avoid rebuilding garble in the main benchmark

Similar to what testscript does, we can reuse the test binary by telling
TestMain to run the main function rather than the Go tests.

This saves a few hundred milliseconds out of each benchmark run.

bench_test.go

@@ -11,7 +11,6 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -53,19 +52,12 @@ func BenchmarkBuild(b *testing.B) {
 	}
 	tdir := b.TempDir()
 
-	garbleBin := filepath.Join(tdir, "garble")
-	if runtime.GOOS == "windows" {
-		garbleBin += ".exe"
-	}
-	err := exec.Command("go", "build", "-o="+garbleBin).Run()
-	qt.Assert(b, err, qt.IsNil)
-
 	// We collect extra metrics.
 	var memoryAllocs, cachedTime, systemTime int64
 
 	outputBin := filepath.Join(tdir, "output")
 	sourceDir := filepath.Join(tdir, "src")
-	err = os.Mkdir(sourceDir, 0o777)
+	err := os.Mkdir(sourceDir, 0o777)
 	qt.Assert(b, err, qt.IsNil)
 
 	writeSourceFile := func(name string, content []byte) {
@@ -85,6 +77,7 @@ func BenchmarkBuild(b *testing.B) {
 		gocache, err := os.MkdirTemp(tdir, "gocache-*")
 		qt.Assert(b, err, qt.IsNil)
 		env := append(os.Environ(),
+			"RUN_GARBLE_MAIN=true",
 			"GOCACHE="+gocache,
 			"GARBLE_WRITE_ALLOCS=true",
 		)
@@ -97,7 +90,7 @@ func BenchmarkBuild(b *testing.B) {
 				writeSourceFile("rebuild.go", []byte(fmt.Sprintf("package main\nvar v%d int", i)))
 			}
 
-			cmd := exec.Command(garbleBin, args...)
+			cmd := exec.Command(os.Args[0], args...)
 			cmd.Env = env
 			cmd.Dir = sourceDir
 

main_test.go

@@ -31,6 +31,9 @@ import (
 var proxyURL string
 
 func TestMain(m *testing.M) {
+	if os.Getenv("RUN_GARBLE_MAIN") == "true" {
+		os.Exit(main1())
+	}
 	os.Exit(testscript.RunMain(garbleMain{m}, map[string]func() int{
 		"garble": main1,
 	}))