Re: [Qemu-trivial] [Qemu-devel] [PATCH] Fix compiler warning (always return a value)

[Markus Armbruster <armbru@redhat.com>]

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Fri, Oct 28, 2011 at 8:40 AM, Markus Armbruster <armbru@redhat.com> wrote:
>> Stefan Hajnoczi <stefanha@gmail.com> writes:
>>
>>> On Mon, Oct 24, 2011 at 10:18:43PM +0200, Stefan Weil wrote:
>>>> For compilations with -DNDEBUG, the default case did not return
>>>> a value which caused a compiler warning.
>>>>
>>>> Signed-off-by: Stefan Weil <sw@weilnetz.de>
>>>> ---
>>>>  hw/ppce500_spin.c |   11 ++++++++---
>>>>  1 files changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/hw/ppce500_spin.c b/hw/ppce500_spin.c
>>>> index cccd940..5b5ffe0 100644
>>>> --- a/hw/ppce500_spin.c
>>>> +++ b/hw/ppce500_spin.c
>>>> @@ -168,17 +168,22 @@ static uint64_t spin_read(void *opaque, target_phys_addr_t addr, unsigned len)
>>>>  {
>>>>      SpinState *s = opaque;
>>>>      uint8_t *spin_p = &((uint8_t*)s->spin)[addr];
>>>> +    uint64_t result = 0;
>>>>
>>>>      switch (len) {
>>>>      case 1:
>>>> -        return ldub_p(spin_p);
>>>> +        result = ldub_p(spin_p);
>>>> +        break;
>>>>      case 2:
>>>> -        return lduw_p(spin_p);
>>>> +        result = lduw_p(spin_p);
>>>> +        break;
>>>>      case 4:
>>>> -        return ldl_p(spin_p);
>>>> +        result = ldl_p(spin_p);
>>>> +        break;
>>>>      default:
>>>>          assert(0);
>>>
>>> I would replace assert(3) with abort(3).  If this ever happens the
>>> program is broken - returning 0 instead of an undefined value doesn't
>>> help.
>>
>> Why is it useful to make failed assertions stop the program regardless
>> of NDEBUG only when the assertion happens to be "can't reach"?
>
> My point is that it should not be an assertion.  The program has a
> control flow path which should never be taken.  In any "safe"
> programming environment the program will terminate with a diagnostic.

In the not-so-safe C programming environment, we can disable that safety
check by defining NDEBUG.

Disabling safety checks is almost always a bad idea.

> That's exactly what we need to do here too.  assert(3) is the wrong
> tool for this; we're not even asserting anything.

We do, actually: "can't reach".  That's as good an assertion as any.