From: Robert Griesemer <gri@google.com>
Date: Mon, 20 Oct 2025 17:25:23 +0000 (-0700)
Subject: go/types, types2: in resolveUnderlying, only compute path when needed
X-Git-Tag: go1.26rc1~536
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=5a42af7f6c9ba057171d8a5af6f4b721b8eff10a;p=gostls13.git

go/types, types2: in resolveUnderlying, only compute path when needed

When following a RHS chain, the (TypeName) Object path is only needed
when there is a cycle (i.e., an error), in which case we can be slow.
Rather than always compute the path, only compute it in the error case.
In the same vain, allocate the seen map lazily, only when needed.

This code could use a test (it doesn't seem to be encountered by our
test suite), but I haven't found a case to provoke the error yet.

Change-Id: Iff6313394442a251adc56580f746928ec13450fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/712321
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
---

diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
index 856f33028a..99e2743484 100644
--- a/src/cmd/compile/internal/types2/named.go
+++ b/src/cmd/compile/internal/types2/named.go
@@ -593,9 +593,7 @@ func (n *Named) resolveUnderlying() {
 		return
 	}
 
-	seen := make(map[*Named]int)
-	var path []Object
-
+	var seen map[*Named]int // allocated lazily
 	var u Type
 	for rhs := Type(n); u == nil; {
 		switch t := rhs.(type) {
@@ -607,9 +605,15 @@ func (n *Named) resolveUnderlying() {
 
 		case *Named:
 			if i, ok := seen[t]; ok {
+				// compute cycle path
+				path := make([]Object, len(seen))
+				for t, j := range seen {
+					path[j] = t.obj
+				}
+				path = path[i:]
 				// Note: This code may only be called during type checking,
 				//       hence n.check != nil.
-				n.check.cycleError(path[i:], firstInSrc(path[i:]))
+				n.check.cycleError(path, firstInSrc(path))
 				u = Typ[Invalid]
 				break
 			}
@@ -625,8 +629,10 @@ func (n *Named) resolveUnderlying() {
 				break
 			}
 
+			if seen == nil {
+				seen = make(map[*Named]int)
+			}
 			seen[t] = len(seen)
-			path = append(path, t.obj)
 
 			assert(t.fromRHS != nil || t.allowNilRHS)
 			rhs = t.fromRHS
diff --git a/src/go/types/named.go b/src/go/types/named.go
index 5c5c837987..b436d073e6 100644
--- a/src/go/types/named.go
+++ b/src/go/types/named.go
@@ -596,9 +596,7 @@ func (n *Named) resolveUnderlying() {
 		return
 	}
 
-	seen := make(map[*Named]int)
-	var path []Object
-
+	var seen map[*Named]int // allocated lazily
 	var u Type
 	for rhs := Type(n); u == nil; {
 		switch t := rhs.(type) {
@@ -610,9 +608,15 @@ func (n *Named) resolveUnderlying() {
 
 		case *Named:
 			if i, ok := seen[t]; ok {
+				// compute cycle path
+				path := make([]Object, len(seen))
+				for t, j := range seen {
+					path[j] = t.obj
+				}
+				path = path[i:]
 				// Note: This code may only be called during type checking,
 				//       hence n.check != nil.
-				n.check.cycleError(path[i:], firstInSrc(path[i:]))
+				n.check.cycleError(path, firstInSrc(path))
 				u = Typ[Invalid]
 				break
 			}
@@ -628,8 +632,10 @@ func (n *Named) resolveUnderlying() {
 				break
 			}
 
+			if seen == nil {
+				seen = make(map[*Named]int)
+			}
 			seen[t] = len(seen)
-			path = append(path, t.obj)
 
 			assert(t.fromRHS != nil || t.allowNilRHS)
 			rhs = t.fromRHS