Make j9localmap_DebugLocalBitsForPC the default mapper

j9localmap_LocalBitsForPC is more aggressive than
j9localmap_DebugLocalBitsForPC in the way it analyzes and marks local
variables as object references. Aggressiveness relates to determining
whether a local object is being used at a specific point in the
bytecode execution, and this helps optimize garbage collection and
runtime performance. In contrast, j9localmap_DebugLocalBitsForPC
adopts a conservative approach; it overestimates and plays safe by
marking objects alive even if they are not being used.

j9localmap_DebugLocalBitsForPC resolves the issue seen in
ibmruntimes/Semeru-Runtimes#93.

We have run the following benchmarks: Liberty DT8/DT10 Startup &
Footprint, JRuby and Nashorn. No performance difference is seen between
j9localmap_DebugLocalBitsForPC and j9localmap_LocalBitsForPC.

Signed-off-by: Babneet Singh <[email protected]>

Author: babsingh

runtime/vm/jvminit.c

@@ -1172,7 +1172,7 @@ initializeJavaVM(void * osMainThread, J9JavaVM ** vmPtr, J9CreateJavaVMParams *c
 	vm->reserved1_identifier = (void*)J9VM_IDENTIFIER;
 	vm->internalVMFunctions = GLOBAL_TABLE(J9InternalFunctions);
 	vm->portLibrary = portLibrary;
-	vm->localMapFunction = j9localmap_LocalBitsForPC;
+	vm->localMapFunction = j9localmap_DebugLocalBitsForPC;
 
 	vm->internalVMLabels = (J9InternalVMLabels*)-1001;
 	vm->cInterpreter = J9_BUILDER_SYMBOL(cInterpreter);