On Mon, May 23, 2016 at 05:53:38PM -0600, Martin Sebor wrote:
> Sorry I'm a little late with my comments but I noticed one minor
> problem (I raised bug 71249 for it since the patch has already
> been checked in), and have a question about the hunk below:
Firstly, thanks for looking into this.
> > @@ -1595,6 +1595,32 @@ gimplify_switch_expr (tree *expr_p, gimple_seq
> > *pre_p)
> > gimplify_ctxp->case_labels.create (8);
> >
> > gimplify_stmt (&SWITCH_BODY (switch_expr), &switch_body_seq);
> > +
> > + /* Possibly warn about unreachable statements between switch's
> > + controlling expression and the first case. */
> > + if (warn_switch_unreachable
> > + /* This warning doesn't play well with Fortran when optimizations
> > + are on. */
> > + && !lang_GNU_Fortran ()
> > + && switch_body_seq != NULL)
> > + {
> > + gimple_seq seq = switch_body_seq;
> > + if (gimple_code (switch_body_seq) == GIMPLE_BIND)
> > + seq = gimple_bind_body (as_a <gbind *> (switch_body_seq));
> > + gimple *stmt = gimple_seq_first_stmt (seq);
> > + enum gimple_code code = gimple_code (stmt);
> > + if (code != GIMPLE_LABEL && code != GIMPLE_TRY)
>
> Why exempt GIMPLE_TRY? It suppresses the warning in cases like:
>
> switch (i) {
> try { } catch (...) { }
> case 1: ;
> }
>
> (If excluding GIMPLE_TRY is unavoidable, it might be worthwhile
> to add a comment to the code, and perhaps also mention it in
> the documentation to preempt bug reports by nitpickers like me ;)
I think I added that so that we do not warn on
switch (i)
{
int A[3];
default:
break;
}
because at the gimple level that looks like
{
int A[3];
try
{
<D.1751>:
goto <D.1752>;
}
finally
{
A = {CLOBBER};
}
}
Another problem with try/finally is that it doesn't have a location
so we'd jsut print useless
cc1: warning: statement will never be executed [-Wswitch-unreachable]
Though it seems so improbable that I don't really care about this case.
> Finally, while even this simple warning can be useful, it would
> be even more helpful if it could also point out other unreachable
> statements within the body of the switch statements after
> a break/goto/return and before a subsequent label. This could
> be especially valuable with optimization to make possible
> diagnosing non-trivial problems like this:
>
> switch (i) {
> case 3:
> if (i < 3)
> return 1;
> i = 8;
> }
>
> (I realize this might be outside the scope of the feature request
> and starting to creep into the -Wunreachable-code territory.)
This really sounds like the old -Wunreachable stuff and I don't think
it's limited to switches as this warning is. Nowadays we have stuff
like gimple_stmt_may_fallthru so maybe that could be useful, but I'm
not about to plunge into this mess anytime soon ;).
Marek
