cleanup warnings indicated by the -Wunused-value compiler option on linux

jeremymanson@google.com suggested the following patch for fixing the warnings indicated by the -Wunused-value compiler options on Linux:

diff --git a/make/linux/makefiles/gcc.make b/make/linux/makefiles/gcc.make
--- a/make/linux/makefiles/gcc.make
+++ b/make/linux/makefiles/gcc.make
@@ -126,7 +126,7 @@
 # Compiler warnings are treated as errors
 WARNINGS_ARE_ERRORS = -Werror
 
-WARNING_FLAGS = -Wpointer-arith -Wsign-compare -Wundef -Wunused-function
+WARNING_FLAGS = -Wpointer-arith -Wsign-compare -Wundef -Wunused-function -Wunused-value
 
 # Since GCC 4.3, -Wconversion has changed its meanings to warn these implicit
 # conversions which might affect the values. Only enable it in earlier versions.
diff --git a/src/share/vm/c1/c1_IR.cpp b/src/share/vm/c1/c1_IR.cpp
--- a/src/share/vm/c1/c1_IR.cpp
+++ b/src/share/vm/c1/c1_IR.cpp
@@ -506,7 +506,7 @@
   _loop_map(0, 0), // initialized later with correct size
   _compilation(c)
 {
- TRACE_LINEAR_SCAN(2, "***** computing linear-scan block order");
+ TRACE_LINEAR_SCAN(2, tty->print_cr("***** computing linear-scan block order"));
 
   init_visited();
   count_edges(start_block, NULL);
@@ -683,7 +683,7 @@
 }
 
 void ComputeLinearScanOrder::assign_loop_depth(BlockBegin* start_block) {
- TRACE_LINEAR_SCAN(3, "----- computing loop-depth and weight");
+ TRACE_LINEAR_SCAN(3, tty->print_cr("----- computing loop-depth and weight"));
   init_visited();
 
   assert(_work_list.is_empty(), "work list must be empty before processing");
@@ -868,7 +868,7 @@
 }
 
 void ComputeLinearScanOrder::compute_order(BlockBegin* start_block) {
- TRACE_LINEAR_SCAN(3, "----- computing final block order");
+ TRACE_LINEAR_SCAN(3, tty->print_cr("----- computing final block order"));
 
   // the start block is always the first block in the linear scan order
   _linear_scan_order = new BlockList(_num_blocks);
diff --git a/src/share/vm/code/nmethod.cpp b/src/share/vm/code/nmethod.cpp
--- a/src/share/vm/code/nmethod.cpp
+++ b/src/share/vm/code/nmethod.cpp
@@ -2602,7 +2602,8 @@
                       relocation_begin()-1+ip[1]);
       for (; ip < index_end; ip++)
         tty->print_cr(" (%d ?)", ip[0]);
- tty->print_cr(" @" INTPTR_FORMAT ": index_size=%d", ip, *ip++);
+ tty->print_cr(" @" INTPTR_FORMAT ": index_size=%d", ip, *ip);
+ ip++;
       tty->print_cr("reloc_end @" INTPTR_FORMAT ":", ip);
     }
   }
diff --git a/src/share/vm/memory/cardTableModRefBS.cpp b/src/share/vm/memory/cardTableModRefBS.cpp
--- a/src/share/vm/memory/cardTableModRefBS.cpp
+++ b/src/share/vm/memory/cardTableModRefBS.cpp
@@ -395,7 +395,7 @@
   }
   // Touch the last card of the covered region to show that it
   // is committed (or SEGV).
- debug_only(*byte_for(_covered[ind].last());)
+ debug_only((void) (*byte_for(_covered[ind].last()));)
   debug_only(verify_guard();)
 }
 
diff --git a/src/share/vm/memory/universe.cpp b/src/share/vm/memory/universe.cpp
--- a/src/share/vm/memory/universe.cpp
+++ b/src/share/vm/memory/universe.cpp
@@ -532,7 +532,9 @@
   if (vt) vt->initialize_vtable(false, CHECK);
   if (ko->oop_is_instance()) {
     InstanceKlass* ik = (InstanceKlass*)ko;
- for (KlassHandle s_h(THREAD, ik->subklass()); s_h() != NULL; s_h = (THREAD, s_h()->next_sibling())) {
+ for (KlassHandle s_h(THREAD, ik->subklass());
+ s_h() != NULL;
+ s_h = KlassHandle(THREAD, s_h()->next_sibling())) {
       reinitialize_vtable_of(s_h, CHECK);
     }
   }
diff --git a/src/share/vm/runtime/sharedRuntime.cpp b/src/share/vm/runtime/sharedRuntime.cpp
--- a/src/share/vm/runtime/sharedRuntime.cpp
+++ b/src/share/vm/runtime/sharedRuntime.cpp
@@ -2733,7 +2733,7 @@
   // ResourceObject, so do not put any ResourceMarks in here.
   char *s = sig->as_C_string();
   int len = (int)strlen(s);
- *s++; len--; // Skip opening paren
+ s++; len--; // Skip opening paren
   char *t = s+len;
   while( *(--t) != ')' ) ; // Find close paren
 
diff --git a/src/share/vm/services/diagnosticArgument.cpp b/src/share/vm/services/diagnosticArgument.cpp
--- a/src/share/vm/services/diagnosticArgument.cpp
+++ b/src/share/vm/services/diagnosticArgument.cpp
@@ -232,7 +232,7 @@
   } else {
     _value._time = 0;
     _value._nanotime = 0;
- strcmp(_value._unit, "ns");
+ strcpy(_value._unit, "ns");
   }
 }
 
diff --git a/src/share/vm/utilities/taskqueue.hpp b/src/share/vm/utilities/taskqueue.hpp
--- a/src/share/vm/utilities/taskqueue.hpp
+++ b/src/share/vm/utilities/taskqueue.hpp
@@ -340,8 +340,12 @@
   if (dirty_n_elems == N - 1) {
     // Actually means 0, so do the push.
     uint localBot = _bottom;
- // g++ complains if the volatile result of the assignment is unused.
- const_cast<E&>(_elems[localBot] = t);
+ // g++ complains if the volatile result of the assignment is
+ // unused, so we cast the volatile away. We cannot cast directly
+ // to void, because gcc treats that as not using the result of the
+ // assignment. However, casting to E& means that we trigger an
+ // unused-value warning. So, we cast the E& to void.
+ (void) const_cast<E&>(_elems[localBot] = t);
     OrderAccess::release_store(&_bottom, increment_index(localBot));
     TASKQUEUE_STATS_ONLY(stats.record_push());
     return true;
@@ -397,7 +401,12 @@
     return false;
   }
 
- const_cast<E&>(t = _elems[oldAge.top()]);
+ // g++ complains if the volatile result of the assignment is
+ // unused, so we cast the volatile away. We cannot cast directly
+ // to void, because gcc treats that as not using the result of the
+ // assignment. However, casting to E& means that we trigger an
+ // unused-value warning. So, we cast the E& to void.
+ (void) const_cast<E&>(t = _elems[oldAge.top()]);
   Age newAge(oldAge);
   newAge.increment();
   Age resAge = _age.cmpxchg(newAge, oldAge);
@@ -640,8 +649,12 @@
   uint dirty_n_elems = dirty_size(localBot, top);
   assert(dirty_n_elems < N, "n_elems out of range.");
   if (dirty_n_elems < max_elems()) {
- // g++ complains if the volatile result of the assignment is unused.
- const_cast<E&>(_elems[localBot] = t);
+ // g++ complains if the volatile result of the assignment is
+ // unused, so we cast the volatile away. We cannot cast directly
+ // to void, because gcc treats that as not using the result of the
+ // assignment. However, casting to E& means that we trigger an
+ // unused-value warning. So, we cast the E& to void.
+ (void) const_cast<E&>(_elems[localBot] = t);
     OrderAccess::release_store(&_bottom, increment_index(localBot));
     TASKQUEUE_STATS_ONLY(stats.record_push());
     return true;
@@ -665,7 +678,12 @@
   // This is necessary to prevent any read below from being reordered
   // before the store just above.
   OrderAccess::fence();
- const_cast<E&>(t = _elems[localBot]);
+ // g++ complains if the volatile result of the assignment is
+ // unused, so we cast the volatile away. We cannot cast directly
+ // to void, because gcc treats that as not using the result of the
+ // assignment. However, casting to E& means that we trigger an
+ // unused-value warning. So, we cast the E& to void.
+ (void) const_cast<E&>(t = _elems[localBot]);
   // This is a second read of "age"; the "size()" above is the first.
   // If there's still at least one element in the queue, based on the
   // "_bottom" and "age" we've read, then there can be no interference with