Compiler directives

Learn directives like //go:nosplit.

Compiler directives

Comments starting with //go: are instructions to the compiler (no space in between). Common ones:

//go:noinline — don't inline.
//go:build linux — compile the file only under this condition.
//go:embed — embed a file in the binary.
//go:generate — a command run by go generate.
//go:nosplit, //go:noescape — low-level directives for the runtime and assembly.

go:embed and go:build

With the embed package files go into the binary at build time; they don't need to be on disk at run time:

import _ "embed"

//go:embed surum.txt
var surum string

Build constraints go at the top of the file: //go:build linux && amd64. File name suffixes such as _linux.go and _test.go add similar constraints.

Exercise Embedded file system

Try the embed idea without a disk file: create a file system with testing/fstest.MapFS where "surum.txt" contains "1.4.0", then read it with fs.ReadFile and print it.

Show example solution

package main

import (
    "fmt"
    "io/fs"
    "testing/fstest"
)

func main() {
    fsys := fstest.MapFS{"surum.txt": {Data: []byte("1.4.0")}}
    b, err := fs.ReadFile(fsys, "surum.txt")
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(b))
}

Test yourself

What happens if you write <code>// go:noinline</code> (with a space)?
- It works the same
- It fails to compile
- The function is deleted
- It counts as an ordinary comment and has no effect
Show answer

It counts as an ordinary comment and has no effect — Directives must start exactly with //go:; the spaced version is ignored by the compiler.

Build constraint

//go:____ !windows

package main

Show answer

//go:build !windows

package main

Directive and effect

//go:build

Conditional compilation
//go:embed

Embed a file
//go:generate

Code generation command
//go:noinline

Don't inline

Next lesson: Summary - Compiler

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