4

There's no text preprocessing in Go. Like Java and others like it, if I want to remove a block of code I can use a const value and surround the code with an if.

If I do this, does the compiler optimise the code from the AST and out of the generated code? Or is the condition executed each time?

EDIT: If I want to replicate #ifdef what's the best way to do it?

Joe
  • 46,419
  • 33
  • 155
  • 245
  • Any decent compiler will optimize this out. Doing optimizations while constructing the AST (like GCC does) would be moronic, though. –  Jun 28 '12 at 21:12
  • 1
    Yes but does the Go compiler do it? Also, what's wrong with optimising ASTs? – Joe Jun 28 '12 at 21:30

1 Answers1

9

If statements with constant conditions are not the same as #ifdef because the code inside is always compiled no matter what. However, the compilers do optimize it away when possible. Consider this program:

package main

import (
    "fmt"
)

func main() {
    if false {
        fmt.Println("Hello, world!")
    }
}

If we run go tool 6g -S test.go, here's the assembly output for the main function:

--- prog list "main" ---
0000 (test.go:7) TEXT    main+0(SB),$40-0
0001 (test.go:11) RET     ,

The dead code is gone so all it does is return.

If you do need to actually conditionally compile parts of your code, it's best to do it through the build system.

Evan Shaw
  • 23,839
  • 7
  • 70
  • 61
  • Excellent, thanks! But what build system? I saw in one of the videos something along the lines of "in order to make `go build` simple we removed most parts you'd recognise as traditionally part of the build system, such as text preprocessing". From your answer, the `if` is the right way to go, but text processing would be the only way to simulate `#ifdef`, right? – Joe Jun 29 '12 at 08:31
  • 6
    You can pass in tags to `go build` through the -tags flag. Then you can conditionally build files by placing things like `// +build tag` or `// +build !tag` in the file. See this documentation: http://golang.org/pkg/go/build/ – Evan Shaw Jun 30 '12 at 06:31