[Bug c/88895] unnecessary warnings in unreachable code (shift)

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88895

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-01-17
                 CC|                            |msebor at gcc dot gnu.org
          Component|c++                         |c
     Ever confirmed|0                           |1

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  The warning is not specific to C++ but triggers for C code as well
(see the example below).  The reason for the false positive is that the warning
is implemented in  the front-ends and doesn't benefit from any control flow
analysis, and so it triggers before the switch statement has been optimized and
the unreachable labels eliminated.  To avoid it the warning would have to be
deferred until quite a bit later.

$ cat u.c && gcc -S -Wall -Wextra -Wpedantic u.c
static inline int f (int i)
{
  switch (sizeof i)
  {
    case 8: return i >> 56;
    case 4: return i >> 24;
    case 2: return i >> 8;
    case 1: return i;
  }
}

int g (int i)
{
  return f (i);
}
u.c: In function ‘f’:
u.c:5:22: warning: right shift count >= width of type [-Wshift-count-overflow]
    5 |     case 8: return i >> 56;
      |                      ^~