Why is Go compilation slower with `CGO_ENABLED=0` even without C bindings?

Compilation Time Slowdown with CGO_ENABLED=0

When developing network-based Go programs, compiling with CGO_ENABLED=0 can result in a significant slowdown compared to CGO_ENABLED=1. This is observed even when no C bindings are employed, as in the case of a basic 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)
    }
}

Compilation times are significantly slower when CGO_ENABLED=0:

% 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

Causes of the Slowdown

The difference in compilation time arises from the dependency on standard library packages. When CGO_ENABLED=1, these packages are built with the necessary flags, but when CGO_ENABLED=0, they are not.

Standard library packages without flags cannot be used with CGO_ENABLED=0, forcing most of the library to be rebuilt. This significantly increases the compilation time.

Mitigating the Slowdown

One solution is to install pre-built packages with the desired flags using go build -i. However, this approach introduces its own challenges. Installing packages built without CGO_ENABLED=0 speeds up future builds without CGO_ENABLED=0 but slows down builds with CGO_ENABLED=0.

To address this issue, it is advisable to use the -installsuffix and/or -pkgdir flags, which allow for different package installation directories based on the compilation flags used. This enables efficient builds with different flags.

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