content/go1.7-binary-size.article - blog - Git at Google

 Smaller Go 1.7 binaries
 18 Aug 2016

 David Crawshaw
 crawshaw@golang.org

 * Introduction

 Go was designed for writing servers.
 That is how it is most widely used today, and as a result a lot of
 work on the runtime and compiler is focused on issues that matter to
 servers: latency, ease of deployment, precise garbage collection,
 fast startup time, performance.

 As Go gets used for a wider variety of programs, there are new issues that must be considered.
 One of these is binary size.
 It has been on the radar for a long time
 (issue [[https://golang.org/issue/6853][#6853]] was filed over two
 years ago), but the growing interest in using Go for
 deploying binaries on smaller devices — such as the Raspberry Pi or
 mobile devices — means it received some attention for the Go 1.7
 release.

 * Work done in Go 1.7

 Three significant changes in Go 1.7 affect binary size.

 The first is the new SSA backend that was enabled for AMD64 in this release.
 While the primary motivation for SSA was improved performance, the
 better generated code is also smaller.
 The SSA backend shrinks Go binaries by ~5%.
 We expect larger gains for the more RISC-like architectures
 like ARM and MIPS when those backends have been converted to SSA in Go 1.8.

 The second change is method pruning.
 Until 1.6, all methods on all used types were kept, even if some of
 the methods were never called.
 This is because they might be called through an interface, or called
 dynamically using the reflect package.
 Now the compiler discards any unexported methods that do not match an
 interface.
 Similarly the linker can discard other exported methods, those that are only
 accessible through reflection, if the corresponding
 [[https://golang.org/pkg/reflect/#Value.Call][reflection features]]
 are not used anywhere in the program.
 That change shrinks binaries by 5–20%.

 The third change is a more compact format for run-time type
 information used by the reflect package.
 The encoding format was originally designed to make the decoder in
 the runtime and reflect packages as simple as possible. By making this
 code a bit harder to read we can compress the format without affecting
 the run-time performance of Go programs.
 The new format shrinks Go binaries by a further 5–15%.
 Libraries built for Android and archives built for iOS shrink further
 as the new format contains fewer pointers, each of which requires
 dynamic relocations in position independent code.

 In addition, there were many small improvements such as improved
 interface data layout, better static data layout, and simplified
 dependencies. For example, the HTTP client no longer links in the entire HTTP
 server.
 The full list of changes can be found in issue
 [[https://golang.org/issue/6853][#6853]].

 * Results

 Typical programs, ranging from tiny toys to large production programs,
 are about 30% smaller when built with Go 1.7.

 The canonical hello world program goes from 2.3MB to 1.6MB:

 	package main

 	import "fmt"

 	func main() {
 		fmt.Println("Hello, World!")
 	}

 When compiled without debugging information the statically
 linked binary is now under a megabyte.

 .image go1.7-binary-size.png

 A large production program used for testing this cycle, `jujud`, went from 94MB
 to 67MB.

 Position-independent binaries are 50% smaller.

 In a position-independent executable (PIE), a pointer in a read-only
 data section requires a dynamic relocation.
 Because the new format for type information replaces pointers by
 section offsets, it saves 28 bytes per pointer.

 Position-independent executables with debugging information removed
 are particularly important to mobile developers, as this is the kind
 of program shipped to phones.
 Big downloads make for a poor user experience, so the reduction here
 is good news.

 * Future Work

 Several changes to the run-time type information were too late for the
 Go 1.7 freeze, but will hopefully make it into 1.8, further shrinking
 programs, especially position-independent ones.

 These changes are all conservative, reducing binary size without increasing
 build time, startup time, overall execution time, or memory usage.
 We could take more radical steps to reduce binary size: the
 [[http://upx.sourceforge.net/][upx]] tool for compressing executables
 shrinks binaries by another 50% at the cost of increased startup time
 and potentially increased memory use.
 For extremely small systems (the kind that might live on a keychain)
 we could build a version of Go without reflection, though it is
 unclear whether such a restricted language would be sufficiently
 useful.
 For some algorithms in the runtime we could use slower but more
 compact implementions when every kilobyte counts.
 All of these call for more research in later development cycles.

 To the many contributors who helped make Go 1.7 binaries smaller,
 thank you!
	Smaller Go 1.7 binaries
	18 Aug 2016

	David Crawshaw
	crawshaw@golang.org

	* Introduction

	Go was designed for writing servers.
	That is how it is most widely used today, and as a result a lot of
	work on the runtime and compiler is focused on issues that matter to
	servers: latency, ease of deployment, precise garbage collection,
	fast startup time, performance.

	As Go gets used for a wider variety of programs, there are new issues that must be considered.
	One of these is binary size.
	It has been on the radar for a long time
	(issue [[https://golang.org/issue/6853][#6853]] was filed over two
	years ago), but the growing interest in using Go for
	deploying binaries on smaller devices — such as the Raspberry Pi or
	mobile devices — means it received some attention for the Go 1.7
	release.

	* Work done in Go 1.7

	Three significant changes in Go 1.7 affect binary size.

	The first is the new SSA backend that was enabled for AMD64 in this release.
	While the primary motivation for SSA was improved performance, the
	better generated code is also smaller.
	The SSA backend shrinks Go binaries by ~5%.
	We expect larger gains for the more RISC-like architectures
	like ARM and MIPS when those backends have been converted to SSA in Go 1.8.

	The second change is method pruning.
	Until 1.6, all methods on all used types were kept, even if some of
	the methods were never called.
	This is because they might be called through an interface, or called
	dynamically using the reflect package.
	Now the compiler discards any unexported methods that do not match an
	interface.
	Similarly the linker can discard other exported methods, those that are only
	accessible through reflection, if the corresponding
	[[https://golang.org/pkg/reflect/#Value.Call][reflection features]]
	are not used anywhere in the program.
	That change shrinks binaries by 5–20%.

	The third change is a more compact format for run-time type
	information used by the reflect package.
	The encoding format was originally designed to make the decoder in
	the runtime and reflect packages as simple as possible. By making this
	code a bit harder to read we can compress the format without affecting
	the run-time performance of Go programs.
	The new format shrinks Go binaries by a further 5–15%.
	Libraries built for Android and archives built for iOS shrink further
	as the new format contains fewer pointers, each of which requires
	dynamic relocations in position independent code.

	In addition, there were many small improvements such as improved
	interface data layout, better static data layout, and simplified
	dependencies. For example, the HTTP client no longer links in the entire HTTP
	server.
	The full list of changes can be found in issue
	[[https://golang.org/issue/6853][#6853]].

	* Results

	Typical programs, ranging from tiny toys to large production programs,
	are about 30% smaller when built with Go 1.7.

	The canonical hello world program goes from 2.3MB to 1.6MB:

	package main

	import "fmt"

	func main() {
	fmt.Println("Hello, World!")
	}

	When compiled without debugging information the statically
	linked binary is now under a megabyte.

	.image go1.7-binary-size.png

	A large production program used for testing this cycle, `jujud`, went from 94MB
	to 67MB.

	Position-independent binaries are 50% smaller.

	In a position-independent executable (PIE), a pointer in a read-only
	data section requires a dynamic relocation.
	Because the new format for type information replaces pointers by
	section offsets, it saves 28 bytes per pointer.

	Position-independent executables with debugging information removed
	are particularly important to mobile developers, as this is the kind
	of program shipped to phones.
	Big downloads make for a poor user experience, so the reduction here
	is good news.

	* Future Work

	Several changes to the run-time type information were too late for the
	Go 1.7 freeze, but will hopefully make it into 1.8, further shrinking
	programs, especially position-independent ones.

	These changes are all conservative, reducing binary size without increasing
	build time, startup time, overall execution time, or memory usage.
	We could take more radical steps to reduce binary size: the
	[[http://upx.sourceforge.net/][upx]] tool for compressing executables
	shrinks binaries by another 50% at the cost of increased startup time
	and potentially increased memory use.
	For extremely small systems (the kind that might live on a keychain)
	we could build a version of Go without reflection, though it is
	unclear whether such a restricted language would be sufficiently
	useful.
	For some algorithms in the runtime we could use slower but more
	compact implementions when every kilobyte counts.
	All of these call for more research in later development cycles.

	To the many contributors who helped make Go 1.7 binaries smaller,
	thank you!