[PATCH] D130510: Missing tautological compare warnings due to unary operators

[Richard Trieu via Phabricator via cfe-commits]

rtrieu reopened this revision.
rtrieu added a comment.
This revision is now accepted and ready to land.

This warning is now flagging some code which I believe is a false positive.  In particular, when template arguments are involved, their values can be calculated during template instantiation, but they should be treated as variables instead of constants.  For example:

  template <int I, class T>
  void foo(int x) {
      bool b1 = (x & sizeof(T)) == 8;
      bool b2 = (x & I) == 8;
      bool b3 = (x & 4) == 8;
  }
  
  void run(int x) {
      foo<4, int>(8);
  }

In this instantiation, x is and'ed with the value 4 in different ways.  `sizeof(T)` is type-dependent, `I` is value-dependent, and `4` is an integer literal.  With your code, each of them would produce a warning.  However, the first two values of 4 will change in different template instantiations, so the warning is not useful when it is okay for some instantiations to have constant values.  Because of this, warnings should treat dependent expressions as non-constant even when they can be evaluated, so only `b3` should get a warning.  This is causing the warning to be emitted on code heavy in template meta-programming, such as array libraries.  Please revert or fix.

I believe that evaluating the expression would make this warning too broad and would need more testing that what was included here.  Only handling UnaryOperator with IntegerLiteral sub-expression makes more sense for the warning, and adding in any new cases if we find them.


Repository:
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https://reviews.llvm.org/D130510