PR28901 -Wunused-variable ignores unused const initialised variables in C

12 years ago it was decided that -Wunused-variable shouldn't warn about
static const variables because some code used const static char rcsid[]
strings which were never used but wanted in the code anyway. But as the
bug points out this hides some real bugs. These days the usage of rcsids
is not very popular anymore. So this patch changes the default to warn
about unused static const variables in C with -Wunused-variable. And it
adds a new option -Wno-unused-const-variable to turn this warning off.
For C++ this new warning is off by default, since const variables can be
used as #defines in C++. New testcases for the new defaults in C and C++
are included testing the new warning and suppressing it with an unused
attribute or using -Wno-unused-const-variable.

gcc/ChangeLog

       PR c/28901
       * toplev.c (check_global_declaration): Check and use
       warn_unused_const_variable.
       * doc/invoke.texi (Warning Options): Add -Wunused-const-variable.
       (-Wunused-variable): Remove non-constant. For C implies
       -Wunused-const-variable.
       (-Wunused-const-variable): New.

gcc/c-family/ChangeLog

       PR c/28901
       * c.opt (Wunused-variable): Option from common.opt.
       (Wunused-const-variable): New option.

gcc/cp/ChangeLog

       PR c/28901
       * cp-objcp-common.c (cxx_warn_unused_global_decl): Remove hard-coded
       VAR_P TREE_READONLY override.

gcc/testsuite/ChangeLog

       PR c/28901
       * g++.dg/warn/unused-variable-1.C: New test.
       * g++.dg/warn/unused-variable-2.C: Likewise.
       * gcc.dg/unused-4.c: Adjust warning for static const.
       * gcc.dg/unused-variable-1.c: New test.
       * gcc.dg/unused-variable-2.c: Likewise.

From-SVN: r227742

13 files changed, 89 insertions, 13 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9059ffa..0aa46d0 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,13 @@
+2015-09-11  Mark Wielaard  <mjw@redhat.com>
+
+	PR c/28901
+	* toplev.c (check_global_declaration): Check and use
+	warn_unused_const_variable.
+	* doc/invoke.texi (Warning Options): Add -Wunused-const-variable.
+	(-Wunused-variable): Remove non-constant. For C implies
+	-Wunused-const-variable.
+	(-Wunused-const-variable): New.
+
 2015-09-14  Richard Biener  <rguenther@suse.de>
 
 	* doc/match-and-simplify.texi: Update for changed syntax
diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 3acc84f..0163b15 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,9 @@
+2015-09-11  Mark Wielaard  <mjw@redhat.com>
+
+	PR c/28901
+	* c.opt (Wunused-variable): Option from common.opt.
+	(Wunused-const-variable): New option.
+
 2015-09-09  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/53184
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index d519d7a..47ba070 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -912,6 +912,14 @@ Wunused-result
 C ObjC C++ ObjC++ Var(warn_unused_result) Init(1) Warning
 Warn if a caller of a function, marked with attribute warn_unused_result, does not use its return value
 
+Wunused-variable
+C ObjC C++ ObjC++ LangEnabledBy(C ObjC C++ ObjC++,Wunused)
+; documented in common.opt
+
+Wunused-const-variable
+C ObjC C++ ObjC++ Var(warn_unused_const_variable) Warning LangEnabledBy(C ObjC,Wunused-variable)
+Warn when a const variable is unused
+
 Wvariadic-macros
 C ObjC C++ ObjC++ CPP(warn_variadic_macros) CppReason(CPP_W_VARIADIC_MACROS) Var(cpp_warn_variadic_macros) Init(0) Warning LangEnabledBy(C ObjC C++ ObjC++,Wpedantic || Wtraditional)
 Warn about using variadic macros
diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index a9952fc..4101a2e 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2015-09-11  Mark Wielaard  <mjw@redhat.com>
+
+	PR c/28901
+	* cp-objcp-common.c (cxx_warn_unused_global_decl): Remove hard-coded
+	VAR_P TREE_READONLY override.
+
 2015-09-10  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/67318
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/ChangeLog b/gcc/testsuite/ChangeLog
index 65dde8c..a285ee2 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,12 @@
+2015-09-11  Mark Wielaard  <mjw@redhat.com>
+
+	PR c/28901
+	* g++.dg/warn/unused-variable-1.C: New test.
+	* g++.dg/warn/unused-variable-2.C: Likewise.
+	* gcc.dg/unused-4.c: Adjust warning for static const.
+	* gcc.dg/unused-variable-1.c: New test.
+	* gcc.dg/unused-variable-2.c: Likewise.
+
 2015-09-13  David Edelsohn  <dje.gcc@gmail.com>
 
 	* gfortran.dg/private_type_3.f90: Require visibility.
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..95e4c52 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 && 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);
 }