It's a completely meaningless comparison in practice.

D's contract mechanism (i.e. in/out, and invariant blocks) can provide very strong guarantees to the compiler (as a specified part of the language as opposed to a GCC pragma/builtin), which the LLVM D compiler definitely uses.

All of the on by defaults that D has are usually there for a reason, i.e. floats are NaN initialized and bounds checking is on by default: These are very good idiot-proofing which can often be ignored unless profiling suggests there is a tangible issue.