I've been working on this on and off. Here's a braindump of what's needed:
1. Port OS-specific things.
This is rather small and I've done most of this already.
2. C Standard Library.
Microsoft's CRT is a royal PITA. It doesn't support a lot of things
required or expected, and other things have subtle differences.
Which is why I wrote a new standard library to make point 1 easier.
At first it was only a set of standard-compliant headers and
implementation stubs to get it to compile, but I've since
implemented certain parts. The hardest part for this route is, for
me at least, libm, and I don't know any modern BSD-licensed libm
that could be used.
The other way would be to further try and shoehorn MSVCRT and give
up C99- and full C++11-compatibility.
3. Implement full dllimport/dllexport semantics so that libc++ can be
used as a DLL.
Clang only support rudimentary dllimport/dllexport for functions.
Making it work with classes is a rather large non-trivial change.
I've been working on this and apart from vtable-handling and
cleanups it's mostly finished. Depending on that last point I may
put up some patches for review soon.
This also requires changes to how libc++ applies those attributes.
Using them like ELF visibility doesn't work correctly for templates.
4. Weak externals for PE to enable replacing operator new/delete.
This was impossible with how dllimport was represented in LLVM
before. If my patches are excepted, it should be doable, but I
haven't looked into it again.
5. Port an ABI library like libc++abi.
I've ported some parts of libc++abi, but it's incomplete and
untested. It also needs an unwind library, which quickly leads
to the next point:
6. Exception handling
This is probably the biggest hurdle. There is some low-level support
for 64-bit SEH directives in LLVM already. 32-bit has the additional
Borland patent issue for SEH exception handling. The data structures
used by MSVC are also undocumented.
Implementing this for 64-bit by using the DWARF exception tables and
providing an appropriate personality routine is the easiest case,
and would also allow catching SEH exceptions from the OS.
Maybe it's also possible to implement such DWARF-based table-based
exception handling for 32-bit, without any way to catch SEH
7. Some minor bugfixes I encountered along the way, like proper COMDAT
linkage for EH sections, proper varargs handling. I'll try to get
those committed soon.