+-----------+
          | sysy file |
          +-----------+
                |
                v
  +------------------------------+
  | antlr4 pre genernated parser |
  +------------------------------+
                |
                |
                v
      +----------------------+
      | frontend gen llvm ir |
      +----------------------+
                |
                |
                v
        +-----------------+
        | pass on llvm ir | <--- cfg node now using shared_ptr and have many copy/destroy
        +-----------------+      while optimizing
                |                this consumes most of compile time
                |
                v
      +---------------------+
      | backend distributer | <--- the backend only take around 0.8% of compile time
      +---------------------+      no need for parallelization
          |     |     |  
          |     |     | <--- compile tasks allocated to multi-threads if parallel issued
          |     |     |      serialize issue or parallel issue
          |    ...   ...
          |
          v
  +-------------------+
  | risc-lang ir      |
  | just Uop and VReg |  <-- schedule on ir, assign register, plan stack map
  +-------------------+
            |
            |
            |
            v            +--------------+
      +-------------+    | other thread |
      |  riscv asm  |    +--------------+
      +-------------+           v
            |                   v
            |                   |                  +--------------+
            +-------------------+------------- <<< | other thread |
                                |                  +--------------+
                                |
                                | <--- do trick on generation
                                |
                                v
                +---------------------------------+
                | asm optimization and generation |
                +---------------------------------+
                                |
                                | <--- schedule on asm and reduce redundant codes
                                v
                          +----------+
                          | asm file |
                          +----------+