On Thu, 25 Nov 2021, Richard Biener wrote:
> On Wed, 24 Nov 2021, Jason Merrill wrote:
>
> > On 11/24/21 11:15, Marek Polacek wrote:
> > > On Wed, Nov 24, 2021 at 04:21:31PM +0100, Richard Biener via Gcc-patches
> > > wrote:
> > >> This resurrects -Wunreachable-code and implements a warning for
> > >> trivially unreachable code as of CFG construction. Most problematic
> > >> with this is the C/C++ frontend added 'return 0;' stmt in main
> > >> which the patch handles for C++ like the C frontend already does
> > >> by using BUILTINS_LOCATION.
> > >>
> > >> Another problem for future enhancement is that after CFG construction
> > >> we no longer can point to the stmt making a stmt unreachable, so
> > >> this implementation tries to warn on the first unreachable
> > >> statement of a region. It might be possible to retain a pointer
> > >> to the stmt that triggered creation of a basic-block but I'm not
> > >> sure how reliable that would be.
> > >>
> > >> So this is really a simple attempt for now, triggered by myself
> > >> running into such a coding error. As always, the perfect is the
> > >> enemy of the good.
> > >>
> > >> It does not pass bootstrap (which enables -Wextra), because of the
> > >> situation in g++.dg/Wunreachable-code-5.C where the C++ frontend
> > >> prematurely elides conditions like if (! GATHER_STATISTICS) that
> > >> evaluate to true - oddly enough it does _not_ do this for
> > >> conditions evaluating to false ... (one of the
> > >> c-c++-common/Wunreachable-code-2.c cases).
> > >
> > > I've taken a look into the C++ thing. This is genericize_if_stmt:
> > > if we have
> > >
> > > if (0)
> > > return;
> > >
> > > then cond is integer_zerop, then_ is a return_expr, but since it has
> > > TREE_SIDE_EFFECTS, we create a COND_EXPR. For
> > >
> > > if (!0)
> > > return;
> > >
> > > we do
> > > 170 else if (integer_nonzerop (cond) && !TREE_SIDE_EFFECTS (else_))
> > > 171 stmt = then_;
> > > which elides the if completely.
> > >
> > > So it seems it would help if we avoided eliding the if stmt if
> > > -Wunreachable-code is in effect. I'd be happy to make that change,
> > > if it sounds sane.
>
> Yes, that seems to work.
>
> > Sure.
> >
> > Currently the front end does various constant folding as part of
> > genericization, as I recall because there were missed optimizations without
> > it. Is this particular one undesirable because it's at the statement level
> > rather than within an expression?
>
> It's undesirable because it short-circuits control flow and thus
>
> if (0)
> return;
> foo ();
>
> becomes
>
> return;
> foo ();
>
> which looks exactly like a case we want to diagnose (very likely a
> programming error).
>
> So yes, it applies to the statement level and there only to control
> statements.
So another case in GCC is
if (WORDS_BIG_ENDIAN == BYTES_BIG_ENDIAN)
...
else
{
/* Assert that we're only dealing with the PDP11 case. */
gcc_assert (!BYTES_BIG_ENDIAN);
gcc_assert (WORDS_BIG_ENDIAN);
cpp_define (pfile, "__BYTE_ORDER__=__ORDER_PDP_ENDIAN__");
where that macro expands to
((void)(!(!0) ? fancy_abort
("/home/rguenther/src/trunk/gcc/cppbuiltin.c", 180, __FUNCTION__), 0 :
0));
((void)(!(0) ? fancy_abort
("/home/rguenther/src/trunk/gcc/cppbuiltin.c", 181, __FUNCTION__), 0 :
0));
cpp_define (pfile, "__BYTE_ORDER__=__ORDER_PDP_ENDIAN__");
and the frontend elides the COND_EXPRs making the cpp_define
unreachable. That's only exposed because we no longer elide the
if (1) guardingt this else path ...
Also this is a case where we definitely do not want to
diagnose that either the else or the true path is statically
unreachable.
Richard.
