6987115: Non-tiered compilation policy creates unnecessary C1 threads

Summary: Fixed NonTieredCompPolicy::compiler_count() to return correct thread count.
Reviewed-by: twisti, kvn

src/share/vm/runtime/compilationPolicy.cpp

@@ -129,16 +129,31 @@ void NonTieredCompPolicy::initialize() {
   }
 }
 
+// Note: this policy is used ONLY if TieredCompilation is off.
+// compiler_count() behaves the following way:
+// - with TIERED build (with both COMPILER1 and COMPILER2 defined) it should return
+//   zero for the c1 compilation levels, hence the particular ordering of the
+//   statements.
+// - the same should happen when COMPILER2 is defined and COMPILER1 is not
+//   (server build without TIERED defined).
+// - if only COMPILER1 is defined (client build), zero should be returned for
+//   the c2 level.
+// - if neither is defined - always return zero.
 int NonTieredCompPolicy::compiler_count(CompLevel comp_level) {
-#ifdef COMPILER1
-  if (is_c1_compile(comp_level)) {
+  assert(!TieredCompilation, "This policy should not be used with TieredCompilation");
+#ifdef COMPILER2
+  if (is_c2_compile(comp_level)) {
     return _compiler_count;
+  } else {
+    return 0;
   }
 #endif
 
-#ifdef COMPILER2
-  if (is_c2_compile(comp_level)) {
+#ifdef COMPILER1
+  if (is_c1_compile(comp_level)) {
     return _compiler_count;
+  } else {
+    return 0;
   }
 #endif