go/doc/go1.8.html

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<h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.8</h2>

<p><strong>
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.
</strong></p>

<p>
The latest Go release, version 1.8, arrives six months after <a href="go1.7">Go 1.7</a>.
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 <a href="/doc/go1compat.html">promise of compatibility</a>.
We expect almost all Go programs to continue to compile and run as before.
</p>

<p>
The release <a href="#ports">adds support for 32-bit MIPS</a>,
<a href="#compiler">updates the compiler back end</a> to generate more efficient code,
<a href="#gc">reduces GC pauses</a> by eliminating stop-the-world stack rescanning,
<a href="#h2push">adds HTTP/2 Push support</a>,
<a href="#httpshutdown">adds HTTP graceful shutdown</a>,
and <a href="#sort">simplifies sorting slices</a>.
</p>

<h2 id="language">Changes to the language</h2>

<p>
  When explicitly converting structs, tags are now ignored for structural type identity.
  See <a href="https://golang.org/cl/24190">change 24190</a> for details.
</p>

<p> <!-- CL 17711 -->
  The language specification now only requires that implementations
  support up to 16-bit exponents in constants.  This does not affect
  either the <code>gc</code> or <code>gccgo</code> compilers, both of
  which still support 32-bit exponents.
</p>

<h2 id="ports">Ports</h2>

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

<p>
Go now requires DragonFly BSD 4.4.4 or later. <!-- CL 29491 -->
</p>

<p>
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.
</p>

<p>
  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.
</p>


<h3 id="known_issues">Known Issues</h3>

<p>
There are some instabilities on FreeBSD and NetBSD that are known but not understood.
These can lead to program crashes in rare cases.
See
<a href="https://golang.org/issue/15658">issue 15658</a>,
<a href="https://golang.org/issue/16396">issue 16396</a>, and
<a href="https://golang.org/issue/16511">issue 16511</a>.
Any help in solving these issues would be appreciated.
</p>

<h2 id="tools">Tools</h2>

<h3 id="cmd_asm">Assembler</h3>

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

<p>
For 64-bit PPC systems, the common vector scalar instructions have been
added. See <a href="https://golang.org/cl/30510">change 30510</a>.
</p>

<h3 id="tool_yacc">Yacc</h3>

<p> <!-- CL 27324, CL 27325 -->
The <code>yacc</code> (previously available via
“<code>go</code> <code>tool</code> <code>yacc</code>”)
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
<code>go get <a href="https://godoc.org/golang.org/x/tools/cmd/goyacc">golang.org/x/tools/cmd/goyacc</a></code>.
</p>

<h3 id="compiler">Compiler Toolchain</h3>

<p>
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.
</p>

<p>
The new back end, based on
<a href="https://en.wikipedia.org/wiki/Static_single_assignment_form">SSA</a>,
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
<a href="https://golang.org/test/bench/go1/">our benchmark programs</a> by N-M%.
TODO: ARM binary size & perf numbers.
</p>

<p>
The temporary <code>-ssa=0</code> flag introduced in Go 1.7 to disable
the new backend has been removed in Go 1.8.
</p>

<p>
  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.
</p>

<p>
  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.
</p>

<p>
  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.
</p>


<h3 id="cmd_cgo">Cgo</h3>

<p>TODO</p>

<h3 id="gccgo">Gccgo</h3>

<p>TODO</p>

<h3 id="cmd_go">Go command</h3>

<p>
The <a href="/cmd/go/"><code>go</code></a> command's basic operation
is unchanged, but there are a number of changes worth noting.
</p>

<p>
A new “<code>go</code> <code>bug</code>” command helps users file bug reports.
</p>

<h3 id="cmd_doc">Go doc</h3>

<p> <!-- CL 25419 -->
  The “<code>go</code> <code>doc</code>” command
  now groups constants and variables with their type,
  following the behavior of
  <a href="/cmd/godoc/"><code>godoc</code></a>.
</p>

<p> <!-- CL 25420 -->
  In order to improve the readability of the <code>doc</code>'s
  output, each summary of the first-level items is guaranteed to
  occupy a single line.
</p>

<p> <!-- CL 31852 -->
  Documentation for interface methods is now only shown when requested
  explicitly.
</p>

<h3 id="plugin">Plug-ins</h3>

<p>
  Go now supports a “<code>plugin</code>” build mode for generating
  plug-ins written in Go, and a
  new <a href="/pkg/plugin/"><code>plugin</code></a> package for
  loading such plug-ins at run time. Plug-ins support is only currently
  available on Linux and Darwin.
</p>

<h2 id="runtime">Runtime</h2>

<h3 id="liveness">Argument Liveness</h3>

<p>
  <!-- Issue 15843 --> 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 <a href="/pkg/runtime/#KeepAlive"><code>runtime.KeepAlive</code></a>
  function added in Go 1.7.
</p>

<h3 id="memstats">MemStats Documentation</h3>

<p> <!-- CL 28972 -->
  The runtime's <a href="/pkg/runtime/#MemStats"><code>MemStats</code></a>
  type has been more thoroughly documented.
</p>

<h2 id="performance">Performance</h2>

<p>
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.
</p>

<p>
There have been significant optimizations bringing more than 10% improvements
to implementations in the
TODO TODO:
<a href="/pkg/foo/"><code>foo</code></a>,
<a href="/pkg/bar/"><code>bar</code></a>,
and
<a href="/pkg/quux/"><code>quux</code></a>
packages.
</p>

<h3 id="gc">Garbage Collector</h3>

<p>
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
<a href="https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md">
document on eliminating stop-the-world stack re-scanning</a> for details.
More work remains for Go 1.9.
</p>

<h3 id="defer">Defer</h3>

<!-- CL 29656, CL 29656 -->
<p><code>defer</code> calls are now about twice as fast.</p>

<h3 id="cgoperf">Cgo</h3>

<p>Calls from Go into C are now TODO% faster.</p>

<h2 id="library">Standard library</h2>

<h3 id="examples">Examples</h3>

<p>
Examples have been added to the documentation across many packages.
</p>

<h3 id="sortslice">Sort</h3>

<p>
The <a href="/pkg/sort/">sort</a> package
now includes a convenience function
<a href="/pkg/sort/#Slice"><code>Slice</code></a> to sort a
slice given a <em>less</em> function.

In many cases this means that writing a new sorter type is not
necessary.
</p>

<p>
Also new are
<a href="/pkg/sort/#SliceStable"><code>SliceStable</code></a> and
<a href="/pkg/sort/#SliceIsSorted"><code>SliceIsSorted</code></a>.
</p>

<h3 id="h2push">HTTP/2 Push</h3>

<p>
The <a href="/pkg/net/http/">net/http</a> package now includes a
mechanism to
send HTTP/2 server pushes from a
<a href="/pkg/net/http/#Handler"><code>Handler</code></a>.
Similar to the existing <code>Flusher</code> and <code>Hijacker</code>
interfaces, an HTTP/2
<a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a>
now implements the new
<a href="/pkg/net/http/#Pusher"><code>Pusher</code></a> interface.
</p>

<h3 id="httpshutdown">HTTP Server Graceful Shutdown</h3>

<h3 id="minor_library_changes">Minor changes to the library</h3>

<p>
As always, there are various minor changes and updates to the library,
made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
in mind.
</p>

<dl id="archive_tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
  <dd>

    <p> <!-- CL 28471, CL 31440, CL 31441, CL 31444, CL 28418, CL 31439 -->
      The tar implementation corrects many bugs in corner cases of the file format.
      The <a href="/pkg/archive/tar/#Reader"><code>Reader</code></a>
      is now able to process tar files in the PAX format with entries larger than 8GB.
      The <a href="/pkg/archive/tar/#Writer"><code>Writer</code></a>
      no longer produces invalid tar files in some situations involving long pathnames.
    </p>

  </dd>
</dl>

<dl id="archive_zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
  <dd>

    <p> <!-- CL 18274 -->
      The zip <code>Reader</code> now supports modification times in
      the NTFS, UNIX, and Extended Time Stamp metadata fields.
      <!-- CL 30811 -->
      When writing zip files, the Extended Time Stamp field is written
      if TODO: validate implementation.
    </p>

  </dd>
</dl>

<dl id="compress_flate"><dt><a href="/pkg/compress/flate/">compress/flate</a></dt>
  <dd>

    <p> <!-- CL 31640, CL 31174, CL 32149 -->
      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 <code>DEFLATE</code> 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.
    </p>

    <p>
      The encoder, when operating in
      <a href="/pkg/compress/flate/#NoCompression"><code>NoCompression</code></a>
      mode, now produces a consistent output that is not dependent on
      the size of the input buffer to the
      <a href="/pkg/compress/flate/#Writer.Write"><code>Write</code></a>
      method.
    </p>

    <p> <!-- CL 28216 -->
      The decoder, upon encountering an error, now returns any
      buffered data it had uncompressed along with the error.
    </p>

  </dd>
</dl>


<dl id="compress_gzip"><dt><a href="/pkg/compress/gzip/">compress/gzip</a></dt>
  <dd>

    <p>
      The <a href="/pkg/compress/gzip/#Writer"><code>Writer</code></a>
      now encodes a zero <code>MTIME</code> field when
      the <a href="/pkg/compress/gzip/#Header"><code>Header.ModTime</code></a>
      field is the zero value.

      In previous releases of Go, the <code>Writer</code> would encode
      a non-sensible value.

      Similarly,
      the <a href="/pkg/compress/gzip/#Reader"><code>Reader</code></a>
      now updates the <code>Header.ModTime</code> field only if the
      encoded <code>MTIME</code> field is non-zero.
    </p>

  </dd>
</dl>

<dl id="context"><dt><a href="/pkg/context/">context</a></dt>
  <dd>
    <p> <!-- CL 30370 -->
      The <a href="/pkg/context#DeadlineExceeded"><code>DeadlineExceeded</code></a>
      error now implements
      <a href="/pkg/net/#Error"><code>net.Error</code></a>
      and reports true for both the <code>Timeout</code> and
      <code>Temporary</code> methods.
    </p>
  </dd>
</dl>

<dl id="debug_pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
  <dd>
    <p> <!-- CL 22720, CL 27212, CL 22181, CL 22332, CL 22336, Issue 15345 -->
      TODO: describe Issue 15345 and CL 22181, CL 22332, CL 22336
      <pre>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</pre>
      </p>
  </dd>
</dl>

<dl id="encoding_base64"><dt><a href="/pkg/encoding/base64/">encoding/base64</a></dt>
  <dd>
    <p> <!-- CL 24964 -->
      The new <a href="/pkg/encoding/base64/#Encoding.Strict"><code>Encoding.Strict</code></a> method returns an <code>Encoding</code> that causes the decoder to return an error when the trailing padding bits are not zero.
    </p>
  </dd>
</dl>

<dl id="encoding_binary"><dt><a href="/pkg/encoding/binary/">encoding/binary</a></dt>
  <dd>
    <p> <!-- CL 28514 -->
      The package now supports boolean values.
      TODO: add docs in encoding/binary package, then link to package docs
      here.
    </p>
  </dd>
</dl>

<dl id="encoding_json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
  <dd>

    <p> <!-- CL 18692  -->
      <a href="/pkg/encoding/json/#UnmarshalTypeError"><code>UnmarshalTypeError</code></a>
      now includes the struct and field name.
    </p>

    <p> <!-- CL 31932 -->
      A <code>nil</code> <a href="/pkg/encoding/json/#Marshaler"><code>Marshaler</code></a>
      now marshals as a JSON "<code>null</code>" value.
    </p>

    <p> <!-- CL 21811 -->
      A <a href="/pkg/encoding/json/#RawMessage"><code>RawMessage</code></a> value now
      marshals the same as its pointer type.
    </p>

    <p> <!-- CL 30371 -->
      Numbers are now formatted in the same floating point format as they are in ES6.
    </p>

    <p> <!-- CL 30944 -->
      Implementations
      of <a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>
      are now called with the literal "<code>null</code>" and can
      decide what to do with it.
    </p>

  </dd>
</dl>

<dl id="encoding_pem"><dt><a href="/pkg/encoding/pem/">encoding/pem</a></dt>
  <dd>
    <p> <!-- CL 27391 -->
      The PEM decoder is now strict about the format of the ending line.
    </p>
  </dd>
</dl>

<dl id="expvar"><dt><a href="/pkg/expvar/">expvar</a></dt>
  <dd>
    <p> <!-- CL 30917 -->
      The new methods
      <a href="/pkg/expvar/#Int.Value"><code>Int.Value</code></a>,
      <a href="/pkg/expvar/#String.Value"><code>String.Value</code></a>,
      <a href="/pkg/expvar/#Float.Value"><code>Float.Value</code></a>, and
      <a href="/pkg/expvar/#Func.Value"><code>Func.Value</code></a>
      report the current value of an exported variable.
    </p>

    <p> <!-- CL 24722 -->
      The new
      function <a href="/pkg/expvar/#Handler"><code>Handler</code></a>
      returns the package's HTTP handler, to enable installing it in
      non-standard locations.
      </p>
  </dd>
</dl>


<dl id="image_png"><dt><a href="/pkg/image/png/">image/png</a></dt>
  <dd>
    <p> <!-- CL 32143, CL 32140 -->
      The PNG decoder now supports truecolor and grayscale transparency.
    </p>
    <p> <!-- CL 29872 -->
      The PNG encoder is now faster and creates smaller output
      when encoding paletted images.
      </p>
  </dd>
</dl>

<dl id="math_rand"><dt><a href="/pkg/math/rand/">math/rand</a></dt>
  <dd>

    <p>
      There is a new
      method <a href="/pkg/math/rand/#Rand.Uint64"><code>Rand.Uint64</code></a>
      to return <code>uint64</code> values. <!-- CL 27253 -->
    </p>

  </dd>
</dl>

<dl id="mime_quotedprintable"><dt><a href="/pkg/mime/quotedprintable/">mime/quotedprintable</a></dt>
  <dd>

    <p>
      The package's parsing has been relaxed in two ways to accept
      more input seen in the wild. First, it now accepts
      a <code>=</code> sign even if it's not followed by two hex
      digits. <!-- CL 32174 -->

      Second, it accepts a trailing soft line-break at the end of a
      message. <!-- CL 27530 -->. That is, the final byte of the
      message may be a <code>=</code> sign and it will now be ignored.
    </p>

  </dd>
</dl>


<dl id="net_http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>

    <p>
      TODO
    </p>

  </dd>
</dl>

<dl id="net_mail"><dt><a href="/pkg/net/mail/">net/mail</a></dt>
  <dd>

    <p> <!-- CL 32176 -->
      Empty quoted strings are now allowed in the name part of an
      address. That is, this is now a legal address:
      <code>"" &lt;gopher@example.com&gt;</code>
    </p>

    <p> <!-- CL 31581 -->
      The <a href="/pkg/net/mail/#ParseDate"><code>ParseDate</code></a>
      function has been exported.
    </p>
    
  </dd>
</dl>

<dl id="net_smtp"><dt><a href="/pkg/net/smtp/">net/smtp</a></dt>
  <dd>

    <p> <!-- CL 33143 -->
      If implementations of the <a href="/pkg/net/smtp/#Auth"><code>Auth</code></a>
      interface return an empty <code>toServer</code> value, the package
      no longer sends trailing whitespace after the SMTP <code>AUTH</code>
      command, which some servers rejected.
    </p>

  </dd>
</dl>

<dl id="net_url"><dt><a href="/pkg/net/url/">net/url</a></dt>
  <dd>

    <p> <!-- CL 31322 --> The new functions
      <a href="/pkg/net/url/#PathEscape"><code>PathEscape</code></a>
      and
      <a href="/pkg/net/url/#PathUnescape"><code>PathUnescape</code></a>
      are similar to the query escaping and unescaping functions but
      don't treat spaces specially.</p>

    <p> <!-- CL 28933 --> The new methods
      <a href="/pkg/net/url/#URL.Hostname"><code>URL.Hostname</code></a>
      and
      <a href="/pkg/net/url/#URL.Port"><code>URL.Port</code></a>
      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.
    </p>

    <p> <!-- CL 28343 --> The existing method
      <a href="/pkg/net/url/#URL.ResolveReference"><code>URL.ResolveReference</code></a>
      now properly handles paths with escaped bytes without losing
      the escaping.
    </p>

    <p> <!-- CL 31467 -->
      The <code>URL</code> type now implements
      <a href="/pkg/encoding/#BinaryMarshaler"><code>encoding.BinaryMarshaler</code></a> and
      <a href="/pkg/encoding/#BinaryUnmarshaler"><code>encoding.BinaryUnmarshaler</code></a>.
    </p>

    <p> <!-- CL 29610, CL 31582 -->
      <code>Parse</code> now rejects relative URLs containing a "<code>:</code>" in
      the first path segment. Such paths should be prefixed with "<code>./</code>".
      The <code>URL.String</code> method now prepends "<code>./</code>" to such paths.
    </p>

  </dd>
</dl>

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p> <!-- CL 30088 -->
      The new function
      <a href="/pkg/reflect/#Swapper"><code>Swapper</code></a> was
      added to support
      the <a href="#sortslice">new <code>sort.Slice</code>
      support</a>.
    </p>
  </dd>
</dl>


<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
  <dd>
    <p> <!-- CL 31210 -->
      The <a href="/pkg/strconv/#Unquote"><code>Unquote</code></a>
      function now strips carriage returns (<code>\r</code>) in
      backquoted raw strings.
    </p>
  </dd>
</dl>

<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
  <dd>

    <p> <!-- CL 20118 --> The new function
      <a href="/pkg/time/#Until"><code>Until</code></a> complements
      the analogous <code>Since</code> function.
    </p>

    <p> <!-- CL 29338 --> <code>ParseDuration</code> now accepts long fractional parts.</p>

    <p> <!-- CL 33429 -->
      <code>Parse</code> now validates days that are too small, in
      addition to days that are too large.
    </p>

    <p> <!-- CL 33029 -->
      The <code>tzdata</code> database has been updated to version
      2016i for systems that don't already have a local time zone
      database.
    </p>

    <p>
  </dd>
</dl>

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- CL 29970 -->
      The new method
      <a href="/pkg/testing/#T.Name"><code>T.Name</code></a>
      (and <code>B.Name</code>) returns the name of the current
      test or benchmark.
    </p>

    <p><!-- CL 31724 -->
      The new method
      <a href="/pkg/testing/#T.Context"><code>T.Context</code></a>
      (and <code>B.Context</code>) returns
      a <a href="/pkg/context/#Context"><code>Context</code></a> for
      the current running test or benchmark.
    </p>

    <p><!-- CL 32483 -->
      The new function
      <a href="/pkg/testing/#CoverMode"><code>CoverMode</code></a>
      reports what the test coverage mode is set to.
    </p>

    <p><!-- CL 32615 -->
      Tests and benchmarks are now marked as failed if the race
      detector is enabled and a data race occurs during execution.
    </p>
   
  </dd>
</dl>

<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
  <dd>
    <p><!-- CL 30935 -->
      <code>SimpleFold</code> now returns its argument unchanged
      if the provided input was an invalid rune.
    </p>
  </dd>
</dl>
                             



<dl id="foo"><dt><a href="/pkg/foo/">foo</a></dt>
  <dd>
    <p> <!-- CL nnn -->

    </p>
  </dd>
</dl>