Go 1.8 Release Notes

DRAFT RELEASE NOTES - Introduction to Go 1.8

Go 1.8 is not yet released. These are work-in-progress release notes. Go 1.8 is expected to be released in February 2017.

The latest Go release, version 1.8, arrives six months after Go 1.7. Most of its changes are in the implementation of the toolchain, runtime, and libraries. There are two minor changes to the language specification. 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 support for 32-bit MIPS, updates the compiler back end to generate more efficient code, reduces GC pauses by eliminating stop-the-world stack rescanning, adds HTTP/2 Push support, adds HTTP graceful shutdown, adds more context support, enables profiling mutexes, and simplifies sorting slices.

Changes to the language

When explicitly converting a value from one struct type to another, as of Go 1.8 the tags are ignored. Thus two structs that differ only in their tags may be converted from one to the other:

func example() {
type T1 struct {
X int json:"foo"
type T2 struct {
X int json:"bar"
var v1 T1
var v2 T2
v1 = T1(v2) // now legal
The language specification now only requires that implementations support up to 16-bit exponents in floating-point constants. This does not affect either the “gc” or gccgo compilers, both of which still support 32-bit exponents.


Go now supports 32-bit MIPS on Linux for both big-endian (linux/mips) and little-endian machines (linux/mipsle).

On DragonFly BSD, Go now requires DragonFly 4.4.4 or later.

The Plan 9 port’s networking support is now much more complete and matches the behavior of Unix and Windows with respect to deadlines and cancelation.

Go 1.8 now only supports OS X 10.8 or later. This is likely the last Go release to support 10.8. Compiling Go or running binaries on older OS X versions is untested.

Go 1.8 will be the last release to support Linux on ARMv5E and ARMv6 processors: Go 1.9 will likely require the ARMv6K (as found in the Raspberry Pi 1) or later. To identify whether a Linux system is ARMv6K or later, run go tool dist -check-armv6k (to facilitate testing, it is also possible to just copy the dist command to the system without installing a full copy of Go 1.8) and if the program terminates with output “ARMv6K supported.” then the system implements ARMv6K or later. Go on non-Linux ARM systems already requires ARMv6K or later.

Known Issues

There are some instabilities on FreeBSD and NetBSD that are known but not understood. These can lead to program crashes in rare cases. See issue 15658, issue 16396, and issue 16511. Any help in solving these issues would be appreciated.



For 64-bit x86 systems, the following instructions have been added: VBROADCASTSD, BROADCASTSS, MOVDDUP, MOVSHDUP, MOVSLDUP, VMOVDDUP, VMOVSHDUP, and VMOVSLDUP.



The yacc tool (previously available by running “go tool yacc”) has been removed. As of Go 1.7 it was no longer used by the Go compiler. It has moved to the “tools” repository and is now available at golang.org/x/tools/cmd/goyacc.


The fix tool has a new “context” fix to change imports from “golang.org/x/net/context” to “context”.


The pprof tool can now profile TLS servers and skip certificate validation by using the “https+insecure” URL scheme.

The callgrind output now has instruction-level granularity.

TODO: more. proto? standalone profiles with symbols?

runtime/pprof: output CPU profiles in pprof protobuf format (CL 33071)
runtime/pprof: write profiles in protobuf format. (CL 32257)


cmd/trace: add option to output pprof files (CL 23324)
cmd/trace: fix a runnable goroutine count bug (CL 25552)
cmd/trace: move process-wide GC events to their own row (CL 30017)
internal/trace: fix analysis of EvGoWaiting/EvGoInSyscall events (CL 25572)
cmd/trace: annotate different mark worker types (CL 30702)

Vet is stricter in some ways and looser where it previously caused false positives.

Vet now checks copying of array of locks, duplicate JSON and XML struct field tags, non-space-separated struct tags, deferred calls to HTTP Response.Body.Close before checking errors, indexed arguments in Printf, and improves existing checks.

Compiler Toolchain

Go 1.7 introduced a new compiler back end for 64-bit x86 systems. In Go 1.8, that back end has been developed further and is now used for all architectures.

The new back end, based on static single assignment form (SSA), generates more compact, more efficient code and provides a better platform for optimizations such as bounds check elimination. The new back end reduces the CPU time required by our benchmark programs by 20-30% on 32-bit ARM systems. For 64-bit x86 systems, which already used the SSA backend in Go 1.7, the gains are a more modest 0-10%. Other architectures will likely see improvements closer to the 32-bit ARM numbers.

The temporary -ssa=0 compiler flag introduced in Go 1.7 to disable the new backend has been removed in Go 1.8.

In addition to enabling the new compiler back end for all systems, Go 1.8 also introduces a new compiler front end. The new compiler front end should not be noticeable to users but is the foundation for future performance work.

The compiler and linker have been optimized and run faster in this release than in Go 1.7, although they are still slower than we would like and will continue to be optimized in future releases. Compared to the previous release, Go 1.8 is about 15% faster.


The environment variable PKG_CONFIG may now be used to set the program to run to handle #cgo pkg-config directives. The default is pkg-config, the program always used by earlier releases. This is intended to make it easier to cross-compile cgo code.

The cgo tool now supports a -srcdir option, which is used by the go command.

If cgo code calls C.malloc, and malloc returns NULL, the program will now crash with an out of memory error. C.malloc will never return nil. Unlike most C functions, C.malloc may not be used in a two-result form returning an errno value.

If cgo is used to call a C function passing a pointer to a C union, and if the C union can contain any pointer values, and if cgo pointer checking is enabled (as it is by default), the union value is now checked for Go pointers.


Due to the alignment of Go’s semiannual release schedule with GCC’s annual release schedule, GCC release 6 contains the Go 1.6.1 version of gccgo. We expect that the next release, GCC 7, will contain the Go 1.8 version of gccgo.

Default GOPATH

The GOPATH environment variable now has a default value if it is unset. It defaults to $HOME/go on Unix and %USERPROFILE%/go on Windows.

Go bug

The new “go bug” command starts a bug report on GitHub, prefilled with information about the current system.

Go doc

The “go doc” command now groups constants and variables with their type, following the behavior of godoc.

In order to improve the readability of doc’s output, each summary of the first-level items is guaranteed to occupy a single line.

Documentation for a specific method in an interface definition can now be requested, as in “go doc net.Conn.SetDeadline”.


Go now supports a “plugin” build mode for generating plugins written in Go, and a new plugin package for loading such plugins at run time. Plugin support is only currently available on Linux and macOS.


Argument Liveness

The garbage collector no longer considers arguments live throughout the entirety of a function. For more information, and for how to force a variable to remain live, see the runtime.KeepAlive function added in Go 1.7.

Updating: Code that sets a finalizer on an allocated object may need to add calls to runtime.KeepAlive in functions or methods using that object. Read the KeepAlive documentation and its example for more details.

MemStats Documentation

The runtime’s MemStats type has been more thoroughly documented.


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 and optimizations in the standard library.

There have been optimizations to implementations in the bytes, crypto/aes, crypto/cipher, crypto/elliptic, crypto/sha256, crypto/sha512, encoding/asn1, encoding/csv, encoding/hex, encoding/json, hash/crc32, image/color, image/draw, math, math/big, reflect, regexp, runtime, strconv, strings, syscall, text/template, and unicode/utf8, packages.

Garbage Collector

Garbage collection pauses should be significantly shorter than they were in Go 1.7, usually under 100 microseconds and often as low as 10 microseconds. See the document on eliminating stop-the-world stack re-scanning for details. More work remains for Go 1.9.


The overhead of deferred function calls has been reduced by about half.


The overhead of calls from Go into C has been reduced by about half.

Standard library


Examples have been added to the documentation across many packages.


The sort package now includes a convenience function Slice to sort a slice given a less function. In many cases this means that writing a new sorter type is not necessary.

Also new are SliceStable and SliceIsSorted.

HTTP/2 Push

The net/http package now includes a mechanism to send HTTP/2 server pushes from a Handler. Similar to the existing Flusher and Hijacker interfaces, an HTTP/2 ResponseWriter now implements the new Pusher interface.

HTTP Server Graceful Shutdown

The HTTP Server now has support for graceful shutdown using the new Server.Shutdown method and abrupt shutdown using the new Server.Close method.

More Context Support

Continuing Go 1.7’s adoption of context.Context into the standard library, Go 1.8 adds more context support to existing packages:

The new Server.Shutdown takes a context argument.
There have been significant additions to the database/sql package with context support.
The new T.Context method in the testing package now returns a context for the active test or benchmark.
All nine of the new Lookup methods on the new net.Resolver now take a context.
Mutex Contention Profiling

The runtime and tools now support profiling contended mutexes.

Most users will want to use the new -mutexprofile flag with go test, and then use pprof on the resultant file.

Lower-level support is also available via the new MutexProfile and SetMutexProfileFraction.

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 follow sections list the user visible changes and additions. Optimizations and bug fixes are not listed.

The tar implementation corrects many bugs in corner cases of the file format. The Reader is now able to process tar files in the PAX format with entries larger than 8GB. The Writer no longer produces invalid tar files in some situations involving long pathnames.

The Reader now supports modification times in the NTFS, UNIX, and Extended Time Stamp metadata fields. The Writer now writes Extended Time Stamp fields.

There have been some minor fixes to the encoder to improve the compression ratio in certain situations. As a result, the exact encoded output of DEFLATE may be different from Go 1.7. Since DEFLATE is the underlying compression of gzip, png, zlib, and zip, those formats may have changed outputs.

The encoder, when operating in NoCompression mode, now produces a consistent output that is not dependent on the size of the slices passed to the Write method.

The decoder, upon encountering an error, now returns any buffered data it had uncompressed along with the error.

The Writer now encodes a zero MTIME field when the Header.ModTime field is the zero value. In previous releases of Go, the Writer would encode a nonsensical value. Similarly, the Reader now reports a zero encoded MTIME field as a zero Header.ModTime.

The DeadlineExceeded error now implements net.Error and reports true for both the Timeout and Temporary methods.

The new method Conn.CloseWrite allows TLS connections to be half closed.

The new method Config.Clone clones a TLS configuration.

The new Config.GetConfigForClient callback allows selecting a configuration for a client dynamically, based on the client’s ClientHelloInfo. The ClientHelloInfo struct now has new fields Conn, SignatureSchemes (using the new type SignatureScheme), SupportedProtos, and SupportedVersions.

The new Config.GetClientCertificate callback allows selecting a client certificate based on the server’s TLS CertificateRequest message, represented by the new CertificateRequestInfo.

The new Config.KeyLogWriter allows debugging TLS connections in WireShark and similar tools.

The new Config.VerifyPeerCertificate callback allows additional validation of a peer’s presented certificate.

The crypto/tls package now implements basic countermeasures against CBC padding oracles. There should be no explicit secret-dependent timings, but it does not attempt to normalize memory accesses to prevent cache timing leaks.

The crypto/tls package now supports X25519 and ChaCha20-Poly1305. ChaCha20-Poly1305 is now prioritized unless AES-GCM when hardware support is present.

AES-128-CBC cipher suites with SHA-256 are also now supported.

SystemCertPool is now implemented on Windows.

PSS signatures are now supported.

UnknownAuthorityError now has a Cert field, reporting the untrusted certificate.

Certificate validation is more permissive in a few cases and stricter in a few other cases.

Root certificates will now also be looked for at /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem on Linux, to support RHEL and CentOS.

The package now supports context.Context. There are new methods ending in Context such as DB.QueryContext and DB.PrepareContext that take context arguments. Using the new Context methods ensures that connections are closed and returned to the connection pool when the request is done; enables canceling in-progress queries should the driver support that; and allows the database pool to cancel waiting for the next available connection.

The IsolationLevel can now be set when starting a transaction by setting the isolation level on the Context then passing that Context to DB.BeginContext. An error will be returned if an isolation level is selected that the driver does not support. A read-only attribute may also be set on the transaction with ReadOnlyContext.

Queries now expose the SQL column type information for drivers that support it. Rows can return ColumnTypes which can include SQL type information, column type lengths, and the Go type.

A Rows can now represent multiple result sets. After Rows.Next returns false, Rows.NextResultSet may be called to advance to the next result set. The existing Rows should continue to be used after it advances to the next result set.

NamedArg may be used as query arguments. The new function Named helps create a NamedArg more succinctly.

If a driver supports the new Pinger interface, the DB’s DB.Ping and DB.PingContext methods will use that interface to check whether a database connection is still valid.

The new Context query methods work for all drivers, but Context cancelation is not responsive unless the driver has been updated to use them. The other features require driver support in database/sql/driver. Driver authors should review the new interfaces. Users of existing driver should review the driver documentation to see what it supports and any system specific documentation on each feature.

The package has been fleshed out and is now used by the Go linker. New are Reloc, Section, StringTable, the method COFFSymbol.FullName, and File fields COFFSymbols and StringTable.

The new Encoding.Strict method returns an Encoding that causes the decoder to return an error when the trailing padding bits are not zero.

Read and Write now support booleans.

UnmarshalTypeError now includes the struct and field name.

A nil Marshaler now marshals as a JSON null value.

A RawMessage value now marshals the same as its pointer type.

Marshal encodes floating-point numbers using the same format as in ES6, preferring decimal (not exponential) notation for a wider range of values. In particular, all floating-point integers up to 264 format the same as the equivalent int64 representation.

In previous versions of Go, unmarshaling a JSON null into an Unmarshaler was considered a no-op; now the Unmarshaler’s UnmarshalJSON method is called with the JSON literal null and can define the semantics of that case.

Decode is now strict about the format of the ending line.

Unmarshal now has wildcard support for collecting all attributes using the new “,any,attr” struct tag.

The new methods Int.Value, String.Value, Float.Value, and Func.Value report the current value of an exported variable.

The new function Handler returns the package’s HTTP handler, to enable installing it in non-standard locations.

Scanf, Fscanf, and Sscanf now handle spaces differently and more consistently than previous releases. See the scanning documentation for details.

The new IsPredeclared function reports whether a string is a predeclared identifier.

The new function Default returns the default “typed” type for an “untyped” type.

The alignment of complex64 now matches the Go compiler.

The package now validates the “type” attribute on a <script> tag.

Decode (and DecodeConfig) now supports True Color and grayscale transparency.

Encoder is now faster and creates smaller output when encoding paletted images.

The new method Int.Sqrt calculates ⌊√x⌋.

The new method Float.Scan is a support routine for fmt.Scanner.

Int.ModInverse now supports negative numbers.

The new Rand.Uint64 method returns uint64 values. The new Source64 interface describes sources capable of generating such values directly; otherwise the Rand.Uint64 method constructs a uint64 from two calls to Source’s Int63 method.

ParseMediaType now preserves unnecessary backslash escapes as literals, in order to support MSIE. When MSIE sends a full file path (in “intranet mode”), it does not escape backslashes: “C:\dev\go\foo.txt”, not “C:\dev\go\foo.txt”. If we see an unnecessary backslash escape, we now assume it is from MSIE and intended as a literal backslash. No known MIME generators emit unnecessary backslash escapes for simple token characters like numbers and letters.

The Reader’s parsing has been relaxed in two ways to accept more input seen in the wild. First, it accepts an equals sign (=) not followed by two hex digits as a literal equal sign. Second, it silently ignores a trailing equals sign at the end of an encoded input.

The Conn documentation has been updated to clarify expectations of an interface implementation. Updates in the net/http packages depend on implementations obeying the documentation.

Updating: implementations of the Conn interface should verify they implement the documented semantics. The golang.org/x/net/nettest package will exercise a Conn and validate it behaves properly.

The new method UnixListener.SetUnlinkOnClose sets whether the underlying socket file should be removed from the file system when the listener is closed.

The new Buffers types permits more efficiently writing to the network from multiple discontiguous buffers in memory. On certain machines, for certain types of connections, this is optimized into an OS-specific batch write operation (such as writev).

The new Resolver looks up names and numbers and supports context.Context. The Dialer now has an optional Resolver field.

Interfaces is now supported on Solaris.

The Go DNS resolver now supports resolv.conf’s “rotate” and “option ndots:0” options. The “ndots” option is now respected in the same way as libresolve.

Server changes:

The server now supports graceful shutdown support, mentioned above.
The Server adds configuration options ReadHeaderTimeout and IdleTimeout and documents WriteTimeout.
FileServer and ServeContent now support HTTP If-Match conditional requests, in addition to the previous If-None-Match support.
There are several additions to what a server’s Handler can do:

The Context returned by Request.Context is canceled if the underlying net.Conn closes. For instance, if the user closes their browser in the middle of a slow request, the Handler can now detect that the user is gone. This complements the existing CloseNotifier support. This functionality requires that the underlying net.Conn implements recently-clarified interface documentation.
To serve trailers known after the header has been written, see the new TrailerPrefix mechanism.
A Handler can now abort a response by panicking with the error ErrAbortHandler.
A Write of zero bytes to a ResponseWriter is now defined as a way to test whether a ResponseWriter has been hijacked: if so, the Write returns ErrHijacked without printing an error to the server’s error log.
Client & Transport changes:

The Client now copies most request headers on redirect. See the documentation on the Client type for details.
The Transport now supports international domain names. Consequently, so do Get and other helpers.
The Client now supports 307 and 308 redirects. If the redirect requires resending the request body, the request must have the new Request.GetBody field defined. NewRequest sets Request.GetBody automatically for common body types.
The Transport now rejects requests for URLs with ports containing non-digit characters.
The Transport will now retry non-idempotent requests if no bytes were written before a network failure.
The new Transport.ProxyConnectHeader allows configuration of header values to send to a proxy during a CONNECT request.
The DefaultTransport.Dialer now enables DualStack (“Happy Eyeballs”) support, to use IPv4 as a backup if it looks like IPv6 might be failing.
There is now support for tracing a client request’s TLS handshakes with the new ClientTrace.TLSHandshakeStart and ClientTrace.TLSHandshakeDone.

The ReverseProxy has a new optional hook, ModifyResponse, for modifying the response from the backend before proxying it to the client.

Empty quoted strings are once again allowed in the name part of an address. That is, Go 1.4 and earlier accepted “” gopher@example.com, but Go 1.5 introduced a bug that rejected this address. The address is recognized again.

The Header.Date method has always provided a way to parse the Date: header. A new function ParseDate allows parsing dates found in other header lines, such as the Resent-Date: header.

If an implementation of the Auth interface’s Start method returns an empty toServer value, the package no longer sends trailing whitespace in the SMTP AUTH command, which some servers rejected.

The new functions PathEscape and PathUnescape are similar to the query escaping and unescaping functions but for path elements.

The new methods URL.Hostname and URL.Port return the hostname and port fields of a URL, correctly handling the case where the port may not be present.

The existing method URL.ResolveReference now properly handles paths with escaped bytes without losing the escaping.

The URL type now implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler, making it possible to process URLs in gob data.

Following RFC 3986, Parse now rejects URLs like this_that:other/thing instead of interpreting them as relative paths (this_that is not a valid scheme). To force interpretation as a relative path, such URLs should be prefixed with “./“. The URL.String method now inserts this prefix as needed.

The new function Executable returns the path name of the running executable.

An attempt to call a method on an os.File that has already been closed will now return the new error value os.ErrClosed. Previously it returned a system-specific error such as syscall.EBADF.

On Unix systems, os.Rename will now return an error when used to rename a directory to an existing empty directory. Previously it would fail when renaming to a non-empty directory but succeed when renaming to an empty directory. This makes the behavior on Unix correspond to that on other systems.

On Windows, long absolute paths are now transparently converted to extended-length paths (paths that start with \?). This permits the package to work with files whose path names are longer than 260 characters.

On Windows, os.IsExist will now return true for the system error ERROR_DIR_NOT_EMPTY. This roughly corresponds to the existing handling of the Unix error ENOTEMPTY.

On Plan 9, files that are not served by #M will now have ModeDevice set in the value returned by FileInfo.Mode.

A number of bugs and corner cases on Windows were fixed: Abs now calls Clean paths as documented, Glob now matches “\?\c:*“, EvalSymlinks now correctly handles “C:.”, and Clean now properly handles a leading “..” in the path.

The new function Swapper was added to support sort.Slice.

The Unquote function now strips carriage returns (\r) in backquoted raw strings, following the Go language semantics.

The Getpagesize now returns the system’s size, rather than a constant value. Previously it always returned 4KB.

The signature of Utimes has changed on Solaris to match all the other Unix systems’ signature. Portable code should continue to use os.Chtimes instead.

The X__cmsg_data field has been removed from Cmsghdr.

Template.Execute can now take a reflect.Value as its data argument, and FuncMap functions can also accept and return reflect.Value.

The new function Until complements the analogous Since function.

ParseDuration now accepts long fractional parts.

Parse now rejects dates before the start of a month, such as June 0; it already rejected dates beyond the end of the month, such as June 31 and July 32.

The tzdata database has been updated to version 2016i for systems that don’t already have a local time zone database.

The new method T.Name (and B.Name) returns the name of the current test or benchmark.

The new method T.Context (and B.Context) returns a Context for the current running test or benchmark.

The new function CoverMode reports the test coverage mode.

Tests and benchmarks are now marked as failed if the race detector is enabled and a data race occurs during execution. Previously, individual test cases would appear to pass, and only the overall execution of the test binary would fail.

SimpleFold now returns its argument unchanged if the provided input was an invalid rune. Previously, the implementation failed with an index bounds check panic.

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