internal/core/adt: improve performance for repeated calls to Unify/Fill

This is done by skipping SpawnRef for computed vertices.

Repeatedly applying unification caused a new closedInfo to
be inserted at the node's position, resulting in a growing
list to process when calling Accept (called by Unify, which
is called by Fill). This caused processing time to become
quadratic for that node.

There are more gains to be made, but this should be a big
help already.

Issue #899

Change-Id: I96ec77bb4bd09b69b9da023e0fb2b8ed5c297080
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9462
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/testdata/script/vet_embed.txt

@@ -32,7 +32,6 @@ e: 2
 
 -- expect-foo --
 field c not allowed:
-    ./foo.yaml:1:2
     ./foo.yaml:2:2
     ./schema.cue:1:1
     ./schema.cue:3:7
@@ -42,12 +41,10 @@ field d not allowed:
     ./schema.cue:1:1
     ./schema.cue:3:7
     ./schema.cue:7:1
-    ./stream.yaml:1:2
     ./stream.yaml:2:2
 -- expect-stream --
 field d not allowed:
     ./schema.cue:1:1
     ./schema.cue:3:7
     ./schema.cue:7:1
-    ./stream.yaml:1:2
     ./stream.yaml:2:2

cmd/cue/cmd/testdata/script/vet_expr.txt

@@ -4,7 +4,6 @@ cmp stderr expect-stderr
 -- expect-stderr --
 translations.hello.lang: incomplete value string
 field skip not allowed:
-    ./data.yaml:16:1
     ./data.yaml:20:1
     ./vet.cue:1:1
     ./vet.cue:1:8

internal/core/adt/eval.go

@@ -1359,7 +1359,15 @@ func (n *nodeContext) addVertexConjuncts(env *Environment, closeInfo CloseInfo,
 		defer func() { arc.SelfCount-- }()
 	}
 
-	closeInfo = closeInfo.SpawnRef(arc, IsDef(x), x)
+	// Performance: the following if check filters cases that are not strictly
+	// necessary for correct functioning. Not updating the closeInfo may cause
+	// some position information to be lost for top-level positions of merges
+	// resulting form APIs. These tend to be fairly uninteresting.
+	// At the same time, this optimization may prevent considerable slowdown
+	// in case an API does many calls to Unify.
+	if !inline || arc.IsClosedStruct() || arc.IsClosedList() {
+		closeInfo = closeInfo.SpawnRef(arc, IsDef(x), x)
+	}
 
 	if arc.status == 0 && !inline {
 		// This is a rare condition, but can happen in certain

internal/core/adt/eval_test.go

@@ -22,6 +22,8 @@ import (
 
 	"github.com/rogpeppe/go-internal/txtar"
 
+	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/cuecontext"
 	"cuelang.org/go/internal/core/adt"
 	"cuelang.org/go/internal/core/debug"
 	"cuelang.org/go/internal/core/eval"
@@ -135,3 +137,17 @@ module: "example.com"
 
 	t.Log(ctx.Stats())
 }
+
+func BenchmarkUnifyAPI(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		ctx := cuecontext.New()
+		v := ctx.CompileString("")
+		for j := 0; j < 500; j++ {
+			if j == 400 {
+				b.StartTimer()
+			}
+			v = v.FillPath(cue.ParsePath(fmt.Sprintf("i_%d", i)), i)
+		}
+	}
+}