Go 1.5 Release Notes

Introduction to Go 1.5

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 promise of compatibility.

The biggest developments in the implementation are:

These and a number of other changes to the implementation and tools are discussed below.

The release also contains one small language change involving map literals.

Finally, the timing of the release 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.

Changes to the language

Map literals

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 corrected in Go 1.5. An example will make this clear. As of Go 1.5, this map literal,

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

may be written as follows, without the Point type listed explicitly:

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

The Implementation

No more C

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 cgo. 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 elsewhere.

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 this presentation.

Compiler and tools

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

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

Garbage collector

The garbage collector has been re-engineered for 1.5 as part of the development outlined in the design document. Expected latencies are much lower than with the collector in prior releases, through a combination of advanced algorithms, better scheduling of the collector, and running more of the collection in parallel with the user program. The “stop the world” phase of the collector will almost always be under 10 milliseconds and usually much less.

For systems that benefit from low latency, such as user-responsive web sites, the drop in expected latency with the new collector may be important.

Details of the new collector were presented in a talk at GopherCon 2015.


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 depend 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.

Another potentially breaking change is that the runtime now sets the default number of threads to run simultaneously, defined by GOMAXPROCS, to the number of cores available on the CPU. In prior releases the default was 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 GOMAXPROCS explicitly. For a more detailed discussion of this change, see the design document.


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 gccgo) will serve. For details, see the design document.


Due mostly to the industry’s move away from 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 amd64 architecture, not 386. 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 dragonfly/386 port is no longer supported at all because DragonflyBSD itself no longer supports the 32-bit 386 architecture.

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

Also available as experiments are ppc64 and ppc64le (64-bit PowerPC, big- and little-endian). Both these ports support cgo but only with internal linking.

On FreeBSD, Go 1.5 requires FreeBSD 8-STABLE+ because of its new use of the SYSCALL instruction.

On NaCl, Go 1.5 requires SDK version pepper-41. Later pepper versions are not compatible due to the removal of the sRPC subsystem from the NaCl runtime.

On Darwin, the use of the system X.509 certificate interface can be disabled with the ios build tag.

The Solaris port now has full support for cgo and the packages net and crypto/x509, as well as a number of other fixes and improvements.



As part of the process to eliminate C from the tree, the compiler and linker were translated from C to Go. It was a genuine (machine assisted) translation, so the new programs are essentially the old programs translated rather than new ones with new bugs. We are confident the translation process has introduced few if any new bugs, and in fact uncovered a number of previously unknown bugs, now fixed.

The assembler is a new program, however; it is described below.


The suites of programs that were the compilers (6g, 8g, etc.), the assemblers (6a, 8a, etc.), and the linkers (6l, 8l, etc.) have each been consolidated into a single tool that is configured by the environment variables GOOS and GOARCH. The old names are gone; the new tools are available through the go tool mechanism as go tool compile, go tool asm, and go tool link. Also, the file suffixes .6, .8, etc. for the intermediate object files are also gone; now they are just plain .o files.

For example, to build and link a program on amd64 for Darwin using the tools directly, rather than through go build, one would run:

$ export GOOS=darwin GOARCH=amd64
$ go tool compile program.go
$ go tool link program.o


Because the go/types package has now moved into the main repository (see below), the vet and cover tools have also been moved. They are no longer maintained in the external golang.org/x/tools repository, although (deprecated) source still resides there for compatibility with old releases.


As described above, the compiler in Go 1.5 is a single Go program, translated from the old C source, that replaces 6g, 8g, and so on. Its target is configured by the environment variables GOOS and GOARCH.

The 1.5 compiler is mostly equivalent to the old, but some internal details have changed. One significant change is that evaluation of constants now uses the math/big package rather than a custom (and less well tested) implementation of high precision arithmetic. We do not expect this to affect the results.

For the amd64 architecture only, the compiler has a new option, -dynlink, that assists dynamic linking by supporting references to Go symbols defined in external shared libraries.


Like the compiler and linker, the assembler in Go 1.5 is a single program that replaces the suite of assemblers (6a, 8a, etc.) and the environment variables GOARCH and GOOS configure the architecture and operating system. Unlike the other programs, the assembler is a wholly new program written in Go.

The new assembler is very nearly compatible with the previous ones, but there are a few changes that may affect some assembler source files. See the updated assembler guide for more specific information about these changes. In summary:

First, the expression evaluation used for constants is a little different. It now uses unsigned 64-bit arithmetic and the precedence of operators (+, -, <<, etc.) comes from Go, not C. We expect these changes to affect very few programs but manual verification may be required.

Perhaps more important is that on machines where SP or PC is only an alias for a numbered register, such as R13 for the stack pointer and R15 for the hardware program counter on ARM, a reference to such a register that does not include a symbol is now illegal. For example, SP and 4(SP) are illegal but sym+4(SP) is fine. On such machines, to refer to the hardware register use its true R name.

One minor change is that some of the old assemblers permitted the notation


to define a named constant. Since this is always possible to do with the traditional C-like #define notation, which is still supported (the assembler includes an implementation of a simplified C preprocessor), the feature was removed.

The linker in Go 1.5 is now one Go program, that replaces 6l, 8l, etc. Its operating system and instruction set are specified by the environment variables GOOS and GOARCH.

There are several other changes. The most significant is the addition of a -buildmode option that expands the style of linking; it now supports situations such as building shared libraries and allowing other languages to call into Go libraries. Some of these were outlined in a design document. For a list of the available build modes and their use, run

$ go help buildmode

Another minor change is that the linker no longer records build time stamps in the header of Windows executables. Also, although this may be fixed, Windows cgo executables are missing some DWARF information.

Finally, the -X flag, which takes two arguments, as in

-X importpath.name value

now also accepts a more common Go flag style with a single argument that is itself a name=value pair:

-X importpath.name=value

Although the old syntax still works, it is recommended that uses of this flag in scripts and the like be updated to the new form.

Go command

The go command’s basic operation is unchanged, but there are a number of changes worth noting.

The previous release introduced the idea of a directory internal to a package being unimportable through the go command. In 1.4, it was tested with the introduction of some internal elements in the core repository. As suggested in the design document, that change is now being made available to all repositories. The rules are explained in the design document, but in summary any package in or under a directory named internal may be imported by packages rooted in the same subtree. Existing packages with directory elements named internal may be inadvertently broken by this change, which was why it was advertised in the last release.

Another change in how packages are handled is the experimental addition of support for “vendoring”. For details, see the documentation for the go command and the design document.

There have also been several minor changes. Read the documentation for full details.

Go vet command

The go tool vet command now does more thorough validation of struct tags.

Trace command

A new tool is available for dynamic execution tracing of Go programs. The usage is analogous to how the test coverage tool works. Generation of traces is integrated into go test, and then a separate execution of the tracing tool itself analyzes the results:

$ go test -trace=trace.out path/to/package
$ go tool trace [flags] pkg.test trace.out

The flags enable the output to be displayed in a browser window. For details, run go tool trace -help. There is also a description of the tracing facility in this talk from GopherCon 2015.

Go doc command

A few releases back, the go doc command was deleted as being unnecessary. One could always run “godoc .” instead. The 1.5 release introduces a new go doc command with a more convenient command-line interface than godoc’s. It is designed for command-line usage specifically, and provides a more compact and focused presentation of the documentation for a package or its elements, according to the invocation. It also provides case-insensitive matching and support for showing the documentation for unexported symbols. For details run “go help doc”.


When parsing #cgo lines, the invocation ${SRCDIR} is now expanded into the path to the source directory. This allows options to be passed to the compiler and linker that involve file paths relative to the source code directory. Without the expansion the paths would be invalid when the current working directory changes.

Solaris now has full cgo support.

On Windows, cgo now uses external linking by default.

When a C struct ends with a zero-sized field, but the struct itself is not zero-sized, Go code can no longer refer to the zero-sized field. Any such references will have to be rewritten.


As always, the changes are so general and varied that precise statements about performance are difficult to make. The changes are even broader ranging than usual in this release, which includes a new garbage collector and a conversion of the runtime to Go. Some programs may run faster, some slower. On average the programs in the Go 1 benchmark suite run a few percent faster in Go 1.5 than they did in Go 1.4, while as mentioned above the garbage collector’s pauses are dramatically shorter, and almost always under 10 milliseconds.

Builds in Go 1.5 will be slower by a factor of about two. The automatic translation of the compiler and linker from C to Go resulted in unidiomatic Go code that performs poorly compared to well-written Go. Analysis tools and refactoring helped to improve the code, but much remains to be done. Further profiling and optimization will continue in Go 1.6 and future releases. For more details, see these slides and associated video.

Standard library


The flag package’s PrintDefaults function, and method on FlagSet, have been modified to create nicer usage messages. The format has been changed to be more human-friendly and in the usage messages a word quoted with `backquotes` is taken to be the name of the flag’s operand to display in the usage message. For instance, a flag created with the invocation,

cpuFlag = flag.Int("cpu", 1, "run `N` processes in parallel")

will show the help message,

-cpu N
        run N processes in parallel (default 1)

Also, the default is now listed only when it is not the zero value for the type.

Floats in math/big

The math/big package has a new, fundamental data type, Float, which implements arbitrary-precision floating-point numbers. A Float value is represented by a boolean sign, a variable-length mantissa, and a 32-bit fixed-size signed exponent. The precision of a Float (the mantissa size in bits) can be specified explicitly or is otherwise determined by the first operation that creates the value. Once created, the size of a Float’s mantissa may be modified with the SetPrec method. Floats support the concept of infinities, such as are created by overflow, but values that would lead to the equivalent of IEEE 754 NaNs trigger a panic. Float operations support all IEEE-754 rounding modes. When the precision is set to 24 (53) bits, operations that stay within the range of normalized float32 (float64) values produce the same results as the corresponding IEEE-754 arithmetic on those values.

Go types

The go/types package up to now has been maintained in the golang.org/x repository; as of Go 1.5 it has been relocated to the main repository. The code at the old location is now deprecated. There is also a modest API change in the package, discussed below.

Associated with this move, the go/constant package also moved to the main repository; it was golang.org/x/tools/exact before. The go/importer package also moved to the main repository, as well as some tools described above.


The DNS resolver in the net package has almost always used cgo to access the system interface. A change in Go 1.5 means that on most Unix systems DNS resolution will no longer require cgo, which simplifies execution on those platforms. Now, if the system’s networking configuration permits, the native Go resolver will suffice. The important effect of this change is that each DNS resolution occupies a goroutine rather than a thread, so a program with multiple outstanding DNS requests will consume fewer operating system resources.

The decision of how to run the resolver applies at run time, not build time. The netgo build tag that has been used to enforce the use of the Go resolver is no longer necessary, although it still works. A new netcgo build tag forces the use of the cgo resolver at build time. To force cgo resolution at run time set GODEBUG=netdns=cgo in the environment. More debug options are documented here.

This change applies to Unix systems only. Windows, Mac OS X, and Plan 9 systems behave as before.


The reflect package has two new functions: ArrayOf and FuncOf. These functions, analogous to the extant SliceOf function, create new types at runtime to describe arrays and functions.


Several dozen bugs were found in the standard library through randomized testing with the go-fuzz tool. Bugs were fixed in the archive/tar, archive/zip, compress/flate, encoding/gob, fmt, html/template, image/gif, image/jpeg, image/png, and text/template, packages. The fixes harden the implementation against incorrect and malicious inputs.

Minor changes to the library