name", " \
+                 enabledby_posarg", " enabledby_negargs"), unknown option '" enabledby_name "'"
         } else {
             for (j = 1; j <= n_enabledby_arg_langs; j++) {
                  lang_name = lang_sanitized_name(enabledby_arg_langs[j]);
                  lang_index = lang_numbers[enabledby_arg_langs[j]];
                  if (enables[lang_name,enabledby_array[k]] == "") {