Externally-defined functions with weak linkage should not be
tail-called on ARM or AArch64, as the AAELF spec requires normal calls
to undefined weak functions to be replaced with a NOP or jump to the
next instruction. The behaviour of branch instructions in this
situation (as used for tail calls) is implementation-defined, so we
cannot rely on the linker replacing the tail call with a return.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215890 91177308-0d34-0410-b5e6-96231b3b80d8
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