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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug other/94165] Wrong "warning: this statement may fall through"
 after __builtin_unreachable().
Date: Fri, 13 Mar 2020 12:16:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94165

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This warning is an early warning (during gimplification) and can handle only
some constructs that clearly can't fall thru.
E.g. we do warn for:
extern void foo (const char *) __attribute__((noreturn));

int
bar (int x)
{
  switch (x)
    {
    default:
      foo ("bug");
      x++;
    case 1:
      return x + 2;
    }
}
but if you take the unreachable code after the noreturn call, we won't warn.
The expression statement is similar, there is code after the noreturn call.
Finding noreturn stuff in the middle of sequences would be difficult and
expensive.  Only the cfg pass is then able to figure out that some code is
unreachable.=