llvm dev - Aug 2013 - [LLVMdev] [cfe-dev] MSVC++ ABI compatibility is not a Windows requirement

Chris Lattner <clattner at apple.com> writes:
> 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.
Full disclosure: I'm the guy who filled the PR requesting the feature
the OP's is working on and, when someone tried to close it as WONTFIX, I
strongly opposed. Furthermore, MSVC++ is my Windows compiler and my
projects suffer from unwelcome complexity for working around the lack of
this feature on LLVM.

So I hope it is obvious that I'm not against Reid's proposal, quite the
contrary.

My point is that nowadays Clang++ is not a production-ready Windows
compiler. The missing features are much easier to implement than MS C++
ABI compatibility. For starters, they are well documented. So it
perplexes me to see the flux of work on MS C++ compatibility when:

 * It is an open ended, difficult, uncertain goal. You'll never know for
   sure that it is done.

 * Obviously, if you don't implement the features that every Windows C++
   compiler must have, you'll never be compatible with MS. Currently
   Clang++ cannot create or use C++ dlls (a series of patches were
   submitted but quickly rejected, I hope that Nico was not discouraged
   by this.) OTOH, implementing 32bit SEH is a must-have requirement for
   MS compatibility, but it would benefit Clang on Windows independently
   of the C++ ABI used.

 * For being the best Windows C++ compiler, Clang++ doesn't need to be
   compatible with the MS C++ ABI. However (and this is highly
   detrimental to Clang, IMO) from reading the mailing lists one could
   easily get the impression that until Clang++ gets the MS C++ ABI, it
   is not ready for Windows. It wont surprise me to learn that some
   contributors are working on the MS C++ ABI because they think that
   way.

 * Using its current C++ ABI and debug info format on Windows has the
   advantage of making Clang++ usable together with tools like existing
   debuggers and profilers. For using Clang with Visual Studio's IDE it
   must learn how to emit debug info compatible with MS, and that is
   another can of worms.

 * As the MS C++ ABI is in flux, while at the same time Clang++ is well
   ahead of MS on conformance, some weird cases might arise where
   Clang++ would be forced to extend the ABI on its own at the risk of
   being incompatible with itself after a while (MS variadic templates
   implementation required an ABI change. MS engineers admit that
   planned features might require more changes. Of course, Clang++
   already supports most of those features.)

 * Finally, legal issues remain: supposing that Clang++ gets MS C++ ABI
   compatibility, is it possible to use MS C++ runtime(s) and libraries
   with Clang++? We could end with lots of work invested on a feature
   that only benefits those who are forced to work with third-party C++
   libraries distributed on binary form. I guess that such users are a
   minority.

To recap: lots of work is being invested on a feature which is very hard
to implement, while at the same time Clang lacks basic features. I'll
dare to say that focusing the effort on those features would mean that,
in a year at most, Clang would become the best C++ compiler for Windows.

Needles to say, everyone has the right to decide on what they work. I
fully respect that. But I'll like to know what motivates those who are
working so hard on the MS C++ ABI while Clang++ remains as a compiler
not ready for production on Windows.

On Jul 31, 2013, at 3:13 PM, Óscar Fuentes <ofv at wanadoo.es>
wrote:
> Full disclosure: I'm the guy who filled the PR requesting the feature
> the OP's is working on and, when someone tried to close it as WONTFIX,
I
> strongly opposed. Furthermore, MSVC++ is my Windows compiler and my
> projects suffer from unwelcome complexity for working around the lack of
> this feature on LLVM.
Good to know.
> My point is that nowadays Clang++ is not a production-ready Windows
> compiler. The missing features are much easier to implement than MS C++
> ABI compatibility.  For starters, they are well documented. So it
> perplexes me to see the flux of work on MS C++ compatibility when
Ok, and it is fine for you to share your opinion.  However, as an open source
project, we generally don't have the luxury of telling contributors how they
should spend their time.  Suggesting to Chip (or anyone) that there are
interesting other problems to solve is one thing, but standing in the way of
progress because you think there are higher priorities is another.
> * It is an open ended, difficult, uncertain goal. You'll never know for
>   sure that it is done.
Well sure.  If you rewind 5 years, people were saying the same thing about GCC
compatibility.  :-)
> * Obviously, if you don't implement the features that every Windows C++
>   compiler must have, you'll never be compatible with MS. Currently
>   Clang++ cannot create or use C++ dlls (a series of patches were
>   submitted but quickly rejected, I hope that Nico was not discouraged
>   by this.) OTOH, implementing 32bit SEH is a must-have requirement for
>   MS compatibility, but it would benefit Clang on Windows independently
>   of the C++ ABI used.
Same comment.  Clang still doesn't have openmp or nested function support. 
It is still useful.
> * For being the best Windows C++ compiler, Clang++ doesn't need to be
>   compatible with the MS C++ ABI. However (and this is highly
>   detrimental to Clang, IMO) from reading the mailing lists one could
>   easily get the impression that until Clang++ gets the MS C++ ABI, it
>   is not ready for Windows. It wont surprise me to learn that some
>   contributors are working on the MS C++ ABI because they think that
>   way.
Fair point, but being ABI compatible DOES enable certain applications, for
example, working with existing binaries that can't be recompiled.
> * Using its current C++ ABI and debug info format on Windows has the
>   advantage of making Clang++ usable together with tools like existing
>   debuggers and profilers. For using Clang with Visual Studio's IDE it
>   must learn how to emit debug info compatible with MS, and that is
>   another can of worms.
> * As the MS C++ ABI is in flux, while at the same time Clang++ is well
>   ahead of MS on conformance, some weird cases might arise where
>   Clang++ would be forced to extend the ABI on its own at the risk of
>   being incompatible with itself after a while (MS variadic templates
>   implementation required an ABI change. MS engineers admit that
>   planned features might require more changes. Of course, Clang++
>   already supports most of those features.)
I don't know enough to comment on this, but I don't think that all
people interested in using clang on windows necessarily care.
>  * Finally, legal issues remain: supposing that Clang++ gets MS C++ ABI
>   compatibility, is it possible to use MS C++ runtime(s) and libraries
>   with Clang++? We could end with lots of work invested on a feature
>   that only benefits those who are forced to work with third-party C++
>   libraries distributed on binary form. I guess that such users are a
>   minority.
You need the technology first, once technology issues are solved, legal issues
can be tackled.  It is certainly true that people within Microsoft would love
great clang support, perhaps in time the right legal agreements can be hammered
out.
> To recap: lots of work is being invested on a feature which is very hard
> to implement, while at the same time Clang lacks basic features. I'll
> dare to say that focusing the effort on those features would mean that,
> in a year at most, Clang would become the best C++ compiler for Windows.
As I said up front, it is fine for you to share your opinion about what is
matter (you're clearly a smart guy and very knowledgeable), please just
don't turn this into blocking progress that you personally aren't
interested in or don't value.

-Chris

First and foremost: this mailing list is simply not an appropriate plate to
discuss legal issues. Full stop. I won't reply or discuss them, and please
don't try to do so yourself. Find a lawyer if you want to discuss such
things.


For the rest of your email, I defer to Chris's comments. If you want to
convince us there is a better way, please contribute excellent patches that
show that better way. Lead by example.
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On 7/31/2013 5:24 PM, Chris Lattner wrote:
> On Jul 31, 2013, at 3:13 PM, Óscar Fuentes <ofv at wanadoo.es> wrote:
>> * For being the best Windows C++ compiler, Clang++ doesn't need to
be
>>    compatible with the MS C++ ABI. However (and this is highly
>>    detrimental to Clang, IMO) from reading the mailing lists one could
>>    easily get the impression that until Clang++ gets the MS C++ ABI, it
>>    is not ready for Windows. It wont surprise me to learn that some
>>    contributors are working on the MS C++ ABI because they think that
>>    way.
> Fair point, but being ABI compatible DOES enable certain applications, for
example, working with existing binaries that can't be recompiled.
I would like to add that some of the Windows APIs require C++ ABI and 
are not (easily) usable from C, so some applications are only viable if 
Clang can be generally compatible with the MS C++ ABI.

-- 
Joshua Cranmer
Thunderbird and DXR developer
Source code archæologist

On Thu, Aug 1, 2013 at 1:06 AM, Óscar Fuentes <ofv at wanadoo.es> wrote:
>  * To understand why they chose to work on MS C++ ABI compatibility
>    instead of other most basic missing features that preclude using
>    Clang for serious C++ development. I'll be grateful if anyone
>    involved on the MS C++ ABI comments on this (it was already mentioned
>    that Wine is interested on that feature, but I don't understand
why.)
>
Clang is also being increasingly used by tools and having proper MSVC C++
ABI support is very valuable for extracting all kinds of information for
interop purposes (record layouts, vftable layouts, mangling, etc).

>  * To ensure that everybody knows that supporting the MS C++ ABI is not
>    required at all for being a Windows compiler.

It is if you need to use closed source libraries from vendors that only
publish MSVC builds.

-- 
João Matos
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On Wed, Jul 31, 2013 at 5:06 PM, Óscar Fuentes <ofv at wanadoo.es> wrote:
> prompts me to ask if it is right to submit to the mailing lists patches
> implementing a feature that is known to be legally
"controversial".
>
I have no idea what you are talking about, but if you have legal or other
questions, or want to discuss 'legal controversies', this isn't the
right
forum. Please do not do it again here.

We do not discuss legal issues on this mailing list. This isn't about you,
or this subject, or anything. It's simply not an appropriate place *for
anyone* to ask about or formulate opinions about legal topics.
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On Wed, Jul 31, 2013 at 5:06 PM, Óscar Fuentes <ofv at wanadoo.es> wrote:
> prompts me to ask if it is right to submit to the mailing lists patches
> implementing a feature that is known to be legally
"controversial".
>
I have no idea what you are talking about, but if you have legal or other
questions, or want to discuss 'legal controversies', this isn't the
right
forum. Please do not do it again here.

We do not discuss legal issues on this mailing list. This isn't about you,
or this subject, or anything. It's simply not an appropriate place *for
anyone* to ask about or formulate opinions about legal topics.
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