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,
produces smaller binaries (TODO: link/numbers),
<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>

<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.
</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>


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

<p>
There are some instabilities on FreeBSD 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>
and <a href="https://golang.org/issue/16396">issue 16396</a>.
Any help in solving these FreeBSD-specific 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>
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” repo 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>
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>
  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>
  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>

<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 core 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>

<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">Core library</h2>

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

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

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

<p>
The <a href="/pkg/sort/">sort</a> package
now includes a convenience function
<a href="/pkg/sort/#Slice"><code>sort.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>sort.SliceStable</code></a> and
<a href="/pkg/sort/#SliceIsSorted"><code>sort.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>
      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="compress_flate"><dt><a href="/pkg/compress/flate/">compress/flate</a></dt>
  <dd>

    <p>
      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, ping, 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>

  </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 MTIME field is non-zero.
    </p>

  </dd>
</dl>