Go 1.9 (2017-08-24)

Go 1.9 was released on August 24, 2017. This release includes two language changes (type aliases), introduces important new features in the standard library such as sync.Map and math/bits, and improves time package support for monotonic time.

Major Changes

Language

• Type Aliases: Go 1.9 introduces type alias syntax, primarily to support large-scale code refactoring (such as moving a type from one package to another) and enable smooth transitions.
  • Syntax: type T1 = T2. Declares T1 as an alias for T2, meaning they denote the same type.
  • Difference: This is different from a type definition (type T1 T2), which creates a new, distinct type.

Toolchain

• Parallel Compilation: The Go compiler now supports parallel compilation of functions within a package. This significantly speeds up compilation by utilizing multi-core CPUs. This is an optimization on top of the existing package-level parallel compilation.
• Test Helper: The testing package introduces the Helper method. t.Helper() marks the current function as a test helper function. When a test fails and prints logs, Go will skip this function and display the line number of the test code that called the helper function, making it easier to locate issues.
• go test -list: Added the -list flag, which accepts a regular expression and lists matching tests, benchmarks, or examples without running them.
• ./... Matching Rule: The ./... pattern no longer matches packages in the vendor directory. To match the vendor directory, you must explicitly use ./vendor/....

Runtime

• GC Performance: Performance of large object allocation is significantly improved, and runtime.ReadMemStats latency with large heaps is greatly reduced (< 100µs).

Standard Library

• sync.Map: The standard library adds sync.Map, a concurrent-safe Map implementation.

  • Use Case: Caching scenarios where keys are stable and reads far outnumber writes; or when multiple goroutines read, write, and overwrite entries for disjoint sets of keys.
  • Features: Safe for concurrent use without additional locking or coordination.

  var m sync.Map
  m.Store("key", "value")
  v, ok := m.Load("key")

• Monotonic Time: The time package now transparently supports Monotonic Time.

  • Improvement: Time values returned by time.Now() contain a monotonic clock reading in addition to the wall clock time.
  • Impact: Calculating the duration between two time points (t2.Sub(t1)) is now always positive and accurate, even if the system time is adjusted (e.g., via NTP synchronization).

• math/bits Package: Added the math/bits package, providing bit manipulation functions for unsigned integers. The compiler treats many functions in this package (such as Len, OnesCount, RotateLeft, etc.) as intrinsics, compiling them directly to underlying CPU instructions for maximum performance.

References

For more details, please refer to the official release notes: Go 1.9 Release Notes