SA_LOCAL_SELF_COMPUTATION bug

[gloNelson]

Hi,
I read SA_LOCAL_SELF_COMPUTATION on SpotBugs Bug Description page, which mentions that this method performs a nonsensical computation of a local variable with another reference to the same variable (e.g., x&x or x-x).
So, I wrote test cases as below.

int testSA_LOCAL_SELF_COMPUTATION(){
      int flags = 1;
      return flags ^ flags; // warning SA_LOCAL_SELF_COMPUTATION
  }

  int testSA_LOCAL_SELF_COMPUTATION1(){
      int flags = 1;
      return flags & flags; // no warning (False negative)
  }

  int testSA_LOCAL_SELF_COMPUTATION2(){
      int flags = 1;
      return flags - flags; // no warning (False negative)
  }

  int testSA_LOCAL_SELF_COMPUTATION3(){
      int flags = 1;
      return flags | flags; // no warning (False negative)
  }

It can be seen from the results that only ^ will trigger SpotBugs to report SA_LOCAL_SELF_COMPUTATION, while other operators will not.

I tried to search test cases of SA_LOCAL_SELF_COMPUTATION in SpotBugs source code and found several test cases of SA_FIELD_SELF_COMPUTATION which detect self computation of field instead of local variables.
Similar test-cases for SA_FIELD_SELF_COMPUTATION executed and all four output SA_FIELD_SELF_COMPUTATION errors report.

int x = 1;
  int testSA_FIELD_SELF_COMPUTATION(){
      return x ^ x; // warning SA_FIELD_SELF_COMPUTATION
  }

  int testSA_FIELD_SELF_COMPUTATION1(){
      return x & x; // warning SA_FIELD_SELF_COMPUTATION
  }

  int testSA_FIELD_SELF_COMPUTATION2(){
      return x - x; // warning SA_FIELD_SELF_COMPUTATION
  }

  int testSA_FIELD_SELF_COMPUTATION3(){
      return x | x; // warning SA_FIELD_SELF_COMPUTATION
  }

Finally, I realized that the fault is in line 325 of FindSelfComparison.java.

else if (opCode == Const.IXOR && item0.sameValue(item1)) {

The code shows that SpotBugs reports SA_FIELD_SELF_COMPUTATION only when opCode equals Const.IXOR(^).

It is so confusing that self computation detection of local variables only including ^ operator while the detection of field supports much more. Is it a bug?

[gloNelson]

Another problematic code section in FindSelfComparison.java is mentioned in #1473 (False negatives on SA_FIELD_SELF_COMPUTATION and SA_LOCAL_SELF_COMPUTATION) in detail.

Thanks a lot. @KengoTODA

[gloNelson]

I have created a pull request to fix this. Please help me review this :) @KengoTODA Thks!

[gtoison]

Fixed by #1478, thank you!