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Go 1.9 Release Notes

Introduction to Go 1.9

The latest Go release, version 1.9, arrives six months after Go 1.8 and is the tenth release in the Go 1.x series. There are two changes to the language: adding support for type aliases and defining when implementations may fuse floating point operations. Most of the changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.

The release adds transparent monotonic time support, parallelizes compilation of functions within a package, better supports test helper functions, includes a new bit manipulation package, and has a new concurrent map type.

Changes to the language

There are two changes to the language.

Go now supports type aliases to support gradual code repair while moving a type between packages. The type alias design document and an article on refactoring cover the problem in detail. In short, a type alias declaration has the form:

type T1 = T2

This declaration introduces an alias name T1—an alternate spelling—for the type denoted by T2; that is, both T1 and T2 denote the same type.

A smaller language change is that the language specification now states when implementations are allowed to fuse floating point operations together, such as by using an architecture's "fused multiply and add" (FMA) instruction to compute x*y + z without rounding the intermediate result x*y. To force the intermediate rounding, write float64(x*y) + z.


There are no new supported operating systems or processor architectures in this release.

ppc64x requires POWER8

Both GOARCH=ppc64 and GOARCH=ppc64le now require at least POWER8 support. In previous releases, only GOARCH=ppc64le required POWER8 and the big endian ppc64 architecture supported older hardware.


Go 1.9 is the last release that will run on FreeBSD 9.3, which is already unsupported by FreeBSD. Go 1.10 will require FreeBSD 10.3+.

OpenBSD 6.0

Go 1.9 now enables PT_TLS generation for cgo binaries and thus requires OpenBSD 6.0 or newer. Go 1.9 no longer supports OpenBSD 5.9.

Known Issues

There are some instabilities on FreeBSD that are known but not understood. These can lead to program crashes in rare cases. See issue 15658. Any help in solving this FreeBSD-specific issue would be appreciated.

Go stopped running NetBSD builders during the Go 1.9 development cycle due to NetBSD kernel crashes, up to and including NetBSD 7.1. As Go 1.9 is being released, NetBSD 7.1.1 is being released with a fix. However, at this time we have no NetBSD builders passing our test suite. Any help investigating the various NetBSD issues would be appreciated.


Parallel Compilation

The Go compiler now supports compiling a package's functions in parallel, taking advantage of multiple cores. This is in addition to the go command's existing support for parallel compilation of separate packages. Parallel compilation is on by default, but it can be disabled by setting the environment variable GO19CONCURRENTCOMPILATION to 0.

Vendor matching with ./...

By popular request, ./... no longer matches packages in vendor directories in tools accepting package names, such as go test. To match vendor directories, write ./vendor/....


The go tool will now use the path from which it was invoked to attempt to locate the root of the Go install tree. This means that if the entire Go installation is moved to a new location, the go tool should continue to work as usual. This may be overridden by setting GOROOT in the environment, which should only be done in unusual circumstances. Note that this does not affect the result of the runtime.GOROOT function, which will continue to report the original installation location; this may be fixed in later releases.

Compiler Toolchain

Complex division is now C99-compatible. This has always been the case in gccgo and is now fixed in the gc toolchain.

The linker will now generate DWARF information for cgo executables on Windows.

The compiler now includes lexical scopes in the generated DWARF if the -N -l flags are provided, allowing debuggers to hide variables that are not in scope. The .debug_info section is now DWARF version 4.

The values of GOARM and GO386 now affect a compiled package's build ID, as used by the go tool's dependency caching.


The four-operand ARM MULA instruction is now assembled correctly, with the addend register as the third argument and the result register as the fourth and final argument. In previous releases, the two meanings were reversed. The three-operand form, in which the fourth argument is implicitly the same as the third, is unaffected. Code using four-operand MULA instructions will need to be updated, but we believe this form is very rarely used. MULAWT and MULAWB were already using the correct order in all forms and are unchanged.

The assembler now supports ADDSUBPS/PD, completing the two missing x86 SSE3 instructions.


Long lists of arguments are now truncated. This improves the readability of go doc on some generated code.

Viewing documentation on struct fields is now supported. For example, go doc http.Client.Jar.


The new go env -json flag enables JSON output, instead of the default OS-specific output format.


The go test command accepts a new -list flag, which takes a regular expression as an argument and prints to stdout the name of any tests, benchmarks, or examples that match it, without running them.


Profiles produced by the runtime/pprof package now include symbol information, so they can be viewed in go tool pprof without the binary that produced the profile.

The go tool pprof command now uses the HTTP proxy information defined in the environment, using http.ProxyFromEnvironment.


The vet command has been better integrated into the go tool, so go vet now supports all standard build flags while vet's own flags are now available from go vet as well as from go tool vet.


Due to the alignment of Go's semiannual release schedule with GCC's annual release schedule, GCC release 7 contains the Go 1.8.3 version of gccgo. We expect that the next release, GCC 8, will contain the Go 1.10 version of gccgo.


Call stacks with inlined frames

Users of runtime.Callers should avoid directly inspecting the resulting PC slice and instead use runtime.CallersFrames to get a complete view of the call stack, or runtime.Caller to get information about a single caller. This is because an individual element of the PC slice cannot account for inlined frames or other nuances of the call stack.

Specifically, code that directly iterates over the PC slice and uses functions such as runtime.FuncForPC to resolve each PC individually will miss inlined frames. To get a complete view of the stack, such code should instead use CallersFrames. Likewise, code should not assume that the length returned by Callers is any indication of the call depth. It should instead count the number of frames returned by CallersFrames.

Code that queries a single caller at a specific depth should use Caller rather than passing a slice of length 1 to Callers.

runtime.CallersFrames has been available since Go 1.7, so code can be updated prior to upgrading to Go 1.9.


As always, the changes are so general and varied that precise statements about performance are difficult to make. Most programs should run a bit faster, due to speedups in the garbage collector, better generated code, and optimizations in the core library.

Garbage Collector

Library functions that used to trigger stop-the-world garbage collection now trigger concurrent garbage collection. Specifically, runtime.GC, debug.SetGCPercent, and debug.FreeOSMemory, now trigger concurrent garbage collection, blocking only the calling goroutine until the garbage collection is done.

The debug.SetGCPercent function only triggers a garbage collection if one is immediately necessary because of the new GOGC value. This makes it possible to adjust GOGC on-the-fly.

Large object allocation performance is significantly improved in applications using large (>50GB) heaps containing many large objects.

The runtime.ReadMemStats function now takes less than 100µs even for very large heaps.

Core library

Transparent Monotonic Time support

The time package now transparently tracks monotonic time in each Time value, making computing durations between two Time values a safe operation in the presence of wall clock adjustments. See the package docs and design document for details.

New bit manipulation package

Go 1.9 includes a new package, math/bits, with optimized implementations for manipulating bits. On most architectures, functions in this package are additionally recognized by the compiler and treated as intrinsics for additional performance.

Test Helper Functions

The new (*T).Helper and (*B).Helper methods mark the calling function as a test helper function. When printing file and line information, that function will be skipped. This permits writing test helper functions while still having useful line numbers for users.

Concurrent Map

The new Map type in the sync package is a concurrent map with amortized-constant-time loads, stores, and deletes. It is safe for multiple goroutines to call a Map's methods concurrently.

Profiler Labels

The runtime/pprof package now supports adding labels to pprof profiler records. Labels form a key-value map that is used to distinguish calls of the same function in different contexts when looking at profiles with the pprof command. The pprof package's new Do function runs code associated with some provided labels. Other new functions in the package help work with labels.

Minor changes to the library

As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.


The ZIP Writer now sets the UTF-8 bit in the FileHeader.Flags when appropriate.


On Linux, Go now calls the getrandom system call without the GRND_NONBLOCK flag; it will now block until the kernel has sufficient randomness. On kernels predating the getrandom system call, Go continues to read from /dev/urandom.


On Unix systems the environment variables SSL_CERT_FILE and SSL_CERT_DIR can now be used to override the system default locations for the SSL certificate file and SSL certificate files directory, respectively.

The FreeBSD file /usr/local/etc/ssl/cert.pem is now included in the certificate search path.

The package now supports excluded domains in name constraints. In addition to enforcing such constraints, CreateCertificate will create certificates with excluded name constraints if the provided template certificate has the new field ExcludedDNSDomains populated.

If any SAN extension, including with no DNS names, is present in the certificate, then the Common Name from Subject is ignored. In previous releases, the code tested only whether DNS-name SANs were present in a certificate.


The package will now use a cached Stmt if available in Tx.Stmt. This prevents statements from being re-prepared each time Tx.Stmt is called.

The package now allows drivers to implement their own argument checkers by implementing driver.NamedValueChecker. This also allows drivers to support OUTPUT and INOUT parameter types. Out should be used to return output parameters when supported by the driver.

Rows.Scan can now scan user-defined string types. Previously the package supported scanning into numeric types like type Int int64. It now also supports scanning into string types like type String string.

The new DB.Conn method returns the new Conn type representing an exclusive connection to the database from the connection pool. All queries run on a Conn will use the same underlying connection until Conn.Close is called to return the connection to the connection pool.


The new NullBytes and NullRawValue represent the ASN.1 NULL type.


The new Encoding.WithPadding method adds support for custom padding characters and disabling padding.


The new field Reader.ReuseRecord controls whether calls to Read may return a slice sharing the backing array of the previous call's returned slice for improved performance.


The sharp flag ('#') is now supported when printing floating point and complex numbers. It will always print a decimal point for %e, %E, %f, %F, %g and %G; it will not remove trailing zeros for %g and %G.


The package now includes 128-bit FNV-1 and FNV-1a hash support with New128 and New128a, respectively.


The package now reports an error if a predefined escaper (one of "html", "urlquery" and "js") is found in a pipeline and does not match what the auto-escaper would have decided on its own. This avoids certain security or correctness issues. Now use of one of these escapers is always either a no-op or an error. (The no-op case eases migration from text/template.)


The Rectangle.Intersect method now returns a zero Rectangle when called on adjacent but non-overlapping rectangles, as documented. In earlier releases it would incorrectly return an empty but non-zero Rectangle.


The YCbCr to RGBA conversion formula has been tweaked to ensure that rounding adjustments span the complete [0, 0xffff] RGBA range.


The new Encoder.BufferPool field allows specifying an EncoderBufferPool, that will be used by the encoder to get temporary EncoderBuffer buffers when encoding a PNG image. The use of a BufferPool reduces the number of memory allocations performed while encoding multiple images.

The package now supports the decoding of transparent 8-bit grayscale ("Gray8") images.


The new IsInt64 and IsUint64 methods report whether an Int may be represented as an int64 or uint64 value.


The new FileHeader.Size field describes the size of a file in a multipart message.


The new Resolver.StrictErrors provides control over how Go's built-in DNS resolver handles temporary errors during queries composed of multiple sub-queries, such as an A+AAAA address lookup.

The new Resolver.Dial allows a Resolver to use a custom dial function.

JoinHostPort now only places an address in square brackets if the host contains a colon. In previous releases it would also wrap addresses in square brackets if they contained a percent ('%') sign.

The new methods TCPConn.SyscallConn, IPConn.SyscallConn, UDPConn.SyscallConn, and UnixConn.SyscallConn provide access to the connections' underlying file descriptors.

It is now safe to call Dial with the address obtained from (*TCPListener).String() after creating the listener with Listen("tcp", ":0"). Previously it failed on some machines with half-configured IPv6 stacks.


The Cookie.String method, used for Cookie and Set-Cookie headers, now encloses values in double quotes if the value contains either a space or a comma.

Server changes:

  • ServeMux now ignores ports in the host header when matching handlers. The host is matched unmodified for CONNECT requests.
  • Server.WriteTimeout now applies to HTTP/2 connections and is enforced per-stream.
  • HTTP/2 now uses the priority write scheduler by default. Frames are scheduled by following HTTP/2 priorities as described in RFC 7540 Section 5.3.
  • The HTTP handler returned by StripPrefix now calls its provided handler with a modified clone of the original *http.Request. Any code storing per-request state in maps keyed by *http.Request should use Request.Context, Request.WithContext, and context.WithValue instead.

Client & Transport changes:

  • The Transport now supports making requests via SOCKS5 proxy when the URL returned by Transport.Proxy has the scheme socks5.

The new ProcessEnv function returns FastCGI environment variables associated with an HTTP request for which there are no appropriate http.Request fields, such as REMOTE_USER.


The new Server.Client method returns an HTTP client configured for making requests to the test server.

The new Server.Certificate method returns the test server's TLS certificate, if any.


The ReverseProxy now proxies all HTTP/2 response trailers, even those not declared in the initial response header. Such undeclared trailers are used by the gRPC protocol.


The os package now uses the internal runtime poller for file I/O. This reduces the number of threads required for read/write operations on pipes, and it eliminates races when one goroutine closes a file while another is using the file for I/O.

On Windows, Args is now populated without shell32.dll, improving process start-up time by 1-7 ms.


The os/exec package now prevents child processes from being created with any duplicate environment variables. If Cmd.Env contains duplicate environment keys, only the last value in the slice for each duplicate key is used.


Lookup and LookupId now work on Unix systems when CGO_ENABLED=0 by reading the /etc/passwd file.

LookupGroup and LookupGroupId now work on Unix systems when CGO_ENABLED=0 by reading the /etc/group file.


The new MakeMapWithSize function creates a map with a capacity hint.


Tracebacks generated by the runtime and recorded in profiles are now accurate in the presence of inlining. To retrieve tracebacks programmatically, applications should use runtime.CallersFrames rather than directly iterating over the results of runtime.Callers.

On Windows, Go no longer forces the system timer to run at high resolution when the program is idle. This should reduce the impact of Go programs on battery life.

On FreeBSD, GOMAXPROCS and runtime.NumCPU are now based on the process' CPU mask, rather than the total number of CPUs.

The runtime has preliminary support for Android O.


Calling SetGCPercent with a negative value no longer runs an immediate garbage collection.


The execution trace now displays mark assist events, which indicate when an application goroutine is forced to assist garbage collection because it is allocating too quickly.

"Sweep" events now encompass the entire process of finding free space for an allocation, rather than recording each individual span that is swept. This reduces allocation latency when tracing allocation-heavy programs. The sweep event shows how many bytes were swept and how many were reclaimed.


Mutex is now more fair.


The new field Credential.NoSetGroups controls whether Unix systems make a setgroups system call to set supplementary groups when starting a new process.

The new field SysProcAttr.AmbientCaps allows setting ambient capabilities on Linux 4.3+ when creating a new process.

On 64-bit x86 Linux, process creation latency has been optimized with use of CLONE_VFORK and CLONE_VM.

The new Conn interface describes some types in the net package that can provide access to their underlying file descriptor using the new RawConn interface.


The package now chooses values in the full range when generating int64 and uint64 random numbers; in earlier releases generated values were always limited to the [-262, 262) range.

In previous releases, using a nil Config.Rand value caused a fixed deterministic random number generator to be used. It now uses a random number generator seeded with the current time. For the old behavior, set Config.Rand to rand.New(rand.NewSource(0)).


The handling of empty blocks, which was broken by a Go 1.8 change that made the result dependent on the order of templates, has been fixed, restoring the old Go 1.7 behavior.


The new methods Duration.Round and Duration.Truncate handle rounding and truncating durations to multiples of a given duration.

Retrieving the time and sleeping now work correctly under Wine.

If a Time value has a monotonic clock reading, its string representation (as returned by String) now includes a final field "m=±value", where value is the monotonic clock reading formatted as a decimal number of seconds.

The included tzdata timezone database has been updated to version 2017b. As always, it is only used if the system does not already have the database available.