[C] Fix PR57258: unused variable warning is emitted for volatile variables

[Mingjie Xing]

Hi,

Changes:

        * c/c-decl.c (pop_scope): Skip volatile variables while emit
warnings for unused variables.

Tested on i686-pc-linux-gnu.

OK?

Mingjie

Index: gcc/c/c-decl.c
===================================================================
--- gcc/c/c-decl.c	(revision 204285)
+++ gcc/c/c-decl.c	(working copy)
@@ -1183,6 +1183,8 @@ pop_scope (void)
 	      && !DECL_IN_SYSTEM_HEADER (p)
 	      && DECL_NAME (p)
 	      && !DECL_ARTIFICIAL (p)
+	      /* A volatile variable might be used in some non-obvious way.  */
+	      && !TREE_THIS_VOLATILE (p)
 	      && scope != file_scope
 	      && scope != external_scope)
 	    {