cmd/go: always track visited packages in setPGOProfilePath

Currently we only track visited (copied) packages when a copy is
required. When a copy is not required, we will rewalk each package's
entire dependency graph every time we see it, which is terribly
inefficient.

Pull the visited package check up a level so that we visit packages only
once regardless of how many times they are visited.

Fixes #60455.
Fixes #60428.

Change-Id: I4e9b31eeeaa170db650c461a5de2ca984b9aba0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/498735
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: prattmic

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

@@ -2950,17 +2950,21 @@ func setPGOProfilePath(pkgs []*Package) {
 				continue // no profile
 			}
 
-			copied := make(map[*Package]*Package)
+			// Packages already visited. The value should replace
+			// the key, as it may be a forked copy of the original
+			// Package.
+			visited := make(map[*Package]*Package)
 			var split func(p *Package) *Package
 			split = func(p *Package) *Package {
+				if p1 := visited[p]; p1 != nil {
+					return p1
+				}
+
 				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")
 					}
@@ -2969,9 +2973,11 @@ func setPGOProfilePath(pkgs []*Package) {
 					// 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
+					p1.Internal.ForMain = pmain.ImportPath
+					visited[p] = p1
 					p = p1
-					p.Internal.ForMain = pmain.ImportPath
+				} else {
+					visited[p] = p
 				}
 				p.Internal.PGOProfile = file
 				updateBuildInfo(p, file)