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 is one minor change 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, and simplifies sorting slices.

Changes to the language

When explicitly converting structs, tags are now ignored for structural type identity. See language specification for details.

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.

Ports

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

Go now requires DragonFly BSD 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 continue supporting 10.8. Compiling Go or running binaries on older OS X versions is untested.

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.

Tools

Assembler

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

For 64-bit PPC systems, the common vector scalar instructions have been added: LXS, LXSDX, LXSI, LXSIWAX, LXSIWZX, LXV, LXVD2X, LXVDSX, LXVW4X, MFVSR, MFVSRD, MFVSRWZ, MTVSR, MTVSRD, MTVSRWA, MTVSRWZ, STXS, STXSDX, STXSI, STXSIWX, STXV, STXVD2X, STXVW4X, XSCV, XSCVDPSP, XSCVDPSPN, XSCVDPSXDS, XSCVDPSXWS, XSCVDPUXDS, XSCVDPUXWS, XSCVSPDP, XSCVSPDPN, XSCVSXDDP, XSCVSXDSP, XSCVUXDDP, XSCVUXDSP, XSCVX, XSCVXP, XVCV, XVCVDPSP, XVCVDPSXDS, XVCVDPSXWS, XVCVDPUXDS, XVCVDPUXWS, XVCVSPDP, XVCVSPSXDS, XVCVSPSXWS, XVCVSPUXDS, XVCVSPUXWS, XVCVSXDDP, XVCVSXDSP, XVCVSXWDP, XVCVSXWSP, XVCVUXDDP, XVCVUXDSP, XVCVUXWDP, XVCVUXWSP, XVCVX, XVCVXP, XXLAND, XXLANDC, XXLANDQ, XXLEQV, XXLNAND, XXLNOR, XXLOR, XXLORC, XXLORQ, XXLXOR, XXMRG, XXMRGHW, XXMRGLW, XXPERM, XXPERMDI, XXSEL, XXSI, XXSLDWI, XXSPLT, and XXSPLTW.

Yacc

The yacc tool (previously available via “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 and is available via go get golang.org/x/tools/cmd/goyacc.

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 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 N-M%. TODO: ARM binary size & perf 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 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.

Due to changes across the compiler toolchain and standard library, binaries built with this release should typically be smaller than binaries built with Go 1.7, sometimes by as much as TODO numbers.

Cgo

TODO

Gccgo

TODO

Go command

The go command's basic operation is unchanged, but there are a number of changes worth noting.

A new “go bug” command helps users file bug reports.

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 the doc's output, each summary of the first-level items is guaranteed to occupy a single line.

Documentation for interface methods is now only shown when requested explicitly.

Plugins

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.

Runtime

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.

MemStats Documentation

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

Performance

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 significant optimizations bringing more than 10% improvements to implementations in the TODO TODO: foo, bar, and quux packages.

Garbage Collector

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

Defer

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

Cgo

Calls from Go into C are now TODO% faster.

Standard library

Examples

Examples have been added to the documentation across many packages.

Sort

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. The related and more abrupt Server.Close is also new.

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.

archive/tar

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.

archive/zip

The zip Reader now supports modification times in the NTFS, UNIX, and Extended Time Stamp metadata fields. When writing zip files, the Extended Time Stamp field is written if TODO: validate implementation.

compress/flate

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.

compress/gzip

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 updates the Header.ModTime field only if the encoded MTIME field is non-zero.

context

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

database/sql

The package now supports context.Context. There are new methods ending in Context such as DB.QueryContext and DB.PrepareContext that support Context. By using the new Context methods it ensures connections are closed and returned to the connection pool when the request is done. It also enables canceling in-progress queries should the driver support it. Finally, using the Context methods 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.

Multiple result sets are now supported on Rows. After Rows.Next returns false Rows.NextResultSet may be called to advance to the next result set. The existing Rows should be continued to be used after it advances to the next result set.

NamedParam may be used as query arguments. The new function Param helps create a NamedParam more succinctly.

Drivers that support the new Pinger interface can now check if the server is still alive when the the DB.Ping or DB.PingContext is called.

The new Context query methods work for all drivers, but Context cancelation is not responsive unless the driver has been updated to used 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.

debug/pe

TODO: describe Issue 15345 and CL 22181, CL 22332, CL 22336 

pkg debug/pe, method (*COFFSymbol) FullName(StringTable) (string, error)
pkg debug/pe, method (StringTable) String(uint32) (string, error)
pkg debug/pe, type File struct, COFFSymbols []COFFSymbol
pkg debug/pe, type File struct, StringTable StringTable
pkg debug/pe, type Reloc struct
pkg debug/pe, type Reloc struct, SymbolTableIndex uint32
pkg debug/pe, type Reloc struct, Type uint16
pkg debug/pe, type Reloc struct, VirtualAddress uint32
pkg debug/pe, type Section struct, Relocs []Reloc
pkg debug/pe, type StringTable []uint8

encoding/base64

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

encoding/binary

The package now supports boolean values. TODO: add docs in encoding/binary package, then link to package docs here.

encoding/json

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.

Implementations of Unmarshaler are now called with the literal "null" and can decide how to handle it.

encoding/pem

The PEM decoder is now strict about the format of the ending line.

expvar

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.

image/png

The PNG decoder now supports truecolor and grayscale transparency.

The PNG encoder is now faster and creates smaller output when encoding paletted images.

math/rand

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

mime/quotedprintable

The package's parsing has been relaxed in two ways to accept more input seen in the wild. First, it now accepts a = sign even if it's not followed by two hex digits. Second, it accepts a trailing soft line-break at the end of a message. . That is, the final byte of the message may be a = sign and it will now be ignored.

net/http

Server changes:

There are several additions to what a server's Handler can do:

Client & Transport changes:

net/mail

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.

net/smtp

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.

net/url

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 are accessors to the hostname and port fields of a URL and deal with the case where the port may or 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.

os

The new function Executable returns the running executable path name.

reflect

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

strconv

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

time

The new function Until complements the analogous Since function.

ParseDuration now accepts long fractional parts.

Parse now validates days that are too small, in addition to days that are too large.

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

testing

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.

unicode

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