Why Does Enabling CGO_ENABLED Slow Down Compilation, Even Without C Bindings?

Why CGO_ENABLED Can Slow Down Compilation, Even Without C Bindings

When creating programs that involve network usage, enabling CGO_ENABLED for compilation can result in a significant slowdown. This happens even if the program does not interact with C code. Consider the following simple HTTP server:

package main

import (
    "flag"
    "fmt"
    "log"
    "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hi! glad you requested %s.\n", r.URL.Path[1:])
}

func main() {
    port := flag.Int("port", 9000, "")
    flag.Parse()

    http.HandleFunc("/", handler)
    err := http.ListenAndServe(fmt.Sprintf(":%d", *port), nil)
    if err != nil {
        log.Fatal(err)
    }
}

A comparison of compilation times reveals a stark difference:

% time go build
go build  0.46s user 0.06s system 131% cpu 0.396 total
% time CGO_ENABLED=0 go build
CGO_ENABLED=0 go build  3.93s user 0.15s system 143% cpu 2.849 total

The reason for this is that packages in the standard library are compiled without flags. When CGO_ENABLED is set, it changes the compilation flags, which means that these pre-built packages can no longer be used. Hence, a large portion of the standard library must be recompiled.

While installing packages via go build -i can alleviate this issue, it comes with its own drawbacks. Installing packages built with CGO_ENABLED=0 will enhance the speed of future builds, but it will also slow down builds that do not use it.

To resolve this conflict, consider using -installsuffix and/or -pkgdir flags, which create separate directories for different compilation modes, optimizing build efficiency and eliminating the delay associated with compiling with CGO_ENABLED.

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