[Qemu-devel] Re: [PATCH 00/25] VMState cleanups and conversion of network drivers

[Juan Quintela]

Jamie Lokier <address@hidden> wrote:
> Juan Quintela wrote:
>>  * add VARRAY_UINT16_UNSAFE: unsafe here means that type checking is off
>>    (a.k.a. as a cast in C).  In this case the problem is that the last
>>    element of one struct is int foo[0], and we allocate the right size
>>    for the array we want.  Problem?  I haven't been able to abuse^Wuse
>>    gcc + cpp + magic to typecheck that for vmstate:
>> 
>>    We have
>>    struct FOO {
>>        int32_t foo[0];
>>    }
>>    We want to "compare the type of foo (t1) with int32_t (t2)
>> 
>>         ((t1(*)[n])0 - (t2*)0)
>>    This one don't work, because we don't have 'n'
>>         ((t1(**))0 - (t2*)0)
>>    This don't work either because t1 is one array.
>>         ((t1(*)[])0 - (t2*)0)
>>    Too clever, imposible cast to on array type.
>>    I tried some other variants, but have not able to get one that compiles.
>
> Since you mention GCC, is it ok to use GCC extensions?

I would bet that qemu don't compile with anyother compiler without big
changes :)

>  __typeof__(t1)
> often does the trick for this sort of thing where t1 alone does not
> compile, even if t1 is a type.
>
> __builtin_types_compatible_p(), __builtin_choose_exper and
> __attribute__((__error__)) are good for informative error messages.

Thanks, will take a look.

I have good error messages, I don't have good ideas about how to trick
the compiler.  Error messages are clear.

Example:

struct FOO {
       unt16_t foo[0];
}
#define vmstate_offset_pointer(_state, _field, _type)                \
    (offsetof(_state, _field) +                                      \
     type_check_pointer(_type, typeof_field(_state, _field)))

Called with (after doing substitutions)
type_check_pointer(uint16_t, struct FOO, foo);

Give that, we try several definitions for type_check_pointer:

#define type_check_pointer(t1,t2) ((t1**)0 - (t2*)0)

Gives the following error:

/scratch/qemu/hw/eeprom93xx.c:144: error: invalid operands to binary -
                      (have ‘uint16_t **’ and ‘uint16_t (*)[]’)

Another try:

#define type_check_pointer(t1,t2) ((t1(*)[])0 - (t2*)0)

gives

/scratch/qemu/hw/eeprom93xx.c:148: error: arithmetic on pointer to an 
incomplete type

Another one:

#define type_check_pointer(t1,t2) ((t1(*)[0])0 - (t2*)0)

gives:

/scratch/qemu/hw/eeprom93xx.c:151: error: initializer element is not constant
/scratch/qemu/hw/eeprom93xx.c:151: error: (near initialization for 
‘vmstate_eeprom’)

And at this point, I got confused :(

After reading the info of __builtin_types_compatible_p(), it could
perhas be useful.  Thinking and testing a bit more about it.

Later, Juan.