> On Dec 5, 2017, at 8:30 PM, Thomas Roughton via swift-dev > <swift-dev@swift.org> wrote: > > I've been working on getting Swift running properly on 64-bit Windows and > wanted to get some feedback/ideas on a specific issue. > > In swift/Runtime/HeapObject.h, there is a TwoWordPair::Return type intended > to return two word-sized values in registers. On Windows, structs are > returned indirectly. For some platforms (ARM, i386, 32-bit Windows) this is > worked around by packing the results into a 64-bit int. > > We can apply a similar solution on Win64 by packing the results into an > __m128 and adopting the __vectorcall calling convention to ensure that the > value is passed in a register. However, adopting a new calling convention for > methods that interact with TwoWordPair::Return has a fairly major fallout; > I've started work in a branch (see the most recent three commits on > https://github.com/troughton/swift/tree/x64-vectorcall > <https://github.com/troughton/swift/tree/x64-vectorcall>), but it feels very > messy. The main issue is that __vectorcall using a different mangling scheme, > which means we need to special-case in quite a few different places. > > Another alternative would be to adopt the Swift calling convention for > swift_allocBox. If this doesn't cause other issues, it seems cleaner and > would have a much smaller impact on the code-base. However, there's currently > an issue blocking using the Swift calling convention on Windows; it gets sent > to MicrosoftMangle in Clang, which doesn't know how to mangle the Swift > calling convention (https://reviews.llvm.org/D31372 > <https://reviews.llvm.org/D31372>). I'd like to resolve this, and it seems > like there are two possible implementations: > > Pick an arbitrary prefix to use for SwiftCC and then mangle the rest of the > string following the Microsoft convention, knowing that tools won't know how > to deal with it. > Alternatively, for functions that use SwiftCC, use the Itanium mangling. This > would require a more major refactoring in Clang but might be easier to > demangle. > What are people's thoughts on these two issues?
I think it wouldn't be the worst thing to just ignore SwiftCC when mangling for the MSVC ABI for now, at least when mangling a top-level function type. John.
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