internal/core/adt: use pointers for snapshot and result in nodeContext

These fields are only used in evalv2's disjunction algorithm,
so they are entirely unused in evalv3, the current default.
Given how often nodeContext structs are allocated,
this helps reduce the memory usage of the new allocator.

                │     old      │                new                │
                │    sec/op    │   sec/op     vs base              │
    VetAutomata   724.0m ± 13%   674.2m ± 9%  -6.88% (p=0.050 n=8)

                │     old      │                new                 │
                │     B/op     │     B/op      vs base              │
    VetAutomata   700.7Mi ± 0%   631.9Mi ± 0%  -9.81% (p=0.000 n=8)

                │     old     │             new              │
                │  allocs/op  │  allocs/op   vs base         │
    VetAutomata   4.478M ± 0%   4.478M ± 0%  ~ (p=0.316 n=8)

Performance on evalv2 is unaffected; adt.TestEvalV2 still takes 3.9s,
and Unity on evalv2 shows noise in the 0-5% range.
Unity on evalv3 shows improvements between 0 and 10% for wall times
as well as for Go.AllocBytes counters.

Updates #3334.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Idab308cb88980b9618c9c4164fae41b870442bd9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217121
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mvdan

internal/core/adt/disjunct.go

@@ -166,9 +166,9 @@ func (n *nodeContext) expandDisjuncts(
 	// save nodeContext.
 
 	if recursive || len(n.disjunctions) > 0 {
-		n.snapshot = clone(*n.node)
+		n.snapshot = clone(n.node)
 	} else {
-		n.snapshot = *n.node
+		n.snapshot = n.node
 	}
 
 	defaultOffset := len(n.usedDefault)
@@ -206,8 +206,9 @@ func (n *nodeContext) expandDisjuncts(
 
 		if recursive {
 			*n = m
-			n.result = *n.node // XXX: n.result = snapshotVertex(n.node)?
-			n.node = &n.result
+			nn := *n.node
+			n.result = &nn // XXX: n.result = snapshotVertex(n.node)?
+			n.node = n.result
 			n.disjuncts = append(n.disjuncts, n)
 		}
 		if n.node.BaseValue == nil {
@@ -246,7 +247,7 @@ func (n *nodeContext) expandDisjuncts(
 				case d.expr != nil:
 					for _, v := range d.expr.Values {
 						cn := dn.clone()
-						*cn.node = clone(dn.snapshot)
+						*cn.node = *clone(dn.snapshot)
 						cn.node.state = cn
 
 						c := MakeConjunct(d.env, v.Val, d.cloneID)
@@ -271,7 +272,7 @@ func (n *nodeContext) expandDisjuncts(
 				case d.value != nil:
 					for i, v := range d.value.Values {
 						cn := dn.clone()
-						*cn.node = clone(dn.snapshot)
+						*cn.node = *clone(dn.snapshot)
 						cn.node.state = cn
 
 						cn.addValueConjunct(d.env, v, d.cloneID)
@@ -425,7 +426,7 @@ func (n *nodeContext) expandDisjuncts(
 				if last {
 					flags |= IgnoreOptional
 				}
-				if Equal(n.ctx, &v.result, &d.result, flags) {
+				if Equal(n.ctx, v.result, d.result, flags) {
 					m := maybeDefault
 					for _, u := range d.usedDefault {
 						m = combineDefault(m, u.nestedMode)
@@ -497,7 +498,9 @@ func mode(hasDefault, marked bool) defaultMode {
 // and can be used as is, or Structs is assumed to not yet be computed at the
 // time that a clone is needed and must be nil. Conjuncts no longer needed and
 // can become nil. All other fields can be copied shallowly.
-func clone(v Vertex) Vertex {
+func clone(v *Vertex) *Vertex {
+	v2 := *v
+	v = &v2
 	v.state = nil
 	if a := v.Arcs; len(a) > 0 {
 		v.Arcs = make([]*Vertex, len(a))
@@ -515,7 +518,7 @@ func clone(v Vertex) Vertex {
 				a := *arc
 				a.state = arc.state.clone()
 				a.state.node = &a
-				a.state.snapshot = clone(a)
+				a.state.snapshot = clone(arc)
 				v.Arcs[i] = &a
 			}
 		}

internal/core/adt/eval.go

@@ -308,7 +308,7 @@ func (c *OpContext) unify(v *Vertex, flags combinedFlags) {
 		switch len(n.disjuncts) {
 		case 0:
 		case 1:
-			x := n.disjuncts[0].result
+			x := *n.disjuncts[0].result
 			x.state = nil
 			x.cyclicReferences = n.node.cyclicReferences
 			*v = x
@@ -973,21 +973,20 @@ func (n *nodeContext) createDisjunct() *Disjunction {
 	p := 0
 	hasDefaults := false
 	for i, x := range n.disjuncts {
-		v := new(Vertex)
-		*v = x.result
+		v := *x.result
 		v.state = nil
 		switch x.defaultMode {
 		case isDefault:
 			a[i] = a[p]
-			a[p] = v
+			a[p] = &v
 			p++
 			hasDefaults = true
 
 		case notDefault:
 			hasDefaults = true
 			fallthrough
 		case maybeDefault:
-			a[i] = v
+			a[i] = &v
 		}
 	}
 	// TODO: disambiguate based on concrete values.
@@ -1102,11 +1101,11 @@ type nodeContext struct {
 	hasDisjunction bool
 
 	// snapshot holds the last value of the vertex before calling postDisjunct.
-	snapshot Vertex
+	snapshot *Vertex
 
 	// Result holds the last evaluated value of the vertex after calling
 	// postDisjunct.
-	result Vertex
+	result *Vertex
 }
 
 type conjunct struct {