Re: Missed compiler optimization issue in function select_rtable_names_for_explain

XChy <xxs_chy@xxxxxxxxxxx> · Wed, 22 May 2024 18:12:57 +0800



How is the memset in select_rtable_names_for_explain a dead-store?  Even memset
      calls could be optimized away from the EXPLAIN codepath I have a feeling it
would have to be many in a tight loop for it to be measurable even?

--
Daniel Gustafsson

    
    For the first question, I don't mean that the memset is the dead store. I mean that the stores with value "0" after the memset are dead:
    ```

        dpns.subplans = NIL;

          dpns.ctes = NIL;

          dpns.appendrels = NULL;
    ```

    
    since the memset has written zeroes to
      the object "dpns", and these members are known to be zero.
    

    For the second question, you are right,
      I don't really profile it or measure the performance impact for
      it. I just think it's worthwhile to improve codegen quality
      without affecting readability, as adopting performance tips from
      some static analyzer.

    
    Best regards, Hongyu.