The latest Go release, version 1.4, arrives as scheduled six months after 1.3
and contains only one tiny language change,
a backwards-compatible simple form of for-range loop.
The release focuses primarily on implementation work, improving the garbage collector
and preparing the ground for a fully concurrent collector to be rolled out in the
next few releases.
Stacks are now contiguous, reallocated when necessary rather than linking on new
"segments";
this release therefore eliminates the notorious "hot stack split" problem.
There are some new tools available including support in the go command
for build-time source code generation
and TODO.
The release also adds support for ARM processors on Android and Native Client (NaCl)
and AMD64 on Plan 9.
As always, Go 1.4 keeps the promise
of compatibility,
and almost everything
will continue to compile and run without change when moved to 1.4.
Up until Go 1.3, for-range loop had two forms
for k, v := range x {
...
}
and
for k := range x {
...
}
If one was not interested in the loop values, only the iteration itself, it was still
necessary to mention a variable (probably the blank identifier, as in
for _ = range x), because
the form
for range x {
...
}
was not syntactically permitted.
This situation seemed awkward, so as of Go 1.4 the variable-free form is now legal. The pattern arises rarely but the code can be cleaner when it does.
Updating: The change is strictly backwards compatible to existing Go
programs, but tools that analyze Go parse trees may need to be modified to accept
this new form as the
Key field of RangeStmt
may now be nil.
Go 1.4 can build binaries for ARM processors running the Android operating system.
It can also build a .so library that can be loaded by an Android application
using the supporting packages in the go.mobile repository.
A brief description of the plans for this experimental port are available
here.
The previous release introduced Native Client (NaCl) support for the 32-bit x86
(GOARCH=386)
and 64-bit x86 using 32-bit pointers (GOARCH=amd64p32).
The 1.4 release adds NaCl support for ARM (GOARCH=arm).
This release adds support for the Plan 9 operating system on AMD64 processors,
provided the kernel supports the nsec system call and uses 4K pages.
The unsafe package allows one
to defeat Go's type system by exploiting internal details of the implementation
or machine representation of data.
It was never explicitly specified what use of unsafe meant
with respect to compatibility as specified in the
Go compatibility guidelines.
The answer, of course, is that we can make no promise of compatibility
for code that does unsafe things.
We have clarified this situation in the documentation included in the release.
The Go compatibilty guidelines and the
docs for the unsafe package
are now explicit that unsafe code is not guaranteed to remain compatible.
Updating: Nothing technical has changed; this is just a clarification of the documentation.
Up to Go 1.4, the runtime (garbage collector, concurrency support, interface management, maps, slices, strings, ...) was mostly written in C, with some assembler support. In 1.4, much of the code has been translated to Go so that the garbage collector can scan the stacks of programs in the runtime and get accurate information about what variables are active. This change was large but should have no semantic effect on programs.
This rewrite allows the garbage collector in 1.4 to be fully precise, meaning that it is aware of the location of all active pointers in the program. This means the heap will be smaller as there will be no false positives keeping non-pointers alive. Other related changes also reduce the heap size, which is smaller by 10%-30% overall relative to the previous release.
A consequence is that stacks are no longer segmented, eliminating the "hot split" problem. When a stack limit is reached, a new, larger stack is allocated, all active frames for the goroutine are copied there, and any pointers into the stack are updated. Performance can be noticeably better in some cases and is always more predictable. Details are available in the design document.
The use of contiguous stacks means that stacks can start smaller without triggering performance issues, so the default starting size for a goroutine's stack in 1.4 has been reduced to 2048 bytes from 8192 bytes. TODO: It may be bumped to 4096 for the release.
As preparation for the concurrent garbage collector scheduled for the 1.5 release, writes to pointer values in the heap are now done by a function call, called a write barrier, rather than directly from the function updating the value. In this next release, this will permit the garbage collector to mediate writes to the heap while it is running. This change has no semantic effect on programs in 1.4, but was included in the release to test the compiler and the resulting performance.
The implementation of interface values has been modified. In earlier releases, the interface contained a word that was either a pointer or a one-word scalar value, depending on the type of the concrete object stored. This implementation was problematical for the garbage collector, so as of 1.4 interface values always hold a pointer. In running programs, most interface values were pointers anyway, so the effect is minimal, but programs that store integers (for example) in interfaces will see more allocations.
TODO gccgo news
Go's package system makes it easy to structure programs into components with clean boundaries, but there are only two forms of access: local (unexported) and global (exported). Sometimes one wishes to have components that are not exported, for instance to avoid acquiring clients of interfaces to code that is part of a public repository but not intended for use outside the program to which it belongs.
The Go language does not have the power to enforce this distinction, but as of Go 1.4 the
go command introduces
a mechanism to define "internal" packages that may not be imported by packages outside
the source subtree in which they reside.
To create such a package, place it in a directory named internal or in a subdirectory of a directory
named internal.
When the go command sees an import of a package with internal in its path,
it verifies that the package doing the import
is within the tree rooted at the parent of the internal directory.
For example, a package .../a/b/c/internal/d/e/f
can be imported only by code in the directory tree rooted at .../a/b/c.
It cannot be imported by code in .../a/b/g or in any other repository.
For Go 1.4, the internal package mechanism is enforced for the main Go repository; from 1.5 and onward it will be enforced for any repository.
Full details of the mechanism are in the design document.
Code often lives in repositories hosted by public services such as github.com,
meaning that the import paths for packages begin with the name of the hosting service,
github.com/rsc/pdf for example.
One can use
an existing mechanism
to provide a "custom" or "vanity" import path such as
rsc.io/pdf, but
that creates two valid import paths for the package.
That is a problem: one may inadvertently import the package through the two
distinct paths in a single program, which is wasteful;
miss an update to a package because the path being used is not recognized to be
out of date;
or break clients using the old path by moving the package to a different hosting service.
Go 1.4 introduces an annotation for package clauses in Go source that identify a canonical
import path for the package.
If an import is attempted using a path that is not canonical,
the go command
will refuse to compile the importing package.
The syntax is simple: put an identifying comment on the package line. For our example, the package clause would read:
package pdf // import "rsc.io/pdf"
With this in place,
the go command will
refuse to compile a package that imports github.com/rsc/pdf,
ensuring that the code can be moved without breaking users.
The check is at build time, not download time, so if go get
fails because of this check, the mis-imported package has been copied to the local machine
and should be removed manually.
Further information is in the design document.
The go command has a new subcommand,
go generate,
to automate the running of tools to generate source code before compilation.
For example, it can be used to run the yacc
compiler-compiler on a .y file to produce the Go source file implementing the grammar,
or to automate the generation of String methods for typed constants using the new
stringer
tool in the go.tools repository.
For more information, see the design document.
Build constraints, also known as build tags, control compilation by including or excluding files
(see the documentation /go/build).
Compilation can also be controlled by the name of the file itself by "tagging" the file with
a suffix (before the .go or .s extension) with an underscore
and the name of the architecture or operating system.
For instance, the file gopher_arm.go will only be compiled if the target
processor is an ARM.
Before Go 1.4, a file called just arm.go was similarly tagged, but this behavior
can break sources when new architectures are added, causing files to suddenly become tagged.
In 1.4, therefore, a file will be tagged in this manner only if the tag (architecture or operating
system name) is preceded by an underscore.
Updating: Packages that depend on the old behavior will no longer compile correctly.
Files with names like windows.go or amd64.go should either
have explicit build tags added to the source or be renamed to something like
os_windows.go or support_amd64.go.
There were a number of minor changes to the
cmd/go
command worth noting.
cgo is being used to build the package,
the go command now refuses to compile C source files,
since the relevant C compilers
(6c etc.)
are intended to be removed from the installation in some future release.
(They are used today only to build part of the runtime.)
It is difficult to use them correctly in any case, so any extant uses are likely incorrect,
so we have disabled them.
go test
subcommand has a new flag, -o, to set the name of the resulting binary,
corresponding to the same flag in other subcommands.
The non-functional -file flag has been removed.
go test
subcommand will compile and link all *_test.go files in the package,
even when there are no Test functions in them.
It previously ignored such files.
go build
subcommand's
-a flag has been changed for non-development installations.
For installations running a released distribution, the -a flag will no longer
rebuild the standard library and commands, to avoid overwriting the installation's files.
TODO cgo news
TODO godoc news
In the main Go source repository, the source code for the packages was kept in
the directory src/pkg, which made sense but differed from
other repositories, including the Go sub-repositories such as go.tools.
In Go 1.4, the pkg level of the source tree is now gone, so for example
the fmt package's source, once kept in
directory src/pkg/fmt, now lives one level higher in src/fmt.
Updating: Tools like godoc that discover source code
need to know about the new location. All tools and services maintained by the Go team
have been updated.
TODO misc news misc: deleted editor support; refer to https://code.google.com/p/go-wiki/wiki/IDEsAndTextEditorPlugins instead (CL 105470043)
Most programs will run about the same speed or slightly faster in 1.4 than in 1.3; some will be slightly slower. There are many changes, making it hard to be precise about what to expect.
As mentioned above, much of the runtime was translated to Go from C, which led to some reduction in heap sizes. It also improved performance slightly because the Go compiler is better at optimization, due to things like inlining, than the C compiler used to build the runtime.
The garbage collector was sped up, leading to measurable improvements for garbage-heavy programs. On the other hand, the new write barriers slow things down again, typically by about the same amount but, depending on their behavior, some programs may be somewhat slower or faster.
Library changes that affect performance are documented below.
TODO new packages
TODO major changes
encoding/gob: remove unsafe (CL 102680045) syscall: now frozen (CL 129820043)
The following list summarizes a number of minor changes to the library, mostly additions. See the relevant package documentation for more information about each change.
cmd/6l, liblink: use pc-relative addressing for all memory references, so that linking Go binaries at high addresses works (CL 125140043). This cuts the maximum size of a Go binary's text+data+bss from 4GB to 2GB. asm: make textflag.h available outside of cmd/ld (CL 128050043) bufio: handling of empty tokens at EOF changed, may require scanner change (CL 145390043) compress/flate, compress/gzip, compress/zlib: Reset support (https://codereview.appspot.com/97140043) crypto/tls: add support for ALPN (RFC 7301) (CL 108710046) crypto/tls: support programmatic selection of server certificates (CL 107400043) encoding/asn1: optional elements with a default value will now only be omitted if they have that value (CL 86960045) fmt: print type *map[T]T as &map[k:v] (CL 154870043) encoding/csv: do not quote empty strings, quote \. (CL 164760043) net/http: add Request.BasicAuth method (CL 76540043) net/http: add Transport.DialTLS hook (CL 137940043) net/http/httputil: add ReverseProxy.ErrorLog (CL 132750043) os: implement symlink support for windows (CL 86160044) reflect: add type.Comparable (CL 144020043) reflect: Value is one word smaller runtime: implement monotonic clocks on windows (CL 108700045) runtime: MemStats.Mallocs now counts very small allocations missed in Go 1.3. This may break tests using runtime.ReadMemStats or testing.AllocsPerRun by giving a more accurate answer than Go 1.3 did (CL 143150043). runtime/race: freebsd is supported (CL 107270043) runtime: add PauseEnd array to MemStats and GCStats (CL 153670043) swig: Due to runtime changes Go 1.4 will require SWIG 3.0.3 (not yet released) sync/atomic: add Value (CL 136710045) syscall: Setuid, Setgid are disabled on linux platforms. On linux those syscalls operate on the calling thread, not the whole process. This does not match the semantics of other platforms, nor the expectations of the caller, so the operations have been disabled until issue 1435 is resolved (CL 106170043) testing: add Coverage (CL 98150043) testing: add TestMain support (CL 148770043) text/scanner: add IsIdentRune field of Scanner. (CL 108030044) text/template: allow comparison of signed and unsigned integers (CL 149780043) time: use the micro symbol (ยต (U+00B5)) to print microsecond duration (CL 105030046) go.sys subrepo created: http://golang.org/s/go1.4-syscall