7104561: UseRDPCForConstantTableBase doesn't work after shorten branches changes

Reviewed-by: never, kvn

src/cpu/sparc/vm/vm_version_sparc.cpp

 /*
- * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -211,7 +211,7 @@ void VM_Version::initialize() {
 #ifdef COMPILER2
   // T4 and newer Sparc cpus have fast RDPC.
   if (has_fast_rdpc() && FLAG_IS_DEFAULT(UseRDPCForConstantTableBase)) {
-//    FLAG_SET_DEFAULT(UseRDPCForConstantTableBase, true);
+    FLAG_SET_DEFAULT(UseRDPCForConstantTableBase, true);
   }
 
   // Currently not supported anywhere.

src/share/vm/opto/machnode.cpp

@@ -484,6 +484,13 @@ int MachConstantNode::constant_offset() {
   // Bind the offset lazily.
   if (offset == -1) {
     Compile::ConstantTable& constant_table = Compile::current()->constant_table();
+    // If called from Compile::scratch_emit_size assume the worst-case
+    // for load offsets: half the constant table size.
+    // NOTE: Don't return or calculate the actual offset (which might
+    // be zero) because that leads to problems with e.g. jumpXtnd on
+    // some architectures (cf. add-optimization in SPARC jumpXtnd).
+    if (Compile::current()->in_scratch_emit_size())
+      return constant_table.size() / 2;
     offset = constant_table.table_base_offset() + constant_table.find_offset(_constant);
     _constant.set_offset(offset);
   }