| internal/bisect | Package bisect can be used by compilers and other programs to serve as a target for the bisect debugging tool. |
| internal/godebugs | Package godebugs provides a table of known GODEBUG settings, for use by a variety of other packages, including internal/godebug, runtime, runtime/metrics, and cmd/go/internal/load. |
| sync | Package sync provides basic synchronization primitives such as mutual exclusion locks. |
| sync/atomic | Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms. |
| unsafe | Package unsafe contains operations that step around the type safety of Go programs. |