Clang equivalent of GCC's __builtin_darn()

问题:

I'm trying to discover Clang's equivalent to GCC's __builtin_darn() on Power9. Grepping Clang 7.0 sources it looks like LLVM supports it:

llvm_source$ cat llvm/test/MC/PowerPC/ppc64-encoding.s | grep darn -B 1 -A 1

# CHECK-BE: darn 2, 3                       # encoding: [0x7c,0x43,0x05,0xe6]
# CHECK-LE: darn 2, 3                       # encoding: [0xe6,0x05,0x43,0x7c]
            darn 2, 3

However, I can't seen to find the builtin:

llvm_source$ grep -IR darn | grep builtin
llvm_source$

What is Clang equivalent of GCC's __builtin_darn()?