[PATCH] D32927: [libc++] Implement exception_ptr on Windows

[Eric Fiselier via Phabricator via cfe-commits]

EricWF added inline comments.


================
Comment at: test/std/language.support/support.exception/propagation/current_exception.pass.cpp:10-11
 
-// exception_ptr has not been implemented on Windows
+// This test needs to be rewritten for the Windows exception_ptr semantics
+// which copy the exception each time the exception_ptr is copied.
 // XFAIL: LIBCXX-WINDOWS-FIXME
----------------
bcraig wrote:
> EricWF wrote:
> > EricWF wrote:
> > > BillyONeal wrote:
> > > > We don't copy the exception each time the exception_ptr is copied -- exception_ptr models shared_ptr. See: "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.10.25017\crt\src\stl\excptptr.cpp"
> > > > 
> > > > We do make a copy when making the initial exception_ptr.
> > > Ah thanks for the clarification. So `std::current_exception()` always returns a different copy.
> > >  exception_ptr models shared_ptr. 
> > 
> > Oh fudge me, it not only models it but it actually uses MSVC's `shared_ptr` implementation. This seems so much sketchier now.
> You may want to pursue your long term plan instead.
> 
> Rather than use the msvcprt functions, dig into the underlying SEH structures, find the pointer to the exception, and work from there.
> 
> Headers in the older crt source have a lot of useful information, and that may be useful enough to get things going.
> 
> The blog mentions where to fine one of those useful headers (ehdata.h):
> http://workblog.pilin.name/2014/03/decoding-parameters-of-thrown-c.html
> "Welcome - ehdata.h. This header appears only in VS2012 and VS2013 (see "%Program Files (x86)%\Microsoft Visual Studio 12.0\VC\crt\src\ehdata.h"):"
> 
> The other useful header is mtdll.h.  I found it in my local msvc 9.0 install, but didn't find it in my msvc-14.0 install.  I don't have 10.0, 11.0, or 12.0 installed locally right now to see if mtdll.h is present in any of those.  mtdll.h describes the layout of _tiddata, a thread local data store for the CRT.  The interesting fields there are void *_curexception;  and void *_curcontext;.
> 
> You may want to pursue your long term plan instead.

I don't think this should block this initial progress. Libc++ has a history of linking to `libstdc++` without major problems. This should be OK too, although I still don't love doing it.


https://reviews.llvm.org/D32927