https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70800
--- Comment #10 from David Binderman <dcb314 at hotmail dot com> --- (In reply to Eric Gallager from comment #4) > (In reply to David Binderman from comment #3) > > I just discovered that clang++ can be made to find this bug by > > adding flag -Wtautological-compare. > > > > jan22a.cc:6:17: warning: bitwise comparison always > > evaluates to false [-Wtautological-compare] > > if ((n & 0x30) == 1) > > ~~~~~~~~~~~^~~~ > > I think Marek enhanced -Wtautological-compare to catch cases like this, in > which case bug 71852 is related I had a quick look at Marek's patch and bearing in mind I am a complete novice at gcc source code, these lines look suspicious: + /* For BIT_AND only warn if (CST2 & CST1) != CST1, and + for BIT_OR only if (CST2 | CST1) != CST1. */ + if (res == cst) + return; so I suspect the comparison operator of ">" makes sure the warning cannot be produced.
