cmd/compile: print accurate escape reason for non-const-length slices

This change makes `-m -m` print a better explanation for the case
where a slice is marked as escaping and heap-allocated because it
has a non-constant len/cap.

Fixes #24578

Change-Id: I0ebafb77c758a99857d72b365817bdba7b446cc0
Reviewed-on: https://go-review.googlesource.com/102895
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>

src/cmd/compile/internal/gc/esc.go

@@ -665,18 +665,29 @@ func (e *EscState) esc(n *Node, parent *Node) {
 		}
 	}
 
-	// Big stuff escapes unconditionally
+	// Big stuff and non-constant-sized stuff escapes unconditionally.
-	// "Big" conditions that were scattered around in walk have been gathered here
+	// "Big" conditions that were scattered around in walk have been
+	// gathered here.
 	if n.Esc != EscHeap && n.Type != nil &&
 		(n.Type.Width > maxStackVarSize ||
 			(n.Op == ONEW || n.Op == OPTRLIT) && n.Type.Elem().Width >= 1<<16 ||
 			n.Op == OMAKESLICE && !isSmallMakeSlice(n)) {
+
+		// isSmallMakeSlice returns false for non-constant len/cap.
+		// If that's the case, print a more accurate escape reason.
+		var msgVerb, escapeMsg string
+		if n.Op == OMAKESLICE && (!Isconst(n.Left, CTINT) || !Isconst(n.Right, CTINT)) {
+			msgVerb, escapeMsg = "has ", "non-constant size"
+		} else {
+			msgVerb, escapeMsg = "is ", "too large for stack"
+		}
+
 		if Debug['m'] > 2 {
-			Warnl(n.Pos, "%v is too large for stack", n)
+			Warnl(n.Pos, "%v "+msgVerb+escapeMsg, n)
 		}
 		n.Esc = EscHeap
 		addrescapes(n)
-		e.escassignSinkWhy(n, n, "too large for stack") // TODO category: tooLarge
+		e.escassignSinkWhy(n, n, escapeMsg) // TODO category: tooLarge
 	}
 
 	e.esc(n.Left, n)

test/escape_array.go

@@ -120,3 +120,10 @@ func doesMakeSlice(x *string, y *string) { // ERROR "leaking param: x" "leaking
 	b := make([]*string, 65537) // ERROR "make\(\[\]\*string, 65537\) escapes to heap"
 	b[0] = y
 }
+
+func nonconstArray() {
+	n := 32
+	s1 := make([]int, n)    // ERROR "make\(\[\]int, n\) escapes to heap"
+	s2 := make([]int, 0, n) // ERROR "make\(\[\]int, 0, n\) escapes to heap"
+	_, _ = s1, s2
+}

test/escape_because.go

@@ -118,6 +118,13 @@ func transmit(b []byte) []byte { // ERROR "from ~r1 \(return\) at escape_because
 	return b
 }
 
+func f14() {
+	n := 32
+	s1 := make([]int, n)    // ERROR "make\(\[\]int, n\) escapes to heap" "from make\(\[\]int, n\) \(non-constant size\)"
+	s2 := make([]int, 0, n) // ERROR "make\(\[\]int, 0, n\) escapes to heap" "from make\(\[\]int, 0, n\) \(non-constant size\)"
+	_, _ = s1, s2
+}
+
 // The list below is all of the why-escapes messages seen building the escape analysis tests.
 /*
    for i in escape*go ; do echo compile $i; go build -gcflags '-l -m -m' $i >& `basename $i .go`.log ; done