Why is Compilation Time Slower with `CGO_ENABLED=0` in Go?

Slowdown in Compilation Time with CGO_ENABLED=0

When developing network-related Go programs, a significant discrepancy in compilation speed can be observed between builds with CGO_ENABLED=0 and those without. This is evident even in a rudimentary HTTP server example:

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 time benchmarks reveal a substantial slowdown:

% 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

Despite the absence of any direct C bindings, this slowdown persists. The underlying cause lies in the build process.

The Role of Build Flags

CGO_ENABLED is one of many build flags used during compilation to configure optimizations and include C code. When this flag is set to 0, pre-built standard library packages cannot be reused. Consequently, a substantial portion of the standard library must be recompiled, leading to the observed slowdown.

Alternative Approaches

While installing pre-built packages with go build -i can accelerate future builds with CGO_ENABLED=0, it does not address the issue for builds without CGO_ENABLED. To efficiently support both build configurations, separate installation directories should be utilized via the -installsuffix and -pkgdir flags. This segregation ensures that packages built with different flags do not overwrite each other.

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