Re: [Qemu-devel] [PATCH v2 23/52] audio: remove audio_MIN, audio_MAX

[Kővágó Zoltán]

On 2018-12-25 11:40, Philippe Mathieu-Daudé wrote:
> On 12/24/18 9:48 PM, Kővágó Zoltán wrote:
>> On 2018-12-24 18:16, Philippe Mathieu-Daudé wrote:
>>> On 12/24/18 3:16 AM, Zoltán Kővágó wrote:
>>>> Hi Phil,
>>>>
>>>> On 2018-12-24 00:49, Philippe Mathieu-Daudé wrote:
>>>>> Hi Zoltán,
>>>>>
>>>>> On 12/23/18 9:51 PM, Kővágó, Zoltán wrote:
>>>>>> There's already a MIN and MAX macro in include/qemu/osdep.h, use them
>>>>>> instead.
>>>>>>
>>>>>> Signed-off-by: Kővágó, Zoltán <address@hidden>
>>>>>>
>>>>>> ---
>>>>>>
>>>>>> Changes from v1:
>>>>>> * removed audio_MIN, audio_MAX macros
>>>>>> ---
>>>>> [...]>
>>>>>> diff --git a/audio/audio.h b/audio/audio.h
>>>>>> index ccfef9e10a..bcbe56d639 100644
>>>>>> --- a/audio/audio.h
>>>>>> +++ b/audio/audio.h
>>>>>> @@ -148,23 +148,6 @@ static inline void *advance (void *p, int incr)
>>>>>>      return (d + incr);
>>>>>>  }
>>>>>>  
>>>>>> -#ifdef __GNUC__
>>>>>> -#define audio_MIN(a, b) ( __extension__ ({      \
>>>>>> -    __typeof (a) ta = a;                        \
>>>>>> -    __typeof (b) tb = b;                        \
>>>>>> -    ((ta)>(tb)?(tb):(ta));                      \
>>>>>> -}))
>>>>>> -
>>>>>> -#define audio_MAX(a, b) ( __extension__ ({      \
>>>>>> -    __typeof (a) ta = a;                        \
>>>>>> -    __typeof (b) tb = b;                        \
>>>>>> -    ((ta)<(tb)?(tb):(ta));                      \
>>>>>> -}))
>>>>>> -#else
>>>>>> -#define audio_MIN(a, b) ((a)>(b)?(b):(a))
>>>>>> -#define audio_MAX(a, b) ((a)<(b)?(b):(a))
>>>>>> -#endif
>>>>>> -
>>>>>
>>>>> Those MIN/MAX are smarter than the ones in "qemu/osdep.h", I'd keep them
>>>>> moving them there.
>>>>
>>>> Yes, but only when using gcc (or clang when it emulates gcc).
>>>> Unfortunately, they work differently when one of the expressions has
>>>> side effects, which is a disaster waiting to happen (when some poor folk
>>>> finally tries to compile it with a non-gcc compiler).
>>>
>>> We already use the typeof extension:
>>>
>>> $ git grep typeof|wc -l
>>> 145
>>>
>>> For MIN/MAX I'd use rather use the __auto_type extension.
>>>
>>>> Or do we support any compilers beside gcc and clang? Because if not,
>>>> sure, do it, just remove the #ifdef and keep only the gcc version.
>>>
>>> Yes, this is checked by the ./configure script:
>>>
>>> 1850 # Check whether the compiler matches our minimum requirements:
>>> ...
>>> 1859 #   error You need at least Clang v3.4 to compile QEMU
>>> ...
>>> 1864 #  error You need at least GCC v4.8 to compile QEMU
>>> ...
>>> 1867 # error You either need GCC or Clang to compiler QEMU
>>
>> Fair enough. I've tried to check the documentation for supported
>> compilers, but I haven't found any info. Well, I didn't look at the
>> configure script, I admit that.
> 
> You haven't found any info because the minimum requirements are not well
> documented on the wiki.
> 
>>
>> This is what I came up with:
>>
>> #ifndef MIN
>> #define MIN(a, b) ( __extension__ ({      \
>>     __auto_type _a = (a);                 \
>>     __auto_type _b = (b);                 \
>>     _a > _b ? _b : _a;                    \
>> }))
>> #endif
>> #ifndef MAX
>> #define MAX(a, b) ( __extension__ ({      \
>>     __auto_type _a = (a);                 \
>>     __auto_type _b = (b);                 \
>>     _a < _b ? _b : _a;                    \
>> }))
>> #endif
>>
>>
>> I changed it a bit, since a and b are the macro parameters, so they are
>> the variables that we should put into parentheses, and I renamed ta/tb
>> to _a/_b, this way they're less likely to clobber some other variables.
> 
> Sounds good. Now the parentheses around a/b are not necessary.
> 
> I wonder however if it wouldn't be cleaner to previously add:
> 
> #ifdef MIN
> # undef MIN
> #endif
> 
> and for MAX, rather than only use these macros if undefs.

Um, this broke a few things. It also means until now in some cases it
used some random definition of MIN/MAX instead of the one in the header.

/home/dirty_ice/qemu/include/qemu/osdep.h:240:5: error: ‘__auto_type’
used with a bit-field initializer
/home/dirty_ice/qemu/include/qemu/osdep.h:247:35: error: braced-group
within expression allowed only inside a function

The first one is fixable with an explicit cast (ugly but works), but the
second one is more problematic. It means we can't write stuff like

USBPort  uports[MAX(MAXPORTS_2, MAXPORTS_3)];

when not in a function. So we either need a dumb version of MIN/MAX, or
scrape the idea altogether.

> 
>> To be honest, we could probably drop the __extension__ keyword too,
>> we're not compiling with -pedantic. Other than audio_MIN and audio_MAX,
>> it only appears in the DO_UPCAST macro.
> 
> Fine with me!
> 
> If you have this patch ready, you can send it alone out of this series,
> no need to resend this whole series for this patch.

There will be adjustmets to other patches too, so I think I'll just
consolidate them.

Regards,
Zoltan