[libcxx-commits] [PATCH] D73838: [libcxxabi] Fix layout of __cxa_exception for win64, fixing static assert failures

Sun Feb 2 09:32:36 PST 2020

rjmccall added a comment.

In D73838#1853482 <https://reviews.llvm.org/D73838#1853482>, @mstorsjo wrote:

> In D73838#1853443 <https://reviews.llvm.org/D73838#1853443>, @steven_wu wrote:
>
> > Looks like this is an ABI change for WIN64. If that is intended then LGTM
>
>
> I think that's ok - I don't think it's worth complicating this further to preserve compatibility with a previous binary layout which was accidental (using !LP64 layout on a platform with 64 bit pointers).

Yeah, I think it's okay to consider this a bug.

> I presume this is a break for cases where an exception is thrown across libraries that have statically linked different versions of libcxxabi?

You really aren't supposed to redundantly statically link libcxxabi like that; there are things in there that need to be unique.

> For cases where libcxxabi is linked in dynamically, all modules would be using the same copy of libcxxabi (either old or new) and it'd be consistent? I presume the `__cxa_exception` offsets don't end up in the calling application's binary anywhere, unless statically linked?

It's supposed to have a stable layout (modulo the ability to extend it with prefixes).  However, I'm not aware of any direct use of its fields by the compiler.

> Is the layout of `__cxa_exception` supposed to match that of libgcc/libstdc++/libsupc++? Because they don't have e.g. the reference count stuck in a padding hole for 32 bit architectures.

That's interesting.  Most details of the layout are indeed supposed to be portable across compilers and runtimes.

> In D73838#1853450 <https://reviews.llvm.org/D73838#1853450>, @rjmccall wrote:
> 
>> There's a Windows-based target that uses libUnwind-based exceptions?
> 
> 
> MinGW toolchains normally use libgcc/libstdc++, but one can build a toolchain that uses libunwind/libcxxabi/libcxx instead; I distribute such a toolchain at https://github.com/mstorsjo/llvm-mingw. But I don't really give any ABI guarantees; I've e.g. switched unwinding mechanisms (between dwarf, sjlj and SEH) before, to avoid bugs until they're fixed properly.

Okay.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D73838/new/

https://reviews.llvm.org/D73838