cmd/compile: put constants before variables in initialization order

Fixes #66575

Change-Id: I03f4d4577b88ad0a92b260b2efd0cb9fe5082b2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/575075
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/types2/initorder.go

@@ -309,6 +309,14 @@ func (a nodeQueue) Swap(i, j int) {
 
 func (a nodeQueue) Less(i, j int) bool {
 	x, y := a[i], a[j]
+
+	// Prioritize all constants before non-constants. See go.dev/issue/66575/.
+	_, xConst := x.obj.(*Const)
+	_, yConst := y.obj.(*Const)
+	if xConst != yConst {
+		return xConst
+	}
+
 	// nodes are prioritized by number of incoming dependencies (1st key)
 	// and source order (2nd key)
 	return x.ndeps < y.ndeps || x.ndeps == y.ndeps && x.obj.order() < y.obj.order()

src/go/types/initorder.go

@@ -311,6 +311,14 @@ func (a nodeQueue) Swap(i, j int) {
 
 func (a nodeQueue) Less(i, j int) bool {
 	x, y := a[i], a[j]
+
+	// Prioritize all constants before non-constants. See go.dev/issue/66575/.
+	_, xConst := x.obj.(*Const)
+	_, yConst := y.obj.(*Const)
+	if xConst != yConst {
+		return xConst
+	}
+
 	// nodes are prioritized by number of incoming dependencies (1st key)
 	// and source order (2nd key)
 	return x.ndeps < y.ndeps || x.ndeps == y.ndeps && x.obj.order() < y.obj.order()

test/fixedbugs/issue66575.go

@@ -0,0 +1,31 @@
+// run
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var (
+	v0 = initv0()
+	v1 = initv1()
+)
+
+const c = "c"
+
+func initv0() string {
+	println("initv0")
+	if c != "" { // have a dependency on c
+		return ""
+	}
+	return ""
+}
+
+func initv1() string {
+	println("initv1")
+	return ""
+}
+
+func main() {
+	// do nothing
+}

test/fixedbugs/issue66575.out

@@ -0,0 +1,2 @@
+initv0
+initv1