diff -r 1af104d6cf99 -r 381bf869f784 src/cpu/x86/vm/assembler_x86.cpp
--- a/src/cpu/x86/vm/assembler_x86.cpp	Tue Aug 16 16:59:46 2011 -0700
+++ b/src/cpu/x86/vm/assembler_x86.cpp	Wed Aug 17 05:14:43 2011 -0700
@@ -3672,7 +3672,7 @@
     } else {
       if (adr.index_needs_rex()) {
         prefix(REX_X);
-      } else if (reg->encoding() >= 4 ) {
+      } else if (byteinst && reg->encoding() >= 4 ) {
         prefix(REX);
       }
     }