[LLVMdev] Proposing a new 'alloca' parameter attribute to implement the Microsoft C++ ABI

[Chris Lattner]

This thread is odd to me. It seems that the gist of your guys' argument is that you don't know if we will ever get full support, therefore we don't welcome progress towards that (very useful) goal/feature.

If the specific proposal doesn't make make sense from a design standpoint, that's one thing, but saying we shouldn't take it because of licensing issues with MFC or because it is harmful to be partially (but not fully) compatible with MSVC seems just weird to me.

-Chris

On Jul 31, 2013, at 9:04 AM, Stephen Lin <swlin at post.harvard.edu> wrote:

>> Quite the contrary, knowing that Clang's C++ ABI is completely
>> incompatible with MS is a maintenance *simplification*.
> 
> Yes, for example, as explained by Bjarne once, incompatible name
> mangling schemes for ABIs that are not guaranteed to be 100% binary
> compatible is a _feature_, not a bug, since it prevents anyone from
> even beginning to develop a workflow that relies upon linking possibly
> incompatible binaries.
> 
>> 
>> Saying that supporting the MS C++ ABI is an uphill battle is an
>> understatement (and better say MS C++ ABI*S*, because it evolved over
>> time and it is known that it will change on future releases.) As far as
>> I'm concerned, I'll never base my decisions on compiler usage on the
>> advertisement of MS compatibility by Clang++, becase I *know* that for a
>> very long time (maybe forever) whatever MS C++ libraries that work with
>> Clang++ is by luck. That's what happens when you try to implement
>> compatibility with an undocumented, propietary, complex feature.
> 
> This is my point as well; it just seems odd to spend so much effort to
> implement and undocumented ABI with no guarantee of support or
> stability (outside of MSVC major version releases). Furthermore, I
> feel that, by reinforcing the notion that use of the MSVC++ ABI is
> required for development on Windows (when no system-provided libraries
> or ABIs require it), Clang developers are hindering the adoption of
> Clang on Windows rather than promoting it, since it is unlikely that
> any party that is not personally invested in Clang development would
> be willing to depend on a backward engineered implementation of a
> "required" but undocumented ABI for production use. Furthermore, it
> means that Clang will always be behind the curve on Windows, since,
> even if MSVC++ ABI support is fully usable one day, no one will be
> willing to link it with object files from a new major version of MSVC
> without some critical mass of users being guinea pigs first and
> ensuring that there are no major bugaboos.
> 
> I agree that there is value in supporting implementing full MSVC++
> ABI, if it can be done, but it seems like that support can never be
> 100% complete (or, more to the point, known to be 100% complete)
> unless Microsoft itself decides to officially support the
> implementation or completely stabilize and document their C++ ABIs.
> However, I personally think that it is not only easier, there is more
> value in implementing a C++ ABI which is compatible with officially
> supported C and COM MSVC++ ABI subsets and consciously _not_
> compatible in others way (in particular, which does not even attempt
> to link C++ symbols with MSVC++, since that is not required for COM).
> This is something that can be made to work and guaranteed (barring
> some seriously misguided behavior from Microsoft) to continue to work
> stably.
> 
> Furthermore, by providing and controlling its own Windows C++ ABI:
> Clang can possibly offer something that MSVC *cannot* (and does not
> even try to do): a development platform and ecosystem which provides a
> stable-over-time C++ ABI and consistent cross-module usage of RTTI,
> STL, C++ memory management, etc. (which is dicey even within MSVC
> major versions unless the exact same build settings are used, last
> time I checked.) This would give someone an active reason to switch to
> Clang more than anything else Clang currently offers on Windows.
> 
> Stephen
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