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

[Asiri Rathnayake via cfe-commits]

rmaprath added a comment.

In http://reviews.llvm.org/D20784#443708, @EricWF wrote:

> 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.


We do not allow linking a with-exceptions `libc++` library with a no-exceptions `libc++abi` library, that is not a use-case for us. We have either with-exceptions libraries or no-exceptions libraries, there is no overlap. This is why I went with the current approach (modify the no-exceptions libc++ variant to match the no-exceptions libc++abi variant).

Note that our use-case is about code-size, we do not want exception handling routines (as well as exception tables) in our final image. We are not after a system where you can swap out the exceptions behaviour. Sorry if I wasn't clear about this.

> 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.


That too would work for us. I agree this is a bit more cleaner since it does not involve changes to `libc++`, so I will spin a patch for it on top of http://reviews.llvm.org/D20677.

/ Asiri


http://reviews.llvm.org/D20784