go/doc/go1.5.html

<!--{
	"Title": "Go 1.5 Release Notes",
	"Path":  "/doc/go1.5",
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<h2 id="introduction">Introduction to Go 1.5</h2>

<p>
The latest Go release, version 1.5,
is a significant release, including major architectural changes to the implementation.
Despite that, we expect almost all Go programs to continue to compile and run as before,
because the release still maintains the Go 1 <a href="/doc/go1compat.html">promise
of compatibility</a>.
</p>

<p>
The biggest developments in the implementation are:
</p>

<ul>

<li>
The compiler and runtime are now written entirely in Go (with a little assembler).
C is no longer involved in the implementation, and so the C compiler that was
once necessary for building the distribution is gone.
</li>

<li>
The garbage collector is now <a href="/s/go14gc">concurrent</a> and provides dramatically lower
pause times by running, when possible, in parallel with other goroutines.
</li>

<li>
By default, Go programs run with <code>GOMAXPROCS</code> set to the
number of cores available; in prior releases it defaulted to 1.
</li>

<li>
Support for <a href="http://golang.org/s/go14internal">internal packages</a>
is now provided for all repositories, not just the Go core.
</li>

<li>
The <code>go</code> command now provides <a href="/s/go15vendor">experimental
support</a> for "vendoring" external dependencies.
</li>

</ul>

<p>
These and a number of other changes to the implementation and tools
are discussed below.
</p>

<p>
The release also contains one small language change involving map literals.
</p>

<p>
Finally, the timing of the <a href="/s/releasesched">release</a>
strays from the usual six-month interval,
both to provide more time to prepare this major release and to shift the schedule thereafter to
time the release dates more conveniently.
</p>

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

<h3 id="mapliterals">Map literals</h3>

<p>
Due to an oversight, the rule that allowed the element type to be elided from slice literals was not
applied to map keys.
This has been <a href="/cl/2591">corrected</a> in Go 1.5.
An example will make this clear: as of Go 1.5, this map literal,
</p>

<pre>
m := map[Point]string{
    Point{29.935523, 52.891566}:   "Persepolis",
    Point{-25.352594, 131.034361}: "Uluru",
    Point{37.422455, -122.084306}: "Googleplex",
}
</pre>

<p>
may be written as follows, without the <code>Point</code> type listed explicitly:
</p>

<pre>
m := map[Point]string{
    {29.935523, 52.891566}:   "Persepolis",
    {-25.352594, 131.034361}: "Uluru",
    {37.422455, -122.084306}: "Googleplex",
}
</pre>

<h2 id="implementation">The Implementation</h2>

<h3 id="c">No more C</h3>

<p>
The compiler and runtime are now implemented in Go and assembler, without C.
The only C source left in the tree is related to testing or to <code>cgo</code>.
There was a C compiler in the tree in 1.4 and earlier.
It was used to build the runtime; a custom compiler was necessary in part to
guarantee the C code would work with the stack management of goroutines.
Since the runtime is in Go now, there is no need for this C compiler and it is gone.
Details of the process to eliminate C are discussed <a href="/s/go13compiler">elsewhere</a>.
</p>

<p>
The conversion from C was done with the help of custom tools created for the job.
Most important, the compiler was actually moved by automatic translation of
the C code into Go.
It is in effect the same program in a different language.
It is not a new implementation
of the compiler so we expect the process will not have introduced new compiler
bugs.
An overview of this process is available in the slides for
<a href="https://talks.golang.org/2015/gogo.slide">this presentation</a>.
</p>

<h3 id="compiler">Compiler and tools</h3>

<p>
Independent of but encouraged by the move to Go, the names of the tools have changed.
The old names <code>6g</code>, <code>8g</code> and so on are gone; instead there
is just one binary, accessible as <code>go</code> <code>tool</code> <code>compile</code>,
that compiles Go source into binaries suitable for the architecture and operating system
specified by <code>$GOARCH</code> and <code>$GOOS</code>.
Similarly, there is now one linker (<code>go</code> <code>tool</code> <code>link</code>)
and one assembler (<code>go</code> <code>tool</code> <code>asm</code>).
The linker was translated automatically from the old C implementation,
but the assembler is a new native Go implementation discussed
in more detail below.
</p>

<p>
Similar to the drop of the names <code>6g</code>, <code>8g</code>, and so on,
the output of the compiler and assembler are now given a plain <code>.o</code> suffix
rather than <code>.8</code>, <code>.6</code>, etc.
</p>


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

<p>
TODO
</p>

<h3 id="runtime">Runtime</h3>

<p>
In Go 1.5, the order in which goroutines are scheduled has been changed.
The properties of the scheduler were never defined by the language,
but programs that depended on the scheduling order may be broken
by this change.
We have seen a few (erroneous) programs affected by this change.
If you have programs that implicitly depend on the scheduling
order, you will need to update them.
</p>

<p>
Another potentially breaking change is that the runtime now
sets the default number of threads to run simultaneously,
defined by <code>GOMAXPROCS</code>, to the number
of cores available on the CPU.
In prior releases it defaulted to 1.
Programs that do not expect to run with multiple cores may
break inadvertently.
They can be updated by removing the restriction or by setting
<code>GOMAXPROCS</code> explicitly. 
</p>

<h3 id="build">Build</h3>

<p>
Now that the Go compiler and runtime are implemented in Go, a Go compiler
must be available to compile the distribution from source.
Thus, to build the Go core, a working Go distribution must already be in place.
(Go programmers who do not work on the core are unaffected by this change.)
Any Go 1.4 or later distribution (including <code>gccgo</code>) will serve.
For details, see the <a href="/s/go15bootstrap">design document</a>.
</p>

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

<p>
Due mostly to the industry's move away the 32-bit x86 architecture,
the set of binary downloads provided is reduced in 1.5.
A distribution for the OS X operating system is provided only for the
<code>amd64</code> architecture, not <code>386</code>.
Similarly, the ports for Snow Leopard (Apple OS X 10.6) still work but are no
longer released as a download or maintained since Apple no longer maintains that version
of the operating system.
Also, the <code>dragonfly/386</code> port is no longer supported at all
because DragonflyBSD itself no longer supports the 32-bit 386 architecture.
</p>

<p>
There are however several new ports available to be built from source.
These include <code>darwin/arm</code> and <code>darwin/arm64</code>.
The new port <code>linux/arm64</code> is mostly in place, but <code>cgo</code>
is only supported using external linking.
</p>

<p>
On FreeBSD, Go 1.5 requires FreeBSD 8-STABLE+ because of its new use of the <code>SYSCALL</code> instruction.
</p>

<p>
On NaCl, Go 1.5 requires SDK version pepper-39 or above because it now uses the
<code>get_random_bytes</code> system call.
</p>

<pre>

API additions and behavior changes:

flag: new nicer format for PrintDefaults (https://golang.org/cl/7330)
math/big: add arbitrary precision Floats (many cl's)
mime/quotedprintable: new package (https://golang.org/cl/5940 + others)
reflect: add ArrayOf (https://golang.org/cl/4111)
reflect: add FuncOf (https://golang.org/cl/1996)

Tools:

build: external linking support for windows (https://golang.org/cl/7163, 7282, 7283, 7284, 7534, 7535)
cmd/cover: tool now lives in the standard repository (https://golang.org/cl/9560)
cmd/gc: constant arithmetic is based on math/big (https://golang.org/cl/7830, 7851, 7857, 8426, 7858, 7912, 8171)
cmd/go, go/build: add ${SRCDIR} variable expansion to cgo lines (https://golang.org/cl/1756)
cmd/go: add $DOLLAR to generate's variables (https://golang.org/cl/8091)
cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
cmd/go: add -run flag to go generate (https://golang.org/cl/9005)
cmd/go: add $GOLINE to generate's variables (https://golang.org/cl/9007)
cmd/go: add go doc (https://golang.org/cl/9227)
cmd/go: internal enforced even outside standard library (golang.org/s/go14internal; https://golang.org/cl/9156)
cmd/go, testing: add go test -count (https://golang.org/cl/10669)
cmd/go: add preliminary support for vendor directories (https://golang.org/cl/10923)
cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)
cmd/go: add -toolexec build option
cmd/go: drop -ccflags build option
cmd/go: add -asmflags build option
cmd/go: add -buildmode build option
cmd/gc: add -dynlink option (for amd64 only)
cmd/ld: add -buildmode option
cmd/trace: new command to view traces (https://golang.org/cl/3601)

Performance:

cmd/gc: evaluate concrete == interface without allocating (https://golang.org/cl/2096)
cmd/gc: optimize memclr of slices and arrays (https://golang.org/cl/2520)
cmd/gc: transform closure calls to function calls (https://golang.org/cl/4050)
cmd/gc: transitive inlining (https://golang.org/cl/5952)
cmd/gc, runtime: speed up some cases of _, ok := i.(T) (https://golang.org/cl/7697)
cmd/gc: speed up large string switches (https://golang.org/cl/7698)
cmd/gc: inline x := y.(*T) and x, ok := y.(*T) (https://golang.org/cl/7862)
cmd/gc: allocate backing storage for non-escaping interfaces on stack (https://golang.org/cl/8201)
encoding/xml: avoid an allocation for tags without attributes (https://golang.org/cl/4160)
image: many optimizations
runtime: add ARM runtime.cmpstring and bytes.Compare (https://golang.org/cl/8010)
runtime: do not scan maps when k/v do not contain pointers (https://golang.org/cl/3288)
runtime: reduce thrashing of gs between ps (https://golang.org/cl/9872)
sort: number of Sort performance optimizations (https://golang.org/cl/2100, https://golang.org/cl/2614, ...)
strconv: optimize decimal to string conversion (https://golang.org/cl/2105)
strconv: optimize float to string conversion (https://golang.org/cl/5600)
sync: add active spinning to Mutex (https://golang.org/cl/5430)
math/big: faster assembly kernels for amd64 and 386 (https://golang.org/cl/2503, https://golang.org/cl/2560)
math/big: faster "pure Go" kernels for platforms w/o assembly kernels (https://golang.org/cl/2480)
regexp: port RE2's bitstate backtracker to the regexp package (https://golang.org/cl/2153)

Assembler:

New cmd/asm tool (now use go tool asm, not go tool 6a)

Assembler now supports -dynlink option.

ARM assembly syntax has had some features removed.

	- mentioning SP or PC as a hardware register
		These are always pseudo-registers except that in some contexts
		they're not, and it's confusing because the context should not affect
		which register you mean. Change the references to the hardware
		registers to be explicit: R13 for SP, R15 for PC.
	- constant creation using assignment
		The files say a=b when they could instead say #define a b.
		There is no reason to have both mechanisms.
	- R(0) to refer to R0.
		Some macros use this to a great extent. Again, it's easy just to
		use a #define to rename a register.
	
Also expression evaluation now uses uint64s instead of signed integers and the
precedence of operators is now Go-like rather than C-like.

Standard library hardening
35 bugs found by randomized testing with go-fuzz (https://github.com/dvyukov/go-fuzz)
were fixed in fmt, archive/zip, archive/tar, encoding/gob, image/jpeg, image/png,
image/gif, compress/flate, text/template, html/template. The fixes harden implementation
against incorrect and malicious inputs.
</pre>

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

<ul>

<li>
The <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package's
<a href="/pkg/archive/zip/#Writer"><code>Writer</code></a> type now has a
<a href="/pkg/archive/zip/#Writer.SetOffset"><code>SetOffset</code></a>
method to specify the location within the output stream at which to write the archive.
</li>

<li>
The <a href="/pkg/bufio/#Reader"><code>Reader</code></a> in the
<a href="/pkg/bufio/"><code>bufio</code></a> package now has a
<a href="/pkg/bufio/#Reader.Discard"><code>Discard</code></a>
method to discard data from the input.
</li>

<li>
Also in the <a href="/pkg/bytes/"><code>bytes</code></a> package,
the <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
now has a <a href="/pkg/bytes/#Buffer.Cap"><code>Cap</code></a> method
that reports the number of bytes allocated within the buffer.
Similarly, both the <a href="/pkg/bytes/"><code>bytes</code></a>
and <a href="/pkg/strings/"><code>strings</code></a> packages,
the <a href="/pkg/bytes/#Reader"><code>Reader</code></a>
type now has a <a href="/pkg/bytes/#Reader.Size"><code>Size</code></a>
method that reports the original length of the underlying slice or string.
</li>

<li>
Both the <a href="/pkg/bytes/"><code>bytes</code></a> and
<a href="/pkg/strings/"><code>strings</code></a> packages
also now have a <a href="/pkg/bytes/#LastIndexByte"><code>LastIndexByte</code></a>
function that locates the rightmost byte with that value in the argument.
</li>

<li>
TODO crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
</li>

<li>
TODO crypto/cipher: support non-standard nonce lengths for GCM. (https://golang.org/cl/8946)
</li>

<li>
TODO crypto/elliptic: add Name field to CurveParams struct (https://golang.org/cl/2133)
</li>

<li>
TODO crypto/elliptic: Unmarshaling points now automatically checks that the point is on the curve (https://golang.org/cl/2421)
</li>

<li>
TODO crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)
</li>

<li>
TODO crypto/tls: including Certificate Transparency SCTs in the handshake is now supported (https://golang.org/cl/8988)
</li>

<li>
TODO crypto/tls: session ticket keys can now be rotated at runtime (https://golang.org/cl/9072)
</li>

<li>
TODO crypto/tls: servers will now always call GetCertificate to pick a certificate for a connection when Certificates is empty (https://golang.org/cl/8792)
</li>

<li>
TODO crypto/x509: wildcards are now only accepted as the first label (https://golang.org/cl/5691)
</li>

<li>
TODO crypto/x509: unknown critical extensions now cause errors in Verify, not when parsing (https://golang.org/cl/9390)
</li>

<li>
The <a href="/pkg/database/sql/#DB"><code>DB</code></a> type of the
<a href="/pkg/database/sql/"><code>database/sql</code></a> package
now has a <a href="/pkg/database/sql/#DB.Stats"><code>Stats</code></a> method
to retrieve database statistics.
</li>

<li>
The <a href="/pkg/encoding/base64/"><code>encoding/base64</code></a> package
now supports unpadded encodings through two new encoding variables,
<a href="/pkg/encoding/base64/#RawStdEncoding"><code>RawStdEncoding</code></a> and
<a href="/pkg/encoding/base64/#RawURLEncoding"><code>RawURLEncoding</code></a>.
</li>

<li>
Also in the <a href="/pkg/fmt/"><code>fmt</code></a> package,
a value of type <a href="/pkg/reflect/#Value"><code>Value</code></a> now
prints what it holds, rather than use the <code>reflect.Value</code>'s <code>Stringer</code>
method, which produces things like <code>&lt;int Value&gt;</code>.
</li>

<li>
The <a href="/pkg/ast/#EmptyStmt"><code>EmptyStmt</code></a> type
in the <a href="/pkg/go/ast/"><code>go/ast</code></a> package now
has a boolean <code>Implicit</code> field that records whether the
semicolon was implicitly added or was present in the source.
</li>

<li>
For forward compatibility the <a href="/pkg/go/build/"><code>go/build</code></a> package
reserves <code>GOARCH</code> values for  a number of architectures that Go might support one day.
This is not a promise that it will.
</li>

<li>
The <a href="/pkg/io/"><code>io</code></a> package
adds a <a href="/pkg/io/#CopyBuffer"><code>CopyBuffer</code></a> function
that is like <a href="/pkg/io/#Copy"><code>Copy</code></a> but
uses a caller-provided buffer, permitting control of allocation and buffer size.
</li>

<li>
The <a href="/pkg/log/"><code>log</code></a> package
has a new <a href="/pkg/log/#LUTC"><code>LUTC</code></a> flag
that causes time stamps to be printed in the UTC time zone.
It also adds a <a href="/pkg/log/#SetOutput"><code>SetOutput</code></a> function
to set the output destination for the standard logger
and a corresponding method for user-created loggers.
</li>

<li>
In Go 1.4, <a href="/pkg/math/#Max"><code>Max</code></a> was not detecting all possible NaN bit patterns.
This is fixed in Go 1.5, so programs that use <code>math.Max</code> on data including NaNs may behave differently,
but now correctly according to the IEEE754 definition of NaNs.
</li>

<li>
The <a href="/pkg/math/big/"><code>math/big</code></a> package
adds a new <a href="/pkg/math/big/#Jacobi"><code>Jacobi</code></a>
function for integers and a new method
<a href="/pkg/math/big/#Int.ModSqrt"><code>ModSqrt</code></a>
method for the <a href="/pkg/math/big/#Int"><code>Int</code></a> type.
</li>

<li>
The <a href="/pkg/mime/"><code>mime</code></a> package adds an
<a href="/pkg/mime/#ExtensionsByType"><code>ExtensionsByType</code></a>
function that returns the MIME extensions know to be associated with a given MIME type.
</li>

<li>
TODO net: add sequential and RFC 6555-compliant TCP dialing (https://golang.org/cl/8768)
</li>

<li>
TODO net: add Source field to OpError (https://go-review.googlesource.com/9231)
</li>

<li>
TODO net: fix inconsistent errors (https://golang.org/cl/9236)
</li>

<li>
TODO net: add SocketConn, SocketPacketConn (https://golang.org/cl/9275)
</li>

<li>
TODO net: use Go's DNS resolver when system configuration permits (https://golang.org/cl/8945)
</li>

<li>
TODO net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
</li>

<li>
TODO net/http: ignore the Unix epoch time in ServeContent (https://golang.org/cl/7915)
</li>

<li>
TODO net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT (https://golang.org/cl/4933)
</li>

<li>
TODO net/mail: adds AddressParser type (https://golang.org/cl/10392)
</li>

<li>
TODO net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
</li>

<li>
The <a href="/pkg/os/"><code>os</code></a> package
has a new <a href="/pkg/os/#LookupEnv"><code>LookupEnv</code></a> function
that is similar to <a href="/pkg/os/#Getenv"><code>Getenv</code></a>
but can distinguish between an empty environment variable and a missing one.
</li>

<li>
The <a href="/pkg/os/signal/"><code>os/signal</code></a> package
adds new <a href="/pkg/os/signal/#Ignore"><code>Ignore</code></a> and
<a href="/pkg/os/signal/#Reset"><code>Reset</code></a> functions.
</li>

<li>
The <a href="/pkg/runtime/pprof/"><code>runtime/pprof</code></a> package
by default now includes overall memory statistics in all memory profiles.
</li>

<li>
The <a href="/pkg/strings/"><code>strings</code></a> package
has a new <a href="/pkg/strings/#Compare"><code>Compare</code></a> function.
This is present to provide symmetry with the <a href="/pkg/bytes/"><code>bytes</code></a> package
but is otherwise unnecessary as strings support comparison natively.
</li>

<li>
The <a href="/pkg/sync/#WaitGroup"><code>WaitGroup</code></a> function in
package <a href="/pkg/sync/"><code>sync</code></a>
now diagnoses code that races a call to <a href="/pkg/sync/#WaitGroup.Add"><code>Add</code></a>
against a return from <a href="/pkg/sync/#WaitGroup.Wait"><code>Wait</code></a>.
If it detects this condition, <code>WaitGroup</code> panics.
</li>

<li>
In the <a href="/pkg/syscall/"><code>syscall</code></a> package,
the Linux <code>SysProcAttr</code> struct now has a
<code>GidMappingsEnableSetgroups</code> field, made necessary
by security changes in Linux 3.19.
On all Unix systems, the struct also has new <code>Foreground</code> and <code>Pgid</code> fields
to provide more control when exec'ing.
On Darwin, there is now a <code>Syscall9</code> function
to support calls with too many arguments.
</li>

<li>
The <a href="/pkg/testing/quick/"><code>testing/quick</code></a> will now
generate <code>nil</code> values for pointer types,
making it possible to use with recursive data structures.
Also, the package now supports generation of array types.
</li>

<li>
In the <a href="/pkg/text/template/"><code>text/template</code></a> and
<a href="/pkg/html/template/"><code>html/template</code></a> packages,
integer constants too large to be represented as a Go integer now trigger a
parse error. Before, they were silently converted to floating point, losing
precision.
</li>

<li>
Also in the <a href="/pkg/text/template/"><code>text/template</code></a> and
<a href="/pkg/html/template/"><code>html/template</code></a> packages,
a new <a href="/pkg/text/template/#Option"><code>Option</code></a> type
allows customization of the behavior of the template during execution.
The sole implemented option allows control over how a missing key is
handled when indexing a map.
The default, which can now be overridden, is as before: to continue with an invalid value.
</li>

<li>
The <a href="/pkg/time/"><code>time</code></a> package's
<code>Time</code> type has a new method
<a href="/pkg/time/#Time.AppendFormat"><code>AppendFormat</code></a>, 
which can be used to avoid allocation when printing a time value.
</li>

<li>
The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
support throughout the system has been upgraded from version 7.0 to
<a href="http://www.unicode.org/versions/Unicode8.0.0/">Unicode 8.0</a>.
</li>

</ul>