cmd/go: add PGO auto mode

Add "auto" mode for the -pgo build flag. When -pgo=auto is
specified, if there is a default.pgo file in the directory of the
main package, it will be selected and used for the build.

Currently it requires exactly one main package when -pgo=auto is
specified. (We'll support multiple main packages in the future.)

Also apply to other build-related subcommands, "go install", "go
run", "go test", and "go list".

For #55022.

Change-Id: Iab7974ab8932daf0e83506de505e044a8e412466
Reviewed-on: https://go-review.googlesource.com/c/go/+/438737
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -202,6 +202,8 @@
 //		run through go run and go test respectively.
 //	-pgo file
 //		specify the file path of a profile for profile-guided optimization (PGO).
+//		Special name "auto" lets the go command select a file named
+//		"default.pgo" in the main package's directory if that file exists.
 //		Special name "off" turns off PGO.
 //	-pkgdir dir
 //		install and load all packages from dir instead of the usual locations.

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

@@ -2807,7 +2807,9 @@ func PackagesAndErrors(ctx context.Context, opts PackageOpts, patterns []string)
 			// We need to test whether the path is an actual Go file and not a
 			// package path or pattern ending in '.go' (see golang.org/issue/34653).
 			if fi, err := fsys.Stat(p); err == nil && !fi.IsDir() {
-				return []*Package{GoFilesPackage(ctx, opts, patterns)}
+				pkgs := []*Package{GoFilesPackage(ctx, opts, patterns)}
+				setPGOProfilePath(pkgs)
+				return pkgs
 			}
 		}
 	}
@@ -2886,9 +2888,60 @@ func PackagesAndErrors(ctx context.Context, opts PackageOpts, patterns []string)
 	// their dependencies).
 	setToolFlags(pkgs...)
 
+	setPGOProfilePath(pkgs)
+
 	return pkgs
 }
 
+// setPGOProfilePath sets cfg.BuildPGOFile to the PGO profile path.
+// 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
+
+	case "auto":
+		// Locate PGO profile from the main package.
+
+		setError := func(p *Package) {
+			if p.Error == nil {
+				p.Error = &PackageError{Err: errors.New("-pgo=auto requires exactly one main package")}
+			}
+		}
+
+		var mainpkg *Package
+		for _, p := range pkgs {
+			if p.Name == "main" {
+				if mainpkg != nil {
+					setError(p)
+					setError(mainpkg)
+					continue
+				}
+				mainpkg = p
+			}
+		}
+		if mainpkg == nil {
+			// No main package, no default.pgo to look for.
+			return
+		}
+		file := filepath.Join(mainpkg.Dir, "default.pgo")
+		if fi, err := os.Stat(file); err == nil && !fi.IsDir() {
+			cfg.BuildPGOFile = file
+		}
+
+	default:
+		// Profile specified from the command line.
+		// Make it absolute path, as the compiler runs on various directories.
+		if p, err := filepath.Abs(cfg.BuildPGO); err != nil {
+			base.Fatalf("fail to get absolute path of PGO file %s: %v", cfg.BuildPGO, err)
+		} else {
+			cfg.BuildPGOFile = p
+		}
+	}
+}
+
 // CheckPackageErrors prints errors encountered loading pkgs and their
 // dependencies, then exits with a non-zero status if any errors were found.
 func CheckPackageErrors(pkgs []*Package) {

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

@@ -159,6 +159,8 @@ and test commands:
 		run through go run and go test respectively.
 	-pgo file
 		specify the file path of a profile for profile-guided optimization (PGO).
+		Special name "auto" lets the go command select a file named
+		"default.pgo" in the main package's directory if that file exists.
 		Special name "off" turns off PGO.
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.

src/cmd/go/internal/work/init.go

@@ -84,8 +84,6 @@ func BuildInit() {
 	if cfg.BuildRace && cfg.BuildCoverMode != "atomic" {
 		base.Fatalf(`-covermode must be "atomic", not %q, when -race is enabled`, cfg.BuildCoverMode)
 	}
-
-	setPGOProfilePath()
 }
 
 // fuzzInstrumentFlags returns compiler flags that enable fuzzing instrumation
@@ -442,21 +440,3 @@ func compilerRequiredAsanVersion() error {
 	}
 	return nil
 }
-
-func setPGOProfilePath() {
-	switch cfg.BuildPGO {
-	case "":
-		fallthrough // default to "auto"
-	case "off":
-		// Nothing to do.
-	case "auto":
-		base.Fatalf("-pgo=auto is not implemented")
-	default:
-		// make it absolute path, as the compiler runs on various directories.
-		if p, err := filepath.Abs(cfg.BuildPGO); err != nil {
-			base.Fatalf("fail to get absolute path of PGO file %s: %v", cfg.BuildPGO, err)
-		} else {
-			cfg.BuildPGOFile = p
-		}
-	}
-}

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

@@ -0,0 +1,69 @@
+# Test go build -pgo=auto flag.
+
+# use default.pgo for a single main package
+go build -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+# check that pgo applied to dependencies
+stderr 'compile.*-p test/dep.*-pgoprofile=.*default\.pgo'
+
+# use default.pgo for ... with a single main package
+go build -n -pgo=auto ./a/...
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+# error with multiple packages
+! go build -n -pgo=auto ./b/...
+stderr '-pgo=auto requires exactly one main package'
+
+# build succeeds without PGO when default.pgo file is absent
+go build -n -pgo=auto -o nopgo.exe ./nopgo
+stderr 'compile.*nopgo.go'
+! stderr '-pgoprofile'
+
+# other build-related commands
+go install -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+go run -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+go test -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go.*a1_test.go'
+stderr 'compile.*-pgoprofile=.*default\.pgo.*external_test.go'
+
+# go list commands should succeed as usual
+go list -pgo=auto ./a/a1
+
+go list -test -pgo=auto ./a/a1
+
+go list -deps -pgo=auto ./a/a1
+
+-- go.mod --
+module test
+go 1.20
+-- a/a1/a1.go --
+package main
+import _ "test/dep"
+func main() {}
+-- a/a1/a1_test.go --
+package main
+import "testing"
+func TestA(*testing.T) {}
+-- a/a1/external_test.go --
+package main_test
+import "testing"
+func TestExternal(*testing.T) {}
+-- a/a1/default.pgo --
+-- b/b1/b1.go --
+package main
+func main() {}
+-- b/b1/default.pgo --
+-- b/b2/b2.go --
+package main
+func main() {}
+-- b/b2/default.pgo --
+-- nopgo/nopgo.go --
+package main
+func main() {}
+-- dep/dep.go --
+package dep