cmd/compile: unnamed parameters do not escape

Fixes #19687

Change-Id: I2e4769b4ec5812506df4ac5dc6bc6a7c5774ecb0
Reviewed-on: https://go-review.googlesource.com/38600
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -2118,4 +2118,13 @@ func (e *EscState) esctag(fn *Node) {
 		case EscHeap: // touched by escflood, moved to heap
 		}
 	}
+
+	// Unnamed parameters are unused and therefore do not escape.
+	// (Unnamed parameters are not in the Dcl list in the loop above
+	// so we need to mark them separately.)
+	for _, f := range fn.Type.Params().Fields().Slice() {
+		if f.Sym == nil || isblanksym(f.Sym) {
+			f.Note = mktag(EscNone)
+		}
+	}
 }

test/escape5.go

@@ -9,6 +9,8 @@
 
 package foo
 
+import "runtime"
+
 func noleak(p *int) int { // ERROR "p does not escape"
 	return *p
 }
@@ -149,3 +151,15 @@ func f10() {
 	var y = make([]byte, 1<<30) // ERROR "make\(\[\]byte, 1 << 30\) escapes to heap"
 	_ = x[0] + y[0]
 }
+
+// Test for issue 19687 (passing to unnamed parameters does not escape).
+func f11(**int) {
+}
+func f12(_ **int) {
+}
+func f13() {
+	var x *int
+	f11(&x)               // ERROR "&x does not escape"
+	f12(&x)               // ERROR "&x does not escape"
+	runtime.KeepAlive(&x) // ERROR "&x does not escape"
+}