Inlining optimizer (#827)

Inliner with support for identifiers and simple select expressions

Note: optional field selections are not supported for inlining matches
at this time.

cel/inlining.go

@@ -0,0 +1,220 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cel
+
+import (
+	"github.com/google/cel-go/common/ast"
+	"github.com/google/cel-go/common/containers"
+	"github.com/google/cel-go/common/operators"
+	"github.com/google/cel-go/common/overloads"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+// InlineVariable holds a variable name to be matched and an AST representing
+// the expression graph which should be used to replace it.
+type InlineVariable struct {
+	name  string
+	alias string
+	def   *ast.AST
+}
+
+// Name returns the qualified variable or field selection to replace.
+func (v *InlineVariable) Name() string {
+	return v.name
+}
+
+// Alias returns the alias to use when performing cel.bind() calls during inlining.
+func (v *InlineVariable) Alias() string {
+	return v.alias
+}
+
+// Expr returns the inlined expression value.
+func (v *InlineVariable) Expr() ast.Expr {
+	return v.def.Expr()
+}
+
+// Type indicates the inlined expression type.
+func (v *InlineVariable) Type() *Type {
+	return v.def.GetType(v.def.Expr().ID())
+}
+
+// NewInlineVariable declares a variable name to be replaced by a checked expression.
+func NewInlineVariable(name string, definition *Ast) *InlineVariable {
+	return NewInlineVariableWithAlias(name, name, definition)
+}
+
+// NewInlineVariableWithAlias declares a variable name to be replaced by a checked expression.
+// If the variable occurs more than once, the provided alias will be used to replace the expressions
+// where the variable name occurs.
+func NewInlineVariableWithAlias(name, alias string, definition *Ast) *InlineVariable {
+	return &InlineVariable{name: name, alias: alias, def: definition.impl}
+}
+
+// NewInliningOptimizer creates and optimizer which replaces variables with expression definitions.
+//
+// If a variable occurs one time, the variable is replaced by the inline definition. If the
+// variable occurs more than once, the variable occurences are replaced by a cel.bind() call.
+func NewInliningOptimizer(inlineVars ...*InlineVariable) ASTOptimizer {
+	return &inliningOptimizer{variables: inlineVars}
+}
+
+type inliningOptimizer struct {
+	variables []*InlineVariable
+}
+
+func (opt *inliningOptimizer) Optimize(ctx *OptimizerContext, a *ast.AST) *ast.AST {
+	root := ast.NavigateAST(a)
+	for _, inlineVar := range opt.variables {
+		matches := ast.MatchDescendants(root, opt.matchVariable(inlineVar.Name()))
+		// Skip cases where the variable isn't in the expression graph
+		if len(matches) == 0 {
+			continue
+		}
+
+		// For a single match, do a direct replacement of the expression sub-graph.
+		if len(matches) == 1 {
+			opt.inlineExpr(ctx, matches[0], ctx.CopyExpr(inlineVar.Expr()), inlineVar.Type())
+			continue
+		}
+
+		if !isBindable(matches, inlineVar.Expr(), inlineVar.Type()) {
+			for _, match := range matches {
+				opt.inlineExpr(ctx, match, ctx.CopyExpr(inlineVar.Expr()), inlineVar.Type())
+			}
+			continue
+		}
+		// For multiple matches, find the least common ancestor (lca) and insert the
+		// variable as a cel.bind() macro.
+		var lca ast.NavigableExpr = nil
+		ancestors := map[int64]bool{}
+		for _, match := range matches {
+			// Update the identifier matches with the provided alias.
+			aliasExpr := ctx.NewIdent(inlineVar.Alias())
+			opt.inlineExpr(ctx, match, aliasExpr, inlineVar.Type())
+			parent, found := match, true
+			for found {
+				_, hasAncestor := ancestors[parent.ID()]
+				if hasAncestor && (lca == nil || lca.Depth() < parent.Depth()) {
+					lca = parent
+				}
+				ancestors[parent.ID()] = true
+				parent, found = parent.Parent()
+			}
+		}
+
+		// Update the least common ancestor by inserting a cel.bind() call to the alias.
+		inlined := ctx.NewBindMacro(lca.ID(), inlineVar.Alias(), inlineVar.Expr(), lca)
+		opt.inlineExpr(ctx, lca, inlined, inlineVar.Type())
+	}
+	return a
+}
+
+// inlineExpr replaces the current expression with the inlined one, unless the location of the inlining
+// happens within a presence test, e.g. has(a.b.c) -> inline alpha for a.b.c in which case an attempt is
+// made to determine whether the inlined value can be presence or existence tested.
+func (opt *inliningOptimizer) inlineExpr(ctx *OptimizerContext, prev, inlined ast.Expr, inlinedType *Type) {
+	switch prev.Kind() {
+	case ast.SelectKind:
+		sel := prev.AsSelect()
+		if !sel.IsTestOnly() {
+			prev.SetKindCase(inlined)
+			return
+		}
+		opt.rewritePresenceExpr(ctx, prev, inlined, inlinedType)
+	default:
+		prev.SetKindCase(inlined)
+	}
+}
+
+// rewritePresenceExpr converts the inlined expression, when it occurs within a has() macro, to type-safe
+// expression appropriate for the inlined type, if possible.
+//
+// If the rewrite is not possible an error is reported at the inline expression site.
+func (opt *inliningOptimizer) rewritePresenceExpr(ctx *OptimizerContext, prev, inlined ast.Expr, inlinedType *Type) {
+	// If the input inlined expression is not a select expression it won't work with the has()
+	// macro. Attempt to rewrite the presence test in terms of the typed input, otherwise error.
+	ctx.sourceInfo.ClearMacroCall(prev.ID())
+	if inlined.Kind() == ast.SelectKind {
+		inlinedSel := inlined.AsSelect()
+		prev.SetKindCase(
+			ctx.NewPresenceTest(prev.ID(), inlinedSel.Operand(), inlinedSel.FieldName()))
+		return
+	}
+	if inlinedType.IsAssignableType(NullType) {
+		prev.SetKindCase(
+			ctx.NewCall(operators.NotEquals,
+				inlined,
+				ctx.NewLiteral(types.NullValue),
+			))
+		return
+	}
+	if inlinedType.HasTrait(traits.SizerType) {
+		prev.SetKindCase(
+			ctx.NewCall(operators.NotEquals,
+				ctx.NewMemberCall(overloads.Size, inlined),
+				ctx.NewLiteral(types.IntZero),
+			))
+		return
+	}
+	ctx.ReportErrorAtID(prev.ID(), "unable to inline expression type %v into presence test", inlinedType)
+}
+
+// isBindable indicates whether the inlined type can be used within a cel.bind() if the expression
+// being replaced occurs within a presence test. Value types with a size() method or field selection
+// support can be bound.
+//
+// In future iterations, support may also be added for indexer types which can be rewritten as an `in`
+// expression; however, this would imply a rewrite of the inlined expression that may not be necessary
+// in most cases.
+func isBindable(matches []ast.NavigableExpr, inlined ast.Expr, inlinedType *Type) bool {
+	if inlinedType.IsAssignableType(NullType) ||
+		inlinedType.HasTrait(traits.SizerType) ||
+		inlinedType.HasTrait(traits.FieldTesterType) {
+		return true
+	}
+	for _, m := range matches {
+		if m.Kind() != ast.SelectKind {
+			continue
+		}
+		sel := m.AsSelect()
+		if sel.IsTestOnly() {
+			return false
+		}
+	}
+	return true
+}
+
+// matchVariable matches simple identifiers, select expressions, and presence test expressions
+// which match the (potentially) qualified variable name provided as input.
+//
+// Note, this function does not support inlining against select expressions which includes optional
+// field selection. This may be a future refinement.
+func (opt *inliningOptimizer) matchVariable(varName string) ast.ExprMatcher {
+	return func(e ast.NavigableExpr) bool {
+		if e.Kind() == ast.IdentKind && e.AsIdent() == varName {
+			return true
+		}
+		if e.Kind() == ast.SelectKind {
+			sel := e.AsSelect()
+			// While the `ToQualifiedName` call could take the select directly, this
+			// would skip presence tests from possible matches, which we would like
+			// to include.
+			qualName, found := containers.ToQualifiedName(sel.Operand())
+			return found && qualName+"."+sel.FieldName() == varName
+		}
+		return false
+	}
+}