RFR: 8157189: 'iload_w' in shared class is not interpreted correctly

[Jiangli Zhou]

Please review the fix for the ‘iload_w’ issue in shared class.

  webrev: http://cr.openjdk.java.net/~jiangli/8157189/
  bug: https://bugs.openjdk.java.net/browse/JDK-8157189?filter=14921

When CDS archives classes at dump time, it rewrites the ‘iload’ opcode to ‘nofast_iload’ in methods. At runtime, ‘nofast_iload’ bytecode instructions are interpreted the same as ‘iload’ instructions but without patching. The ‘iload’ opcode can be used in conjunction with the ‘wide’ instruction to access a local variable using a two-byte unsigned index. The CDS bytecode rewriter does not check if the current ‘iload’ is ‘iload_w’ during rewriting at dump time, and rewrites all ‘iload’ to ’nofast_iload'. That causes problems when the originally ‘iload_w’ is handled the same as the ‘iload’ at runtime. The fix is to only rewrite the opcode to the nofast version when it is not ‘iload_w’. As the VM does not patch the ‘iload_w’ instruction, so the fix doesn’t introduce any new writes to the shared class either.

Thanks,
Jiangli