The current fix for JDK-8213473 replaces 'testB_mem_imm' with 'testUB_mem_imm' in 'x86_64.ad' which is fine for unsigned bytes, for example:
  if ((flags & 0x80) != 0) ...

But per JLS §5.6.2 & §5.1.2, if we have something like:
  if ((flags & (byte)0x80) != 0) ...

the mask is 32-bit sign-extended and 'LoadB' isn't converted to 'LoadUB' in 'AndINode::Ideal' which is fine.

Unfortunately 'testUB_mem_imm' won't be matched in this case, so I guess we'd need both 'testB_mem_imm' & 'testUB_mem_imm' patterns.