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The current std::list::merge code calls size() before starting to merge any elements, so that the _M_size members can be updated after the merge finishes. The work is done in a try-block so that the sizes can still be updated in an exception handler if any element comparison throws. The _M_size members only exist for the cxx11 ABI, so the initial call to size() and the try-catch are only needed for that ABI. For the old ABI the size() call performs an O(N) list traversal to get a value that isn't even used, and catching exceptions just to rethrow them isn't needed either. This refactors the merge functions to remove the try-catch block and use an RAII type instead. For the cxx11 ABI that type's destructor updates the list sizes, and for the old ABI it's a no-op. libstdc++-v3/ChangeLog: * include/bits/list.tcc (list::merge): Remove call to size() and try-catch block. Use _Finalize_merge instead. * include/bits/stl_list.h (list::_Finalize_merge): New scope guard type to update _M_size members after a merge.
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