cmd/compile: improve escape analysis of known calls

Escape analysis is currently very naive about identifying calls to
known functions: it only recognizes direct calls to a declared
function, or direct calls to a closure.

This CL adds a new "staticValue" helper function that can trace back
through local variables that were initialized and never reassigned
based on a similar optimization already used by inlining. (And to be
used by inlining in a followup CL.)

Updates #41474.

Change-Id: I8204fd3b1e150ab77a27f583985cf099a8572b2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/256458
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -771,10 +771,11 @@ func (e *Escape) call(ks []EscHole, call, where *Node) {
 		var fn *Node
 		switch call.Op {
 		case OCALLFUNC:
-			if call.Left.Op == ONAME && call.Left.Class() == PFUNC {
-				fn = call.Left
-			} else if call.Left.Op == OCLOSURE {
-				fn = call.Left.Func.Closure.Func.Nname
+			switch v := staticValue(call.Left); {
+			case v.Op == ONAME && v.Class() == PFUNC:
+				fn = v
+			case v.Op == OCLOSURE:
+				fn = v.Func.Closure.Func.Nname
 			}
 		case OCALLMETH:
 			fn = asNode(call.Left.Type.FuncType().Nname)

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

@@ -751,6 +751,55 @@ func inlinableClosure(n *Node) *Node {
 	return f
 }
 
+func staticValue(n *Node) *Node {
+	for {
+		n1 := staticValue1(n)
+		if n1 == nil {
+			return n
+		}
+		n = n1
+	}
+}
+
+// staticValue1 implements a simple SSA-like optimization.
+func staticValue1(n *Node) *Node {
+	if n.Op != ONAME || n.Class() != PAUTO || n.Name.Addrtaken() {
+		return nil
+	}
+
+	defn := n.Name.Defn
+	if defn == nil {
+		return nil
+	}
+
+	var rhs *Node
+FindRHS:
+	switch defn.Op {
+	case OAS:
+		rhs = defn.Right
+	case OAS2:
+		for i, lhs := range defn.List.Slice() {
+			if lhs == n {
+				rhs = defn.Rlist.Index(i)
+				break FindRHS
+			}
+		}
+		Fatalf("%v missing from LHS of %v", n, defn)
+	default:
+		return nil
+	}
+	if rhs == nil {
+		Fatalf("RHS is nil: %v", defn)
+	}
+
+	unsafe, _ := reassigned(n)
+	if unsafe {
+		return nil
+	}
+
+	return rhs
+}
+
 // reassigned takes an ONAME node, walks the function in which it is defined, and returns a boolean
 // indicating whether the name has any assignments other than its declaration.
 // The second return value is the first such assignment encountered in the walk, if any. It is mostly

test/escape_closure.go

@@ -50,7 +50,7 @@ func ClosureCallArgs4() {
 }
 
 func ClosureCallArgs5() {
-	x := 0                     // ERROR "moved to heap: x"
+	x := 0 // ERROR "moved to heap: x"
 	// TODO(mdempsky): We get "leaking param: p" here because the new escape analysis pass
 	// can tell that p flows directly to sink, but it's a little weird. Re-evaluate.
 	sink = func(p *int) *int { // ERROR "leaking param: p" "func literal does not escape"
@@ -132,7 +132,7 @@ func ClosureCallArgs14() {
 }
 
 func ClosureCallArgs15() {
-	x := 0                      // ERROR "moved to heap: x"
+	x := 0 // ERROR "moved to heap: x"
 	p := &x
 	sink = func(p **int) *int { // ERROR "leaking param content: p" "func literal does not escape"
 		return *p
@@ -164,3 +164,16 @@ func ClosureLeak2a(a ...string) string { // ERROR "leaking param content: a"
 func ClosureLeak2b(f func() string) string { // ERROR "f does not escape"
 	return f()
 }
+
+func ClosureIndirect() {
+	f := func(p *int) {} // ERROR "p does not escape" "func literal does not escape"
+	f(new(int))          // ERROR "new\(int\) does not escape"
+
+	g := f
+	g(new(int)) // ERROR "new\(int\) does not escape"
+
+	h := nopFunc
+	h(new(int)) // ERROR "new\(int\) does not escape"
+}
+
+func nopFunc(p *int) {} // ERROR "p does not escape"