cmd/go: enable -pgo=auto by default

Updates #58099.
Updates #55022.

Change-Id: I32eacdf9f008d16566e0b30230ecc25d110a9811
Reviewed-on: https://go-review.googlesource.com/c/go/+/474236
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/cmd/go/internal/load/pkg.go

@@ -2939,8 +2939,6 @@ func PackagesAndErrors(ctx context.Context, opts PackageOpts, patterns []string)
 // In -pgo=auto mode, it finds the default PGO profile.
 func setPGOProfilePath(pkgs []*Package) {
 	switch cfg.BuildPGO {
-	case "":
-		fallthrough // default to "off"
 	case "off":
 		return
 

src/cmd/go/internal/work/build.go

@@ -163,7 +163,7 @@ and test commands:
 		build, the go command selects a file named "default.pgo" in the package's
 		directory if that file exists, and applies it to the (transitive)
 		dependencies of the main package (other packages are not affected).
-		Special name "off" turns off PGO.
+		Special name "off" turns off PGO. The default is "auto".
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.
 		For example, when building with a non-standard configuration,
@@ -318,7 +318,7 @@ func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
 	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
 	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")
-	cmd.Flag.StringVar(&cfg.BuildPGO, "pgo", "", "")
+	cmd.Flag.StringVar(&cfg.BuildPGO, "pgo", "auto", "")
 	cmd.Flag.StringVar(&cfg.BuildPkgdir, "pkgdir", "", "")
 	cmd.Flag.BoolVar(&cfg.BuildRace, "race", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildMSan, "msan", false, "")

src/cmd/go/testdata/script/build_pgo_auto.txt

@@ -34,6 +34,21 @@ go list -test -pgo=auto ./a/a1
 
 go list -deps -pgo=auto ./a/a1
 
+# -pgo=auto is the default. Commands without explicit -pgo=auto
+# should work as -pgo=auto.
+go build -n ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+stderr 'compile.*-p test/dep.*-pgoprofile=.*default\.pgo'
+
+go build -n -o nopgo.exe ./nopgo
+stderr 'compile.*nopgo.go'
+! stderr '-pgoprofile'
+
+# -pgo=off should turn off PGO.
+go build -n -pgo=off ./a/a1
+stderr 'compile.*a1.go'
+! stderr '-pgoprofile'
+
 -- go.mod --
 module test
 go 1.20