How to remove unused code at compile time?

Question

We've built a Go package that is used by many of us.

It's imported using the standard import ("package-name") method.

At compile time though, all of our utilities, including the very small ones, end up as very large binaries.

We've extracted all the strings in the utilities and discovered that the entire package is being compiled into each and every utility. Including functions that are not being used by those utilities.

EDIT 1:

Thank you to the people who are responding to this question.

Here's what we are seeing:

main.go

package main

import "play/subplay"

func main() {
    subplay.A()
}

play/subplay.go

package subplay

func A() {
    fmt.Printf("this is function A()")
}

func B() {
    fmt.Printf("secret string")
}

Function B() is never called. Yet, after building the binary, we find the string "secret string" into main.exe.

How can we remove unused code from Go programs at compile time?

Answer 1

The compiler already does this. All code ends up in package files (.a), but in the executable binary the Go tool does not include everything from imported packages, only what is needed (or more precisely: it excludes things that it can prove unreachable).

See possible duplicate Splitting client/server code.

One thing to note here: if an imported package (from which you only want to include things you refer to) imports other packages, this has to be done recursively of course.

For example if you import this package:

package subplay

func A() {}

And call nothing from it:

package main

import _ "play/subplay"

func main() {
}

The result binary (Go 1.8, linux, amd64) will be 960,134 bytes (roughly 1 MB).

If you change subplay to import net/http:

package subplay

import _ "net/http"

func A() {}

But still don't call anything from net/http, the result will be: 5,370,935 bytes (roughly 5 MB)! (Note that net/http also imports 39 other packages!)

Now if you go ahead and start using things from net/http:

package subplay

import "net/http"

func A() {
    http.ListenAndServe("", nil)
}

But in the main package you still don't call subplay.A(), the size of the executable binary does not change: remains 5,370,935 bytes!

And when you change the main package to call subplay.A():

package main

import "play/subplay"

func main() {
    subplay.A()
}

The result binary grows: 5,877,919 bytes!

If you change http.ListenAndServe() to http.ListenAndServeTLS():

func A() {
    http.ListenAndServeTLS("", "", "", nil)
}

Result binary is: 6,041,535 bytes.

As you can see, what gets compiled into the executable binary does depend on what you call / use from the imported packages.

Also don't forget that Go is a statically linked language, the result executable binary has to include everything it needs. See related question: Reason for huge size of compiled executable of Go

Answer 2

You can turn your package into a plugin, if you don’t mind the restrictions (plugins need Go 1.8 and currently only work on Linux).

[edit] Since you are using Windows, this is not an option for you.