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), 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). 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?
- Thomas
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