cmd/go: support multiple main packages with -pgo=auto

In -pgo=auto mode, the go command finds a profile named
default.pgo in the main package's directly, and if found, use it
as the profile for the build. Currently we only support a single
main package when -pgo=auto is used.

When multiple main packages are included in a build, they may
have different default profiles (or some have profiles whereas
some don't), so a common dependent package would need to be built
multiple times, with different profiles (or lack of). This CL
handles this. To do so, we need to split (unshare) the dependency
graph so they can attach different profiles.

Fixes #58099.

Change-Id: I1ad21361967aafbf5089d8d5e89229f95fe31276
Reviewed-on: https://go-review.googlesource.com/c/go/+/472358
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -202,8 +202,10 @@
 //		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.
+//		When the special name "auto" is specified, for each main package in the
+//		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.
 //	-pkgdir dir
 //		install and load all packages from dir instead of the usual locations.

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

@@ -23,6 +23,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"runtime/debug"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -2905,34 +2906,55 @@ func setPGOProfilePath(pkgs []*Package) {
 		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
+		// Locate PGO profiles from the main packages, and
+		// attach the profile to the main package and its
+		// dependencies.
+		// If we're builing multiple main packages, they may
+		// have different profiles. We may need to split (unshare)
+		// the dependency graph so they can attach different
+		// profiles.
 		for _, p := range pkgs {
-			if p.Name == "main" {
-				if mainpkg != nil {
-					setError(p)
-					setError(mainpkg)
-					continue
+			if p.Name != "main" {
+				continue
+			}
+			pmain := p
+			file := filepath.Join(pmain.Dir, "default.pgo")
+			if _, err := os.Stat(file); err != nil {
+				continue // no profile
+			}
+
+			copied := make(map[*Package]*Package)
+			var split func(p *Package) *Package
+			split = func(p *Package) *Package {
+				if len(pkgs) > 1 && p != pmain {
+					// Make a copy, then attach profile.
+					// No need to copy if there is only one root package (we can
+					// attach profile directly in-place).
+					// Also no need to copy the main package.
+					if p1 := copied[p]; p1 != nil {
+						return p1
+					}
+					if p.Internal.PGOProfile != "" {
+						panic("setPGOProfilePath: already have profile")
+					}
+					p1 := new(Package)
+					*p1 = *p
+					// Unalias the Internal.Imports slice, which is we're going to
+					// modify. We don't copy other slices as we don't change them.
+					p1.Internal.Imports = slices.Clone(p.Internal.Imports)
+					copied[p] = p1
+					p = p1
 				}
-				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() {
-			for _, p := range PackageList(pkgs) {
 				p.Internal.PGOProfile = file
+				// Recurse to dependencies.
+				for i, pp := range p.Internal.Imports {
+					p.Internal.Imports[i] = split(pp)
+				}
+				return p
 			}
+
+			// Replace the package and imports with the PGO version.
+			split(pmain)
 		}
 
 	default:
@@ -2979,11 +3001,18 @@ func CheckPackageErrors(pkgs []*Package) {
 	seen := map[string]bool{}
 	reported := map[string]bool{}
 	for _, pkg := range PackageList(pkgs) {
-		if seen[pkg.ImportPath] && !reported[pkg.ImportPath] {
-			reported[pkg.ImportPath] = true
+		// -pgo=auto with multiple main packages can cause a package being
+		// built multiple times (with different profiles).
+		// We check that package import path + profile path is unique.
+		key := pkg.ImportPath
+		if pkg.Internal.PGOProfile != "" {
+			key += " pgo:" + pkg.Internal.PGOProfile
+		}
+		if seen[key] && !reported[key] {
+			reported[key] = true
 			base.Errorf("internal error: duplicate loads of %s", pkg.ImportPath)
 		}
-		seen[pkg.ImportPath] = true
+		seen[key] = true
 	}
 	base.ExitIfErrors()
 }

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

@@ -159,8 +159,10 @@ 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.
+		When the special name "auto" is specified, for each main package in the
+		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.
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.

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

@@ -11,10 +11,6 @@ stderr 'compile.*-p test/dep.*-pgoprofile=.*default\.pgo'
 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'
@@ -54,14 +50,6 @@ 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() {}

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

@@ -0,0 +1,87 @@
+# Test go build -pgo=auto flag with multiple main packages.
+
+go build -n -pgo=auto ./a ./b ./nopgo
+
+# a/default.pgo applies to package a and (transitive)
+# dependencies.
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*a(/|\\\\)a\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep2(/|\\\\)dep2\.go'
+stderr -count=1 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep3(/|\\\\)dep3\.go'
+
+# b/default.pgo applies to package b and (transitive)
+# dependencies.
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*b(/|\\\\)b\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep2(/|\\\\)dep2\.go'
+stderr -count=1 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep3(/|\\\\)dep3\.go'
+
+# nopgo should be built without PGO.
+! stderr 'compile.*-pgoprofile=.*nopgo(/|\\\\)nopgo\.go'
+
+# Dependencies should also be built without PGO.
+# Here we want to match a compile action without -pgoprofile,
+# by matching 3 occurrences of "compile dep.go", among which
+# 2 of them have -pgoprofile (therefore one without).
+stderr -count=3 'compile.*dep(/|\\\\)dep.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep(/|\\\\)dep\.go'
+
+stderr -count=3 'compile.*dep2(/|\\\\)dep2.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep2(/|\\\\)dep2\.go'
+
+stderr -count=3 'compile.*dep3(/|\\\\)dep3.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep3(/|\\\\)dep3\.go'
+
+# go test works the same way
+go test -n -pgo=auto ./a ./b ./nopgo
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*a(/|\\\\)a_test\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*b(/|\\\\)b_test\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+! stderr 'compile.*-pgoprofile=.*nopgo(/|\\\\)nopgo_test\.go'
+
+# Here we have 3 main packages, a, b, and nopgo, where a and b each has
+# its own default.pgo profile, and nopgo has none.
+# All 3 main packages import dep and dep2, both of which then import dep3
+# (a diamond-shape import graph).
+-- go.mod --
+module test
+go 1.20
+-- a/a.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- a/a_test.go --
+package main
+import "testing"
+func TestA(*testing.T) {}
+-- a/default.pgo --
+dummy profile a
+-- b/b.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- b/b_test.go --
+package main
+import "testing"
+func TestB(*testing.T) {}
+-- b/default.pgo --
+dummy profile b
+-- nopgo/nopgo.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- nopgo/nopgo_test.go --
+package main
+import "testing"
+func TestNopgo(*testing.T) {}
+-- dep/dep.go --
+package dep
+import _ "test/dep3"
+-- dep2/dep2.go --
+package dep2
+-- dep3/dep3.go --
+package dep3