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Re: [Xen-devel] [RFC PATCH 0/4] Add missing default labels to switch statements

On 2/23/19 12:08 AM, Julien Grall wrote:

On 22/02/2019 21:00, Stefano Stabellini wrote:
On Fri, 22 Feb 2019, Julien Grall wrote:
BTW, I checked the series with -Wswitch-default:
Warn whenever a switch statement does not have a default case.
Furthermore, using BUG() is a pretty bad idea in switch.
It is and not only in the switch. The reason I put BUG is that I tried
to follow
the existing "error handling" at those places.
It is not because BUG() is been used today in some places that we need to
continue to spread it.

Use of BUG() itself is another topic which will also need to be
So we should not add more of them...
Again, I see this as a dedicated change. So, in the current series I think
it is
acceptable to use the existing way of error handling if any at all.
That's not how it works in upstream. If you know some constructs are wrong, it
is best to try to address partially the problem directly then having so you
reduce the amounts of change afterwards.

So please try to not introduce more BUG() in the code base.
Hi Oleksandr, Julien,

Julien's right that we should not introduce any more BUG()s. In fact,
each of them makes the code less safe, not more safe! The purpose of
MISRAC 16.4 is "defensive programming": write the code in a way that is
more (not less!) resilient to failure.

So, I think it is a good idea to introduce a default label because it
can help us spot unexpected issues. Instead of calling BUG() in the
default handler, which is detrimental, we should return an error when
possible, or just print a warning.
I looked at the first patch and to be honest I can't see how this hence 
our code...

As 16.4 clearly state, even a simple comment would be enough to address
the rule. We just need to explain why a default label is not needed.
Such as:

   /* unreachable because blah and blah */
... as we already have defensive code. Indeed, in most of the switch we 
deal with potential issue by initializing the variable before the 
switch. If you look at the first patch, a lot of "default: break;" is 
introduced. So what's our benefits? How this code is more defensive than 
what we currently have?
...and if you remove those initialized variables and forget
to update the switch with default? Or change the initial value
the way your previous switch assumptions do not work anymore?
I am not trying to defend MISRA here, but things change and we all
make mistakes. This is why defensive programming
Furthermore, how this is going to help us (thanks to -Wswitch) if an 
enumerate is extended and we miss a case that the compiler don't notice 
And if the switch's index is not an enumeration, but a plain integer...?

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