cmd/compile/internal/gc: better handling of self-assignments in esc.go

Teach escape analysis to recognize these assignment patterns
as not causing the src to leak:

	val.x = val.y
	val.x[i] = val.y[j]
	val.x1.x2 = val.x1.y2
	... etc

Helps to avoid "leaking param" with assignments showed above.
The implementation is based on somewhat similiar xs=xs[a:b]
special case that is ignored by the escape analysis.

We may figure out more generalized version of this,
but this one looks like a safe step into that direction.

Updates #14858

Change-Id: I6fe5bfedec9c03bdc1d7624883324a523bd11fde
Reviewed-on: https://go-review.googlesource.com/126395
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -654,6 +654,58 @@ func (e *EscState) esclist(l Nodes, parent *Node) {
 	}
 }
 
+// isSelfAssign reports whether assignment from src to dst can
+// be ignored by the escape analysis as it's effectively a self-assignment.
+func (e *EscState) isSelfAssign(dst, src *Node) bool {
+	if dst == nil || src == nil || dst.Op != src.Op {
+		return false
+	}
+
+	switch dst.Op {
+	case ODOT, ODOTPTR:
+		// Safe trailing accessors that are permitted to differ.
+	case OINDEX:
+		if e.mayAffectMemory(dst.Right) || e.mayAffectMemory(src.Right) {
+			return false
+		}
+	default:
+		return false
+	}
+
+	// The expression prefix must be both "safe" and identical.
+	return samesafeexpr(dst.Left, src.Left)
+}
+
+// mayAffectMemory reports whether n evaluation may affect program memory state.
+// If expression can't affect it, then it can be safely ignored by the escape analysis.
+func (e *EscState) mayAffectMemory(n *Node) bool {
+	// We may want to use "memory safe" black list instead of general
+	// "side-effect free", which can include all calls and other ops
+	// that can affect allocate or change global state.
+	// It's safer to start from a whitelist for now.
+	//
+	// We're ignoring things like division by zero, index out of range,
+	// and nil pointer dereference here.
+	switch n.Op {
+	case ONAME, OCLOSUREVAR, OLITERAL:
+		return false
+	case ODOT, ODOTPTR:
+		return e.mayAffectMemory(n.Left)
+	case OIND, OCONVNOP:
+		return e.mayAffectMemory(n.Left)
+	case OCONV:
+		return e.mayAffectMemory(n.Left)
+	case OINDEX:
+		return e.mayAffectMemory(n.Left) && e.mayAffectMemory(n.Right)
+	case OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
+		return e.mayAffectMemory(n.Left) && e.mayAffectMemory(n.Right)
+	case ONOT, OCOM, OPLUS, OMINUS, OALIGNOF, OOFFSETOF, OSIZEOF:
+		return e.mayAffectMemory(n.Left)
+	default:
+		return true
+	}
+}
+
 func (e *EscState) esc(n *Node, parent *Node) {
 	if n == nil {
 		return
@@ -813,6 +865,22 @@ opSwitch:
 			break
 		}
 
+		// Also skip trivial assignments that assign back to the same object.
+		//
+		// It covers these cases:
+		//	val.x = val.y
+		//	val.x[i] = val.y[j]
+		//	val.x1.x2 = val.x1.y2
+		//	... etc
+		//
+		// These assignments do not change assigned object lifetime.
+		if e.isSelfAssign(n.Left, n.Right) {
+			if Debug['m'] != 0 {
+				Warnl(n.Pos, "%v ignoring self-assignment in %S", e.curfnSym(n), n)
+			}
+			break
+		}
+
 		e.escassign(n.Left, n.Right, e.stepAssignWhere(nil, nil, "", n))
 
 	case OAS2: // x,y = a,b

test/escape_param.go

@@ -58,20 +58,85 @@ func caller2b() {
 	sink = p       // ERROR "p escapes to heap$"
 }
 
+func paramArraySelfAssign(p *PairOfPairs) { // ERROR "p does not escape"
+	p.pairs[0] = p.pairs[1] // ERROR "ignoring self-assignment in p.pairs\[0\] = p.pairs\[1\]"
+}
+
+type PairOfPairs struct {
+	pairs [2]*Pair
+}
+
+type BoxedPair struct {
+	pair *Pair
+}
+
+type WrappedPair struct {
+	pair Pair
+}
+
+func leakParam(x interface{}) { // ERROR "leaking param: x"
+	sink = x
+}
+
+func sinkAfterSelfAssignment1(box *BoxedPair) { // ERROR "leaking param content: box"
+	box.pair.p1 = box.pair.p2 // ERROR "ignoring self-assignment in box.pair.p1 = box.pair.p2"
+	sink = box.pair.p2        // ERROR "box.pair.p2 escapes to heap"
+}
+
+func sinkAfterSelfAssignment2(box *BoxedPair) { // ERROR "leaking param content: box"
+	box.pair.p1 = box.pair.p2 // ERROR "ignoring self-assignment in box.pair.p1 = box.pair.p2"
+	sink = box.pair           // ERROR "box.pair escapes to heap"
+}
+
+func sinkAfterSelfAssignment3(box *BoxedPair) { // ERROR "leaking param content: box"
+	box.pair.p1 = box.pair.p2 // ERROR "ignoring self-assignment in box.pair.p1 = box.pair.p2"
+	leakParam(box.pair.p2)    // ERROR "box.pair.p2 escapes to heap"
+}
+
+func sinkAfterSelfAssignment4(box *BoxedPair) { // ERROR "leaking param content: box"
+	box.pair.p1 = box.pair.p2 // ERROR "ignoring self-assignment in box.pair.p1 = box.pair.p2"
+	leakParam(box.pair)       // ERROR "box.pair escapes to heap"
+}
+
+func selfAssignmentAndUnrelated(box1, box2 *BoxedPair) { // ERROR "leaking param content: box2" "box1 does not escape"
+	box1.pair.p1 = box1.pair.p2 // ERROR "ignoring self-assignment in box1.pair.p1 = box1.pair.p2"
+	leakParam(box2.pair.p2)     // ERROR "box2.pair.p2 escapes to heap"
+}
+
+func notSelfAssignment1(box1, box2 *BoxedPair) { // ERROR "leaking param content: box2" "box1 does not escape"
+	box1.pair.p1 = box2.pair.p1
+}
+
+func notSelfAssignment2(p1, p2 *PairOfPairs) { // ERROR "leaking param content: p2" "p1 does not escape"
+	p1.pairs[0] = p2.pairs[1]
+}
+
+func notSelfAssignment3(p1, p2 *PairOfPairs) { // ERROR "leaking param content: p2" "p1 does not escape"
+	p1.pairs[0].p1 = p2.pairs[1].p1
+}
+
+func boxedPairSelfAssign(box *BoxedPair) { // ERROR "box does not escape"
+	box.pair.p1 = box.pair.p2 // ERROR "ignoring self-assignment in box.pair.p1 = box.pair.p2"
+}
+
+func wrappedPairSelfAssign(w *WrappedPair) { // ERROR "w does not escape"
+	w.pair.p1 = w.pair.p2 // ERROR "ignoring self-assignment in w.pair.p1 = w.pair.p2"
+}
+
 // in -> in
 type Pair struct {
 	p1 *int
 	p2 *int
 }
 
-func param3(p *Pair) { // ERROR "leaking param content: p$"
-	p.p1 = p.p2
+func param3(p *Pair) { // ERROR "param3 p does not escape"
+	p.p1 = p.p2 // ERROR "param3 ignoring self-assignment in p.p1 = p.p2"
 }
 
 func caller3a() {
-	i := 0            // ERROR "moved to heap: i$"
-	j := 0            // ERROR "moved to heap: j$"
-	p := Pair{&i, &j} // ERROR "&i escapes to heap$" "&j escapes to heap$"
+	i := 0
+	j := 0
+	p := Pair{&i, &j} // ERROR "caller3a &i does not escape" "caller3a &j does not escape"
 	param3(&p)        // ERROR "caller3a &p does not escape"
 	_ = p
 }