[libcxx-commits] [libcxxabi] [libcxx] [libcxxabi] Dont throw in make exception ptr if __cxa_init_primary_exception is available (PR #65534)

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itrofimow wrote:

I had to split `typeinfo` header to break the `exception` -> `typeinfo` -> `exception` circular dependency, hence a sizeable +/-, other than that the PR is basically a couple dozen LoC.

The build is failing mostly due to missing changes in abi-lists for various architectures, i'm planning to fix that once/if the general idea is approved.

One question left is the way to determine whether the new `__cxa_init_primary_exception` function is available: right now i'm relying on `LIBCXX_BUILDING_LIBCXXABI`, but i think it's not enough. 
Should i add a new define into libcxxabi, or check for `__cxa_init_primary_exception` presence at runtime via `dlsym`?
Personally i'm leaning towards `dlsym`, since it would not only allow linking with other versions of libcxxabi, but potentially implementing the same optimization when linked against libcxxrt, see https://github.com/libcxxrt/libcxxrt/pull/23.

@philnik777 @EricWF Could you please have a look?

https://github.com/llvm/llvm-project/pull/65534