C++: Promote cpp/unsigned-difference-expression-compared-zero to Code Scanning

[MathiasVP]

All results flagged up by MRVA are either clear TPs, or FPs because of bad code. I consider "bad code" to be code such as:

/** INVARIANT: x >= y */
void foo(unsigned x, unsigned y) {
  if(x - y > 0) {
    // ...
  } else {
    // x >= y and x - y <= 0 so x == y.
  }
}

and rewriting this to something like:

void foo(unsigned x, unsigned y) {
  if(x > y) {
    // ...
  } else {
    // ...
  }
}

or even (if that was the intended meaning):

void foo(unsigned x, unsigned y) {
  if(x != y) {
    // ...
  } else {
    // ...
  }
}

is much clearer and less error-prone. As an added bonus, all of these alerts should be easily auto-fixable.

[jketema]

What was your assessment of the DCA results?

[MathiasVP]

DCA results were all good. There were two clear FPs on DCA which were subsequently fixed in #16988. The remaining ones fit the above description (where a - b > 0 really should be replaced with a > b)

[MathiasVP]

DCA looks good to me. The alerts are the same as the previous DCA run with the exception of two removed FPs that disappeared in #16988 🎉

Performance looks totally unchanged.