[Qemu-devel] Re: [PATCH 1/5] Suppress some gcc warnings with -Wtype-limits

[andrzej zaborowski]

On 5 September 2010 11:44, Blue Swirl <address@hidden> wrote:
> On Sun, Sep 5, 2010 at 9:26 AM, Michael S. Tsirkin <address@hidden> wrote:
>> On Sun, Sep 05, 2010 at 09:06:10AM +0000, Blue Swirl wrote:
>>> On Sun, Sep 5, 2010 at 7:54 AM, Michael S. Tsirkin <address@hidden> wrote:
>>> > On Sat, Sep 04, 2010 at 05:21:24PM +0000, Blue Swirl wrote:
>>> >> In the unsigned number space, the checks can be merged into one,
>>> >> assuming that BLKDBG_EVEN_MAX is less than INT_MAX. Alternatively we
>>> >> could have:
>>> >>  -    if (event < 0 || event >= BLKDBG_EVENT_MAX) {
>>> >>  +    if ((int)event < 0 || event >= BLKDBG_EVENT_MAX) {
>>> >>
>>> >> This would also implement the check that the writer of this code was
>>> >> trying to make.
>>> >> The important thing to note is however is that the check as it is now
>>> >> is not correct.

I agree, assuming that an enum can reach 0x80000000 different values,
perhaps the current code is not ideal.  Still I think calling it
"wrong" is wrong, and calling your patch a "fix" is wrong. (Same as
calling patches that remove a warning a "fix", they are workarounds)

>>> >
>>> > I agree. But it seems to indicate a bigger problem.
>>> >
>>> > If we are trying to pass in a negative value, which is not one
>>> > of enum values, using BlkDebugEvent as type is just confusing,
>>> > we should just pass int instead.
>>>
>>> AFAICT it's only possible to use the values listed in event_names in
>>> blkdebug.c, other values are rejected. So the check should actually be
>>> an assert() or it could even be removed.
>>
>> Sounds good.
>>
>>> >> >> How about adding assert(OMAP_EMIFS_BASE == 0) and commenting out the
>>> >> >> check? Then if the value changes, the need to add the comparison back
>>> >> >> will be obvious.
>>> >> >
>>> >> > This would work but it's weird.  The thing is it's currently a correct
>>> >> > code and the check may be useless but it's the optimiser's task to
>>> >> > remove it, not ours.  The compiler is not able to tell whether the
>>> >> > check makes sense or nott, because the compiler only has access to
>>> >> > preprocessed code.  So why should you let the compiler have anything
>>> >> > to say on it.
>>> >>
>>> >> Good point. I'll try to invent something better.
>>> >
>>> > Use #pragma to supress the warning? Maybe we could wrap this in a macro ..
>>>
>>> Those lines may also desynch silently with changes to OMAP_EMIFS_BASE.
>>>
>>> I think the assertion is still the best way, it ensures that something
>>> will happen if OMAP_EMIFS_BASE changes. We could for example remove
>>> OMAP_EMIFS_BASE entirely (it's only used for the check), but someone
>>> adding a new define could still forget to adjust the check anyway.
>>
>> We could replace it with a macro
>> #define OMAP_EMIFS_VALID(addr) ((target_phys_addr_t)addr < OMAP_EMIFF_BASE)
>> but all this does look artificial. And of course using type casts
>> is always scary ...
>>
>> Would it help to have some inline functions that do the range checking 
>> correctly?
>> We have a couple of range helpers in pci.h, these could be moved out
>> to range.h and we could add some more. As there act on u64 this will get
>> the type limits mostly automatically right.
>
> That seems to be the best solution, I get no warnings with this:

While the resulting code is clean (just as the current code), I think
it really shows that this warning should not be enabled.  At this
point you find yourself working around your compiler and potentially
forcing other write some really strange code to work around the
problem caused by this.

There are many warnings that should not be enabled by default for this
reason (like the uninitialised variable warning) unless they are fixed
to be really intelligent (which is unlikely in this case).

Cheers