Re: Add "-Wimplicit-fallthrough" to default flags

At Tue, 12 May 2020 17:12:51 -0400, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote in
> Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> > Fixed one straggler in contrib, and while testing it I realized why
> > ccache doesn't pay attention to the changes I was doing in the file:
> > ccache compares the *preprocessed* version of the file and only if that
> > differs from the version that was cached last, ccache sends the new one
> > to the compiler; and of course these comments are not present in the
> > preprocessed version, so changing only the comment accomplishes nothing.
> > You have to touch one byte outside of any comments.
>
> Ugh. So the only way ccache could avoid this is to drop the
> preprocessed-file comparison check if -Wimplicit-fallthrough is on.
> Doesn't really sound like something we'd want to ask them to do.
>
> > I bet this is going to bite someone ... maybe we'd be better off going
> > all the way to -Wimplicit-fallthrough=5 and use the
> > __attribute__((fallthrough)) stuff instead.
>
> I'm not really in favor of the __attribute__ solution --- seems too
> gcc-specific. FALLTHROUGH-type comments are understood by other
> sorts of tools besides gcc.
>
> In practice, it doesn't seem like this'll be a huge problem once
> we're past the initial fixup stage. We can revisit it later if
> that prediction proves wrong, of course.

FWIW, I got a warning for jsonpath_gram.c.

> jsonpath_gram.c:1026:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
> if (*++yyp != '\\')
> ^
> jsonpath_gram.c:1029:11: note: here
> default:
> ^~~~~~~

jsonpath_gram.c:1025
> case '\\':
> if (*++yyp != '\\')
> goto do_not_strip_quotes;
> /* Fall through. */
> default:

It is generated code by bison.

$ bison --version
bison (GNU Bison) 3.0.4

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center