Disasm now translates xor x0,x0,x0 as a machine-generated bubble ("-").

It is very convenient for pipeline trace viewing to differentiate
  between compiler NOPs and pipeline bubbles.

spike/disasm.cc

@@ -247,6 +247,7 @@ disassembler_t::disassembler_t()
   DEFINE_ITYPE(jalr);
 
   add_insn(new disasm_insn_t("nop", match_addi, mask_addi | mask_rd | mask_rs1 | mask_imm, {}));
+  add_insn(new disasm_insn_t(" - ", match_xor, mask_xor | mask_rd | mask_rs1 | mask_rs2, {})); // for machine-generated bubbles
   DEFINE_I0TYPE("li", addi);
   DEFINE_I1TYPE("move", addi);
   DEFINE_ITYPE(addi);