[PATCH] D20784: [libcxx][libcxxabi] Decouple no-exceptions libcpp variant from cxa_exception.cpp and cxa_personality.cpp of libcxxabi

[Eric Fiselier via cfe-commits]

EricWF added a comment.

I don't like the direction of this patch, and it seems like the wrong direction for what your trying to achieve as well.
If your trying to support a system with two variants of libc++abi, one that throws and the other that does not, then you'll want libc++ to remain unmodified so it can depend on the selected libc++abi to provide the correct exception handling behavior.

I think these particular symbols should always be defined within libc++abi (sorry if I said otherwise earlier, I didn't understand the implementation). The -fno-exceptions build of libc++abi can build these symbols with very simple definitions that is detached from the rest of the exception handling machinery.

My preference would be to create a new file `cxa_no_exception_definitions.cpp` (bike shedding welcome) which  builds inplace of `cxa_exception.cpp` and contains definitions for these symbols. For example.

- `__cxa_uncaugh_exception()`/`__cxa_uncaught_exceptions()` always returns zero.
- `__cxa_decrement_exception_refcount(ptr)`/`__cxa_increment_refcount(ptr)` terminate if `ptr != nullptr`.
- `__cxa_primary_exception()` returns nullptr.
- `__cxa_rethrow_primary_exception(ptr)` returns as to hit the terminate call in libc++.

This way we are not conflating building libc++ without exceptions with building libc++ against an ABI library which doesn't provide the exception handling API.


http://reviews.llvm.org/D20784