[llvm-dev] Problem with mingw32 DLL build

[Chandler Carruth via llvm-dev]

I'm about to switch over the the conclusion here since I've already gotten
this wrong a few times. =[

However, I want to say for the record that I don't really think we should
keep supporting this kind of DLL builds given the limitations of DLLs when
it comes to correctly implementing these aspects of C++. I suspect we
should really move toward having an LLVM.dll (and a LTO.dll, etc) and only
using static linking within that on Windows. (I know that actually
producing an LLVM.dll is a huge challenge currently due to CMake and other
issues.) Trying to support the fully shared build with the limitations of
DLLs when it only even somewhat works on one platform (mingw32, as other
things prevent it from working w/ MSVC from my understanding of
conversations with Reid) seems like a losing battle and to have a high cost
for very little gain for the community.

On Tue, Mar 1, 2016 at 11:09 PM Reid Kleckner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Tue, Mar 1, 2016 at 1:32 PM, Chandler Carruth <chandlerc at gmail.com>
> wrote:
>>
>> I don't disagree with your analysis, but I'd like to get a build bot that
>> tests DLL builds with a host toolchain other than mingw32 if this is in
>> fact common to those configs (as no other bot broke that i saw)... But
>> that's a separate issue.
>>
>
> We don't support BUILD_SHARED_LIBS=ON with MSVC. It doesn't come with
> tools that make it easy to support this configuration. CMake recently added
> some support for building DLLs that export all symbols, but it's very new
> and requires you to annotate all your exported global data, i.e. pass ids.
>
> The base class isn't *just* providing the static variable. It is also
>> providing the accessor method. I really like using a method instead of raw
>> access to the static data member. It is also providing the name of the
>> pass. So what we'd likely end up with is still having the base class but
>> having a friend declaration and private static data member in the derived
>> class.
>>
>
> Sure, that'd work.
>
>
>> But before going there, I'd like to ask if there is another approach that
>> would work: do you see any ways to effectively cause the static data member
>> to be emitted reliably?
>>
>
> I'm sure we could come up with a way to get the ID emitted reliably at the
> definition of the analysis, but we also need to ensure that the ID is
> unique, which means we need something like the extern template declaration
> in the header.
>
> I think your idea of using CRTP to stamp out the pass name and ID accessor
> is reasonable, and then we can use a normal C++ declaration/definition pair
> in the derived class for the ID.
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