Build constraints are commonly used to separate normal unit tests from integration tests that require external resources, like a database or network access. To do this, add a custom build constraint to the top of the test file:

// +build integration
 
package main
 
import (
    "testing"
)
 
func TestThatRequiresNetworkAccess(t *testing.T) {
    t.Fatal("It failed!")
}

The test file will not compile into the build executable unless the following invocation of go test is used:

go test -tags "integration"

Results:

$ go test
?       bitbucket.org/yourname/yourproject    [no test files]
$ go test -tags "integration"
--- FAIL: TestThatRequiresNetworkAccess (0.00s)
        main_test.go:10: It failed!
FAIL
exit status 1
FAIL    bitbucket.org/yourname/yourproject    0.003s

We can optimize a simple xor function for only architectures that support unaligned reads/writes by creating two files that define the function and prefixing them with a build constraint (for an actual example of the xor code which is out of scope here, see crypto/cipher/xor.go in the standard library):

// +build 386 amd64 s390x

package cipher

func xorBytes(dst, a, b []byte) int { /* This function uses unaligned reads / writes to optimize the operation */ }

and for other architectures:

// +build !386,!amd64,!s390x

package cipher

func xorBytes(dst, a, b []byte) int { /* This version of the function just loops and xors */ }