[Mesa-dev] [PATCH] nir: Make C++ more happy with NIR_SRC_INIT and NIR_DEST_INIT

[Francisco Jerez]

Jason Ekstrand <jason at jlekstrand.net> writes:

> On Fri, Jun 26, 2015 at 3:03 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>
>>> On Fri, Jun 26, 2015 at 12:08 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>>>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>>>
>>>>> In C, if you partially initialize a structure, the rest of the struct gets
>>>>> set to 0.  C++, however, does not have this rule so GCC throws warnings
>>>>> whenver NIR_SRC_INIT or NIR_DEST_INIT is used in C++.
>>>>
>>>> I don't think that's right, in C++ initializers missing from an
>>>> aggregate initializer list are also defined to be initialized
>>>> (value-initialized to be more precise, what would set them to zero in
>>>> this case just like in C).
>>>
>>> Yes, that is correct.  I just did a second attempt that, instead,
>>> defines a static const variable named NIR_SRC_INIT with a partial
>>> initializer.  C++ still gets grumpy and gives me a pile of "missing
>>> initializer" warnings.
>>>
>> That's likely related to the warning flags you have enabled in CXXFLAGS,
>> not to C++ itself.  Maybe you have -Wmissing-field-initializers enabled
>> for C++ only?
>>
>>>>> Since nir.h contains a static inline that uses NIR_SRC_INIT, every C++
>>>>> file that includes nir.h complains about this.
>>>>>
>>>> I suspect the reason why this causes a warning may be that you're using
>>>> compound literals? (which are a C99-specific feature and not part of C++)
>>>>
>>>>> This patch adds a small static inline function that makes a struct,
>>>>> memsets it to 0, and returns it.  NIR_SRC_INIT and NIR_DEST_INIT are then
>>>>> wrappers around this function.
>>>>
>>>> In C++ you could just call the implicitly defined default constructor
>>>> for nir_src or nir_dest, like 'nir_src()'.
>>>
>>> The implicitly defined default constructor does nothing to POD types,
>>> so doing so would explicitly *not* perform the desired action of
>>> zeroing out the data.
>>>
>>
>> Indeed, but 'nir_src()' doesn't only call the implicitly-defined trivial
>> default constructor, it value-initializes the object (See section 8.5/8
>> of the C++14 spec) what for POD types causes all members to be
>> zero-initialized.
>
> It looks like this greatly depends on your C++ version.  If it's C++11
> or above, I believe it does get zero-initialized.  If it's earlier
> than C++11, it doesn't.  At least that's the way I read this:
>
> http://en.cppreference.com/w/cpp/language/value_initialization

Not really, it will get zero-initialized back to C++98.  AFAICT what the
article is trying to say is that in C++98 what is now referred to as
value-initialization used to be called default-initialization in the
spec, but still it had the effect of zero-initializing the structure.
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