Re: [Xen-devel] [PATCH 4/8] tools/xenalyze: Mark unreachable code as unreachable

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2016-02-25 at 15:09 +0000, George Dunlap wrote:
> On 25/02/16 15:03, Ian Campbell wrote:
> > On Thu, 2016-02-25 at 14:48 +0000, George Dunlap wrote:
> > > ...so that coverity knows it's unreachable.
> > 
> > I would not be surprised if Coverity starts complaining about the dead
> > code
> > once this is in place. fprintf + abort is probably what would be wanted
> > to
> > placate it in this case.
> 
> Hrm -- it would be nice to have a way to figure out what coverity likes
> without having to actually check something into the tree...

If this code is truly unreachable (i.e. it is after a while(1) with no
breaks etc) then you should just drop the logging since it will never be
reached, then the __builtin_unreachable() is appropriate.

If, as the log message implies, this is code which _should_ be unreachable
by design but would be reached in the case of a logic error in the
preceding code then what you want is either fprintf()+abort() or maybe
assert().

But Coverity seems to have disproven this possibility, correctly AFAICT
because all of the preceeding cases of the if chain end with a goto, this
removing the logging and leaving the __builtin_unreachable() is the way to
go.

I don't think this is really about what would keep Coverity happy, more to
do with the intended semantics of execution reaching this point.

BTW in my simple test case actually trying to execute
__builtin_unreachable() results in a SEGV, so that logging really isn't
doing anything useful with your patch.

Ian.

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