[Bug c++/95156] New: -Wtautological-compare warns in C but not C++

[Bug c++/95156] New: -Wtautological-compare warns in C but not C++

[simon.marchi at polymtl dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95156

            Bug ID: 95156
           Summary: -Wtautological-compare warns in C but not C++
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: simon.marchi at polymtl dot ca
  Target Milestone: ---

Any reason why this code:

volatile unsigned short insn;

int main()
{
  if ((insn & 0xffb0) == 0xe950) {
      return 1;
  } else {
      return 0;
  }
}

warns when compiled with gcc:

$ gcc-10 test.c -c -Wall
test.c: In function ‘main’:
test.c:5:23: warning: bitwise comparison always evaluates to false
[-Wtautological-compare]
    5 |   if ((insn & 0xffb0) == 0xe950) {
      |  

but not when compiled with g++:

$ g++-10 test.c -c -Wall
<nothing>

?

[Bug c++/95156] -Wtautological-compare warns in C but not C++

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95156

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |11.0
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |10.1.0, 11.0, 9.2.0
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2020-05-15

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
In C mode the warning function, warn_tautological_bitwise_comparison (const
op_location_t &loc, tree_code code, tree lhs, tree rhs), gets a BIT_AND_EXPR
while in C++ mode it's passed a NOP_EXPR that casts the result of the same
BIT_AND_EXPR as in C to int.  The cast seems pointless since the type of the
BIT_AND_EXPR already is int.

I couldn't find a version where G++ did issue the expected warning so it's not
a regression.

(As an aside, the volatile qualifier isn't necessary to reproduce the bug.)