Go 1.9 Release Notes
Introduction to Go 1.9
The latest Go release, version 1.9, arrives six months after Go 1.8 and is the tenth release in the Go 1.x series. There are two changes to the language: adding support for type aliases and defining when implementations may fuse floating point operations. Most of the changes are in the implementation of the toolchain, runtime, and libraries. 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 transparent monotonic time support, parallelizes compilation of functions within a package, better supports test helper functions, includes a new bit manipulation package, and has a new concurrent map type.
Changes to the language
There are two changes to the language.
Go now supports type aliases to support gradual code repair while moving a type between packages. The type alias design document and an article on refactoring cover the problem in detail. In short, a type alias declaration has the form:
type T1 = T2
This declaration introduces an alias name
alternate spelling—for the type denoted by
T2; that is,
T2 denote the same type.
A smaller language change is that the
now states when implementations are allowed to fuse floating
point operations together, such as by using an architecture's "fused
multiply and add" (FMA) instruction to compute
without rounding the intermediate result
To force the intermediate rounding, write
There are no new supported operating systems or processor architectures in this release.
ppc64x requires POWER8
require at least POWER8 support. In previous releases,
GOARCH=ppc64le required POWER8 and the big
ppc64 architecture supported older
Go 1.9 is the last release that will run on FreeBSD 9.3, which is already unsupported by FreeBSD. Go 1.10 will require FreeBSD 10.3+.
Go 1.9 now enables PT_TLS generation for cgo binaries and thus requires OpenBSD 6.0 or newer. Go 1.9 no longer supports OpenBSD 5.9.
There are some instabilities on FreeBSD that are known but not understood. These can lead to program crashes in rare cases. See issue 15658. Any help in solving this FreeBSD-specific issue would be appreciated.
Go stopped running NetBSD builders during the Go 1.9 development cycle due to NetBSD kernel crashes, up to and including NetBSD 7.1. As Go 1.9 is being released, NetBSD 7.1.1 is being released with a fix. However, at this time we have no NetBSD builders passing our test suite. Any help investigating the various NetBSD issues would be appreciated.
The Go compiler now supports compiling a package's functions in parallel, taking
advantage of multiple cores. This is in addition to the
existing support for parallel compilation of separate packages.
Parallel compilation is on by default, but it can be disabled by setting the
Vendor matching with ./...
By popular request,
./... no longer matches packages
vendor directories in tools accepting package names,
test. To match vendor
The go tool will now use the path from which it
was invoked to attempt to locate the root of the Go install tree.
This means that if the entire Go installation is moved to a new
location, the go tool should continue to work as usual.
This may be overridden by setting
GOROOT in the environment,
which should only be done in unusual circumstances.
Note that this does not affect the result of
the runtime.GOROOT function, which
will continue to report the original installation location;
this may be fixed in later releases.
Complex division is now C99-compatible. This has always been the case in gccgo and is now fixed in the gc toolchain.
The linker will now generate DWARF information for cgo executables on Windows.
The compiler now includes lexical scopes in the generated DWARF if the
-N -l flags are provided, allowing
debuggers to hide variables that are not in scope. The
section is now DWARF version 4.
The values of
GO386 now affect a
compiled package's build ID, as used by the
The four-operand ARM
MULA instruction is now assembled correctly,
with the addend register as the third argument and the result
register as the fourth and final argument.
In previous releases, the two meanings were reversed.
The three-operand form, in which the fourth argument is implicitly
the same as the third, is unaffected.
Code using four-operand
will need to be updated, but we believe this form is very rarely used.
MULAWB were already
using the correct order in all forms and are unchanged.
The assembler now supports
ADDSUBPS/PD, completing the
two missing x86 SSE3 instructions.
Long lists of arguments are now truncated. This improves the readability
doc on some generated code.
Viewing documentation on struct fields is now supported.
enables JSON output, instead of the default OS-specific output
command accepts a new
-list flag, which takes a regular
expression as an argument and prints to stdout the name of any
tests, benchmarks, or examples that match it, without running them.
Profiles produced by the
runtime/pprof package now
include symbol information, so they can be viewed
without the binary that produced the profile.
pprof command now
uses the HTTP proxy information defined in the environment, using
has been better integrated into the
vet now supports all standard build
vet's own flags are now available
vet as well as
Due to the alignment of Go's semiannual release schedule with GCC's annual release schedule, GCC release 7 contains the Go 1.8.3 version of gccgo. We expect that the next release, GCC 8, will contain the Go 1.10 version of gccgo.
Call stacks with inlined frames
should avoid directly inspecting the resulting PC slice and instead use
to get a complete view of the call stack, or
to get information about a single caller.
This is because an individual element of the PC slice cannot account
for inlined frames or other nuances of the call stack.
Specifically, code that directly iterates over the PC slice and uses
functions such as
to resolve each PC individually will miss inlined frames.
To get a complete view of the stack, such code should instead use
Likewise, code should not assume that the length returned by
Callers is any indication of the call depth.
It should instead count the number of frames returned by
Code that queries a single caller at a specific depth should use
Caller rather than passing a slice of length 1 to
has been available since Go 1.7, so code can be updated prior to
upgrading to Go 1.9.
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, better generated code, and optimizations in the core library.
Library functions that used to trigger stop-the-world garbage
collection now trigger concurrent garbage collection.
now trigger concurrent garbage collection, blocking only the calling
goroutine until the garbage collection is done.
function only triggers a garbage collection if one is immediately
necessary because of the new GOGC value.
This makes it possible to adjust GOGC on-the-fly.
Large object allocation performance is significantly improved in applications using large (>50GB) heaps containing many large objects.
function now takes less than 100µs even for very large heaps.
Transparent Monotonic Time support
time package now transparently
tracks monotonic time in each
value, making computing durations between two
a safe operation in the presence of wall clock adjustments.
See the package docs and
New bit manipulation package
Go 1.9 includes a new package,
math/bits, with optimized
implementations for manipulating bits. On most architectures,
functions in this package are additionally recognized by the
compiler and treated as intrinsics for additional performance.
Test Helper Functions
methods mark the calling function as a test helper function. When
printing file and line information, that function will be skipped.
This permits writing test helper functions while still having useful
line numbers for users.
is a concurrent map with amortized-constant-time loads, stores, and
deletes. It is safe for multiple goroutines to call a
now supports adding labels to
pprof profiler records.
Labels form a key-value map that is used to distinguish calls of the
same function in different contexts when looking at profiles
runs code associated with some provided labels. Other new functions
in the package help work with labels.
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.
Writernow sets the UTF-8 bit in the
On Linux, Go now calls the
getrandomsystem call without the
GRND_NONBLOCKflag; it will now block until the kernel has sufficient randomness. On kernels predating the
getrandomsystem call, Go continues to read from
On Unix systems the environment variables
SSL_CERT_DIRcan now be used to override the system default locations for the SSL certificate file and SSL certificate files directory, respectively.
The FreeBSD file
/usr/local/etc/ssl/cert.pemis now included in the certificate search path.
The package now supports excluded domains in name constraints. In addition to enforcing such constraints,
CreateCertificatewill create certificates with excluded name constraints if the provided template certificate has the new field
If any SAN extension, including with no DNS names, is present in the certificate, then the Common Name from
Subjectis ignored. In previous releases, the code tested only whether DNS-name SANs were present in a certificate.
The package will now use a cached
Stmtif available in
Tx.Stmt. This prevents statements from being re-prepared each time
The package now allows drivers to implement their own argument checkers by implementing
driver.NamedValueChecker. This also allows drivers to support
Outshould be used to return output parameters when supported by the driver.
Rows.Scancan now scan user-defined string types. Previously the package supported scanning into numeric types like
int64. It now also supports scanning into string types like
DB.Connmethod returns the new
Conntype representing an exclusive connection to the database from the connection pool. All queries run on a
Connwill use the same underlying connection until
Conn.Closeis called to return the connection to the connection pool.
NullRawValuerepresent the ASN.1 NULL type.
The new Encoding.WithPadding method adds support for custom padding characters and disabling padding.
The new field
Reader.ReuseRecordcontrols whether calls to
Readmay return a slice sharing the backing array of the previous call's returned slice for improved performance.
The sharp flag ('
#') is now supported when printing floating point and complex numbers. It will always print a decimal point for
%G; it will not remove trailing zeros for
The package now includes 128-bit FNV-1 and FNV-1a hash support with
The package now reports an error if a predefined escaper (one of "html", "urlquery" and "js") is found in a pipeline and does not match what the auto-escaper would have decided on its own. This avoids certain security or correctness issues. Now use of one of these escapers is always either a no-op or an error. (The no-op case eases migration from text/template.)
Rectangle.Intersectmethod now returns a zero
Rectanglewhen called on adjacent but non-overlapping rectangles, as documented. In earlier releases it would incorrectly return an empty but non-zero
The YCbCr to RGBA conversion formula has been tweaked to ensure that rounding adjustments span the complete [0, 0xffff] RGBA range.
Encoder.BufferPoolfield allows specifying an
EncoderBufferPool, that will be used by the encoder to get temporary
EncoderBufferbuffers when encoding a PNG image. The use of a
BufferPoolreduces the number of memory allocations performed while encoding multiple images.
The package now supports the decoding of transparent 8-bit grayscale ("Gray8") images.
IsUint64methods report whether an
Intmay be represented as an
FileHeader.Sizefield describes the size of a file in a multipart message.
Resolver.StrictErrorsprovides control over how Go's built-in DNS resolver handles temporary errors during queries composed of multiple sub-queries, such as an A+AAAA address lookup.
Resolverto use a custom dial function.
JoinHostPortnow only places an address in square brackets if the host contains a colon. In previous releases it would also wrap addresses in square brackets if they contained a percent ('
The new methods
UnixConn.SyscallConnprovide access to the connections' underlying file descriptors.
It is now safe to call
Dialwith the address obtained from
(*TCPListener).String()after creating the listener with
Listen("tcp", ":0"). Previously it failed on some machines with half-configured IPv6 stacks.
Cookie.Stringmethod, used for
Set-Cookieheaders, now encloses values in double quotes if the value contains either a space or a comma.
ServeMuxnow ignores ports in the host header when matching handlers. The host is matched unmodified for
Server.Servewith added TLS support.
Server.WriteTimeoutnow applies to HTTP/2 connections and is enforced per-stream.
- HTTP/2 now uses the priority write scheduler by default. Frames are scheduled by following HTTP/2 priorities as described in RFC 7540 Section 5.3.
The HTTP handler returned by
StripPrefixnow calls its provided handler with a modified clone of the original
*http.Request. Any code storing per-request state in maps keyed by
LocalAddrContextKeynow contains the connection's actual network address instead of the interface address used by the listener.
Client & Transport changes:
Transportnow supports making requests via SOCKS5 proxy when the URL returned by
Transport.Proxyhas the scheme
ProcessEnvfunction returns FastCGI environment variables associated with an HTTP request for which there are no appropriate
http.Requestfields, such as
Server.Clientmethod returns an HTTP client configured for making requests to the test server.
Server.Certificatemethod returns the test server's TLS certificate, if any.
ReverseProxynow proxies all HTTP/2 response trailers, even those not declared in the initial response header. Such undeclared trailers are used by the gRPC protocol.
ospackage now uses the internal runtime poller for file I/O. This reduces the number of threads required for read/write operations on pipes, and it eliminates races when one goroutine closes a file while another is using the file for I/O.
Argsis now populated without
shell32.dll, improving process start-up time by 1-7 ms.
os/execpackage now prevents child processes from being created with any duplicate environment variables. If
Cmd.Envcontains duplicate environment keys, only the last value in the slice for each duplicate key is used.
LookupIdnow work on Unix systems when
CGO_ENABLED=0by reading the
LookupGroupIdnow work on Unix systems when
CGO_ENABLED=0by reading the
MakeMapWithSizefunction creates a map with a capacity hint.
Tracebacks generated by the runtime and recorded in profiles are now accurate in the presence of inlining. To retrieve tracebacks programmatically, applications should use
runtime.CallersFramesrather than directly iterating over the results of
On Windows, Go no longer forces the system timer to run at high resolution when the program is idle. This should reduce the impact of Go programs on battery life.
runtime.NumCPUare now based on the process' CPU mask, rather than the total number of CPUs.
The runtime has preliminary support for Android O.
SetGCPercentwith a negative value no longer runs an immediate garbage collection.
The execution trace now displays mark assist events, which indicate when an application goroutine is forced to assist garbage collection because it is allocating too quickly.
"Sweep" events now encompass the entire process of finding free space for an allocation, rather than recording each individual span that is swept. This reduces allocation latency when tracing allocation-heavy programs. The sweep event shows how many bytes were swept and how many were reclaimed.
Mutexis now more fair.
The new field
Credential.NoSetGroupscontrols whether Unix systems make a
setgroupssystem call to set supplementary groups when starting a new process.
The new field
SysProcAttr.AmbientCapsallows setting ambient capabilities on Linux 4.3+ when creating a new process.
On 64-bit x86 Linux, process creation latency has been optimized with use of
Conninterface describes some types in the
netpackage that can provide access to their underlying file descriptor using the new
The package now chooses values in the full range when generating
uint64random numbers; in earlier releases generated values were always limited to the [-262, 262) range.
In previous releases, using a nil
Config.Randvalue caused a fixed deterministic random number generator to be used. It now uses a random number generator seeded with the current time. For the old behavior, set
The handling of empty blocks, which was broken by a Go 1.8 change that made the result dependent on the order of templates, has been fixed, restoring the old Go 1.7 behavior.
The new methods
Duration.Truncatehandle rounding and truncating durations to multiples of a given duration.
Retrieving the time and sleeping now work correctly under Wine.
Timevalue has a monotonic clock reading, its string representation (as returned by
String) now includes a final field
valueis the monotonic clock reading formatted as a decimal number of seconds.
tzdatatimezone database has been updated to version 2017b. As always, it is only used if the system does not already have the database available.