PR28901 -Wunused-variable ignores unused const initialised variables

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 3239a85..f8a5aa1 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -862,6 +862,12 @@  c_common_post_options (const char **pfilename)
     warn_shift_negative_value = (extra_warnings
 				 && (cxx_dialect >= cxx11 || flag_isoc99));
 
+  /* -Wunused-variable implies -Wunused-const-variable for C, but not
+     for C++, because const variables take the place of #defines in C++.  */
+  if (warn_unused_const_variable_set == -1)
+    warn_unused_const_variable_set = (warn_unused_variable
+				      && ! c_dialect_cxx ());
+
   /* Declone C++ 'structors if -Os.  */
   if (flag_declone_ctor_dtor == -1)
     flag_declone_ctor_dtor = optimize_size;
diff --git a/gcc/common.opt b/gcc/common.opt
index 94d1d88..c85f1b9 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -735,6 +735,10 @@  Wunused-variable
 Common Var(warn_unused_variable) Warning EnabledBy(Wunused)
 Warn when a variable is unused
 
+Wunused-const-variable
+Common Var(warn_unused_const_variable_set) Init(-1) Warning
+Warn when a const variable is unused
+
 Wcoverage-mismatch
 Common Var(warn_coverage_mismatch) Init(1) Warning
 Warn in case profiles in -fprofile-use do not match
diff --git a/gcc/cp/cp-objcp-common.c b/gcc/cp/cp-objcp-common.c
index 2cab89c..808defd 100644
--- a/gcc/cp/cp-objcp-common.c
+++ b/gcc/cp/cp-objcp-common.c
@@ -62,10 +62,6 @@  cxx_warn_unused_global_decl (const_tree decl)
   if (DECL_IN_SYSTEM_HEADER (decl))
     return false;
 
-  /* Const variables take the place of #defines in C++.  */
-  if (VAR_P (decl) && TREE_READONLY (decl))
-    return false;
-
   return true;
 }
 
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 518d689..7b5e44e 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -290,6 +290,7 @@  Objective-C and Objective-C++ Dialects}.
 -Wunsuffixed-float-constants  -Wunused  -Wunused-function @gol
 -Wunused-label  -Wunused-local-typedefs -Wunused-parameter @gol
 -Wno-unused-result -Wunused-value @gol -Wunused-variable @gol
+-Wunused-const-variable @gol
 -Wunused-but-set-parameter -Wunused-but-set-variable @gol
 -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance @gol
 -Wvla -Wvolatile-register-var  -Wwrite-strings @gol
@@ -4143,9 +4144,20 @@  its return value. The default is @option{-Wunused-result}.
 @item -Wunused-variable
 @opindex Wunused-variable
 @opindex Wno-unused-variable
-Warn whenever a local variable or non-constant static variable is unused
-aside from its declaration.
-This warning is enabled by @option{-Wall}.
+Warn whenever a local or static variable is unused aside from its
+declaration. This option implies @option{-Wunused-const-variable} for C,
+but not for C++. This warning is enabled by @option{-Wall}.
+
+To suppress this warning use the @code{unused} attribute
+(@pxref{Variable Attributes}).
+
+@item -Wunused-const-variable
+@opindex Wunused-const-variable
+@opindex Wno-unused-const-variable
+Warn whenever a constant static variable is unused aside from its declaration.
+This warning is enabled by @option{-Wunused-variable} for C, but not for C++.
+In C++ this is normally not an error since const variables take the place of
+@code{#define}s in C++.
 
 To suppress this warning use the @code{unused} attribute
 (@pxref{Variable Attributes}).
diff --git a/gcc/testsuite/g++.dg/warn/unused-variable-1.C b/gcc/testsuite/g++.dg/warn/unused-variable-1.C
new file mode 100644
index 0000000..cf531c0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/unused-variable-1.C
@@ -0,0 +1,7 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wunused-variable" } */
+
+static int a = 0;	  /* { dg-warning "defined but not used" } */
+static const int b = 0;	  /* Unlike C, this doesn't cause a warning in C++. */
+static int c __attribute__ ((unused)) = 0;
+static const char rcsid[] = "version-string"; /* Likewise. */
diff --git a/gcc/testsuite/g++.dg/warn/unused-variable-2.C b/gcc/testsuite/g++.dg/warn/unused-variable-2.C
new file mode 100644
index 0000000..b608fbc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/unused-variable-2.C
@@ -0,0 +1,7 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wunused-variable -Wunused-const-variable" } */
+
+static int a = 0;	/* { dg-warning "defined but not used" } */
+static const int b = 0;	/* { dg-warning "defined but not used" } */
+static int c __attribute__ ((unused)) = 0;
+static const char rcsid[] __attribute__ ((unused)) = "version-string";
diff --git a/gcc/testsuite/gcc.dg/unused-4.c b/gcc/testsuite/gcc.dg/unused-4.c
index 99e845f..5323600 100644
--- a/gcc/testsuite/gcc.dg/unused-4.c
+++ b/gcc/testsuite/gcc.dg/unused-4.c
@@ -1,6 +1,6 @@ 
 /* { dg-do compile } */
 /* { dg-options "-Wunused -O3" } */
 
-static const int i = 0;
+static const int i = 0;		/* { dg-warning "defined but not used" } */
 static void f() { }		/* { dg-warning "defined but not used" } */
 static inline void g() { }
diff --git a/gcc/testsuite/gcc.dg/unused-variable-1.c b/gcc/testsuite/gcc.dg/unused-variable-1.c
new file mode 100644
index 0000000..cb86c3bc
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/unused-variable-1.c
@@ -0,0 +1,7 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wunused-variable" } */
+
+static int a = 0;	  /* { dg-warning "defined but not used" } */
+static const int b = 0;	  /* { dg-warning "defined but not used" } */
+static int c __attribute__ ((unused)) = 0;
+static const char rcsid[] __attribute__ ((unused)) = "version-string";
diff --git a/gcc/testsuite/gcc.dg/unused-variable-2.c b/gcc/testsuite/gcc.dg/unused-variable-2.c
new file mode 100644
index 0000000..0496466
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/unused-variable-2.c
@@ -0,0 +1,7 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wunused-variable -Wno-unused-const-variable" } */
+
+static int a = 0;	  /* { dg-warning "defined but not used" } */
+static const int b = 0;
+static int c __attribute__ ((unused)) = 0;
+static const char rcsid[] = "version-string";
diff --git a/gcc/toplev.c b/gcc/toplev.c
index 926224a..83cafed 100644
--- a/gcc/toplev.c
+++ b/gcc/toplev.c
@@ -497,10 +497,9 @@  check_global_declaration (tree decl)
 
   /* Warn about static fns or vars defined but not used.  */
   if (((warn_unused_function && TREE_CODE (decl) == FUNCTION_DECL)
-       /* We don't warn about "static const" variables because the
-	  "rcs_id" idiom uses that construction.  */
-       || (warn_unused_variable
-	   && TREE_CODE (decl) == VAR_DECL && ! TREE_READONLY (decl)))
+       || (((warn_unused_variable && ! TREE_READONLY (decl))
+	    || (warn_unused_const_variable_set > 0 && TREE_READONLY (decl)))
+	   && TREE_CODE (decl) == VAR_DECL))
       && ! DECL_IN_SYSTEM_HEADER (decl)
       && ! snode->referred_to_p (/*include_self=*/false)
       /* This TREE_USED check is needed in addition to referred_to_p
@@ -527,7 +526,9 @@  check_global_declaration (tree decl)
     warning_at (DECL_SOURCE_LOCATION (decl),
 		(TREE_CODE (decl) == FUNCTION_DECL)
 		? OPT_Wunused_function
-		: OPT_Wunused_variable,
+		: (TREE_READONLY (decl)
+		   ? OPT_Wunused_const_variable
+		   : OPT_Wunused_variable),
 		"%qD defined but not used", decl);
 }