[edk2-devel] [PATCH v1 0/5] EDK2 Code First: PI Specification: Update EFI_MM_COMMUNICATE_HEADER

[Marvin Häuser]

On 22.06.21 17:34, Laszlo Ersek wrote:
> On 06/18/21 11:37, Marvin Häuser wrote:
>> On 16.06.21 22:58, Kun Qin wrote:
>>> On 06/16/2021 00:02, Marvin Häuser wrote:
>>>> 2) Is it feasible yet with the current set of supported compilers to
>>>> support flexible arrays?
>>> My impression is that flexible arrays are already supported (as seen
>>> in UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h).
>>> Please correct me if I am wrong.
>>>
>>> Would you mind letting me know why this is applicable here? We are
>>> trying to seek ideas on how to catch developer mistakes caused by this
>>> change. So any input is appreciated.
>> Huh, interesting. Last time I tried I was told about incompatibilities
>> with MSVC, but I know some have been dropped since then (2005 and 2008
>> if I recall correctly?), so that'd be great to allow globally.
> I too am surprised to see
> "UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h". The
> flexible array member is a C99 feature, and I didn't even know that we
> disallowed it for the sake of particular VS toolchains -- I thought we
> had a more general reason than just "not supported by VS versions X and Y".
>
> The behavior of OFFSET_OF() would be interesting -- the OFFSET_OF()
> macro definition for non-gcc / non-clang:
>
> #define OFFSET_OF(TYPE, Field) ((UINTN) &(((TYPE *)0)->Field))
>
> borders on undefined behavior as far as I can tell, so its behavior is
> totally up to the compiler. It works thus far okay on Visual Studio, but
> I couldn't say if it extended correctly to flexible array members.

Yes, it's UB by the standard, but this is actually how MS implements 
them (or used to anyway?). I don't see why it'd cause issues with 
flexible arrays, as only the start of the array is relevant (which is 
constant for all instances of the structure no matter the amount of 
elements actually stored). Any specific concern? If so, they could be 
addressed by appropriate STATIC_ASSERTs.

Best regards,
Marvin

>
> Thanks
> Laszlo
>
>> I feel
>> like if the structure is modified anyway, it should probably get a
>> trailing flexible array over the 1-sized hack. What do you think?



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