Quoting Chris Peterson from bug 990764 comment 46: --- In debug builds today, MOZ_MAKE_COMPILER_BELIEVE_IS_UNREACHABLE calls both MOZ_ASSERT_UNREACHABLE and __builtin_unreachable/whatever. However, there is a risk that the compiler might optimize away the MOZ_ASSERT_UNREACHABLE even in debug builds, seeing that the code path leads to __builtin_unreachable. For more predictable debug tests, we could change MOZ_MAKE_COMPILER_BELIEVE_IS_UNREACHABLE to not call __builtin_unreachable in debug builds: #ifdef DEBUG # define MOZ_MAKE_COMPILER_BELIEVE_IS_UNREACHABLE(reason) MOZ_CRASH(reason) #else # define MOZ_MAKE_COMPILER_BELIEVE_IS_UNREACHABLE(reason) __builtin_unreachable() #endif