Illegal Instruction: 4

With optimizations on the Swift compiler of Xcode 7 now crashes with "Illegal Instruction: 4". Apparently something is not yet implemented.

Not yet implemented
UNREACHABLE executed at /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-700.0.38.1/src/swift/lib/SILPasses/Utils/CFG.cpp:522!
0  swift                    0x0000000107d93e0b llvm::sys::PrintStackTrace(__sFILE*) + 43
1  swift                    0x0000000107d9454b SignalHandler(int) + 379
2  libsystem_platform.dylib 0x00007fff9110af1a _sigtramp + 26
3  libsystem_platform.dylib 0x000000000000ffff _sigtramp + 1861243135
4  swift                    0x0000000107d94346 abort + 22
5  swift                    0x0000000107d4de21 llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 481
6  swift                    0x000000010625b160 swift::splitEdge(swift::TermInst*, unsigned int, swift::DominanceInfo*, swift::SILLoopInfo*) + 2992
7  swift                    0x000000010625b294 swift::splitEdgesFromTo(swift::SILBasicBlock*, swift::SILBasicBlock*, swift::DominanceInfo*, swift::SILLoopInfo*) + 212
8  swift                    0x0000000106113dda (anonymous namespace)::SwiftArrayOptPass::run() + 2618
9  swift                    0x000000010608b8e8 swift::SILPassManager::runFunctionPasses(llvm::ArrayRef<swift::SILFunctionTransform*>) + 1608
10 swift                    0x000000010608c56c swift::SILPassManager::runOneIteration() + 988
11 swift                    0x000000010608950c swift::runSILOptimizationPasses(swift::SILModule&) + 492
12 swift                    0x0000000105dad83f performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&) + 9583
13 swift                    0x0000000105dab0b3 frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 2515
14 swift                    0x0000000105da728f main + 1983
15 libdyld.dylib            0x00007fff8a2ec5c9 start + 1
16 libdyld.dylib            0x0000000000000083 start + 1976646331

Does anybody know what causes this and how to avoid it?