Re: [PATCH 000/141] Fix fall-through warnings for Clang

Edward Cree
2020-11-25T22:44:59Z

On 25/11/2020 00:32, Miguel Ojeda wrote:
> I have said *authoring* lines of *this* kind takes a minute per line.
> Specifically: lines fixing the fallthrough warning mechanically and
> repeatedly where the compiler tells you to, and doing so full-time for
> a month.
<snip>
> It is useful since it makes intent clear.
To make the intent clear, you have to first be certain that you
understand the intent; otherwise by adding either a break or a
fallthrough to suppress the warning you are just destroying the
information that "the intent of this code is unknown".
Figuring out the intent of a piece of unfamiliar code takes more
than 1 minute; just because
case foo:
thing;
case bar:
break;
produces identical code to
case foo:
thing;
break;
case bar:
break;
doesn't mean that *either* is correct — maybe the author meant
to write
case foo:
return thing;
case bar:
break;
and by inserting that break you've destroyed the marker that
would direct someone who knew what the code was about to look
at that point in the code and spot the problem.
Thus, you *always* have to look at more than just the immediate
mechanical context of the code, to make a proper judgement that
yes, this was the intent. If you think that that sort of thing
can be done in an *average* time of one minute, then I hope you
stay away from code I'm responsible for!
One minute would be an optimistic target for code that, as the
maintainer, one is already somewhat familiar with. For code
that you're seeing for the first time, as is usually the case
with the people doing these mechanical fix-a-warning patches,
it's completely unrealistic.

A warning is only useful because it makes you *think* about the
code. If you suppress the warning without doing that thinking,
then you made the warning useless; and if the warning made you
think about code that didn't *need* it, then the warning was
useless from the start.

So make your mind up: does Clang's stricter -Wimplicit-fallthrough
flag up code that needs thought (in which case the fixes take
effort both to author and to review) or does it flag up code
that can be mindlessly "fixed" (in which case the warning is
worthless)? Proponents in this thread seem to be trying to
have it both ways.

-ed