在 2018/9/20 22:08, Vincent Lefevre 写道: >> In C++, declaring n1 const avoids the warning regardless of >> optimization levels. > > If the constant propagation is done at -O0, this could explain > the behavior. > > Or do you mean that GCC remembers the type the data come from, > i.e. assuming char is signed, if n1 is of type char, then ~n1 > is necessarily representable in a char, thus can be regarded > as of being of type char in its analysis? >
In C++ adding the `const` qualifier makes `~n1` a constant expression. In C it never is, regardless of qualifiers. BTW, I am quite disappointed with such 'false' warnings, because by performing a compound AND-and-ASSIGN operation on a `char` object I have no interest in bits that don't fit into a `char`, be they ones or zeroes. Perhaps there are scenarios where they shouldn't be ignored, but I can't think of any. -- Best regards, LH_Mouse
