SERVER-51356 Introduce single eval frame model to SBE builders

1 files changed, 222 insertions, 0 deletions

diff --git a/src/mongo/db/query/sbe_stage_builder_eval_frame.h b/src/mongo/db/query/sbe_stage_builder_eval_frame.h
new file mode 100644
index 00000000000..35ffa9e316d
--- /dev/null
+++ b/src/mongo/db/query/sbe_stage_builder_eval_frame.h
@@ -0,0 +1,222 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <stack>
+
+#include "mongo/db/exec/sbe/expressions/expression.h"
+
+namespace mongo::stage_builder {
+
+/**
+ * EvalExpr is a wrapper around an EExpression that can also carry a SlotId. It is used to eliminate
+ * extra project stages. If 'slot' field is set, it contains the result of an expression. The user
+ * of the class can just use this slot instead of projecting an expression into a new slot.
+ */
+class EvalExpr {
+public:
+    EvalExpr() = default;
+
+    EvalExpr(EvalExpr&& e) : _expr(std::move(e._expr)), _slot(e._slot) {
+        e._slot = boost::none;
+    }
+
+    EvalExpr(std::unique_ptr<sbe::EExpression>&& e) : _expr(std::move(e)) {}
+
+    EvalExpr(sbe::value::SlotId s) : _expr(sbe::makeE<sbe::EVariable>(s)), _slot(s) {}
+
+    EvalExpr& operator=(EvalExpr&& e) {
+        if (this == &e) {
+            return *this;
+        }
+
+        _expr = std::move(e._expr);
+        _slot = e._slot;
+        e._slot = boost::none;
+        return *this;
+    }
+
+    EvalExpr& operator=(std::unique_ptr<sbe::EExpression>&& e) {
+        _expr = std::move(e);
+        _slot = boost::none;
+        return *this;
+    }
+
+    EvalExpr& operator=(sbe::value::SlotId s) {
+        _expr = sbe::makeE<sbe::EVariable>(s);
+        _slot = s;
+        return *this;
+    }
+
+    explicit operator bool() const {
+        return static_cast<bool>(_expr);
+    }
+
+    void reset() {
+        _expr.reset();
+        _slot = boost::none;
+    }
+
+    std::unique_ptr<sbe::EExpression> extractExpr() {
+        return std::move(_expr);
+    }
+
+    boost::optional<sbe::value::SlotId> getSlot() const {
+        return _slot;
+    }
+
+private:
+    std::unique_ptr<sbe::EExpression> _expr;
+    boost::optional<sbe::value::SlotId> _slot;
+};
+
+/**
+ * EvalStage contains a PlanStage ('stage') and a vector of slots ('outSlots'). The outSlots vector
+ * allows us to make sure important/relevant slots produced by 'stage' remain visible when 'stage'
+ * is used on the left side of a LoopJoinStage.
+ */
+struct EvalStage {
+    std::unique_ptr<sbe::PlanStage> stage;
+    sbe::value::SlotVector outSlots;
+};
+
+/**
+ * To support non-leaf operators in general, SBE builders maintain a stack of EvalFrames. An
+ * EvalFrame holds a subtree to build on top of (stage), a stack of expressions (exprs) and extra
+ * data useful for particular builder (data).
+ * Initially there is only one EvalFrame on the stack which holds the main tree. Non-leaf operators
+ * can decide to push an EvalFrame on the stack before each of their children is evaluated if
+ * desired. If a non-leaf operator pushes one or more EvalFrames onto the stack, it is responsible
+ * for removing these EvalFrames from the stack later.
+ */
+template <typename T>
+class EvalFrame {
+public:
+    template <typename... Args>
+    EvalFrame(EvalStage stage, Args&&... args)
+        : _data{std::forward<Args>(args)...}, _stage(std::move(stage)) {}
+
+    const EvalExpr& topExpr() const {
+        invariant(!_exprs.empty());
+        return _exprs.top();
+    }
+
+    void pushExpr(EvalExpr expr) {
+        _exprs.push(std::move(expr));
+    }
+
+    EvalExpr popExpr() {
+        invariant(!_exprs.empty());
+        auto expr = std::move(_exprs.top());
+        _exprs.pop();
+        return expr;
+    }
+
+    size_t exprsCount() const {
+        return _exprs.size();
+    }
+
+    const T& data() const {
+        return _data;
+    }
+
+    T& data() {
+        return _data;
+    }
+
+    void setStage(EvalStage stage) {
+        _stage = std::move(stage);
+    }
+
+    const EvalStage& getStage() const {
+        return _stage;
+    }
+
+    EvalStage extractStage() {
+        return std::move(_stage);
+    }
+
+private:
+    T _data;
+    EvalStage _stage;
+    std::stack<EvalExpr> _exprs;
+};
+
+/**
+ * Empty struct for 'data' field in case builder does not need to carry any additional data with
+ * each frame.
+ */
+struct NoExtraFrameData {};
+
+using EvalExprStagePair = std::pair<EvalExpr, EvalStage>;
+
+template <typename Data = NoExtraFrameData>
+class EvalStack {
+public:
+    using Frame = EvalFrame<Data>;
+
+    EvalStack() = default;
+
+    template <typename... Args>
+    void emplaceFrame(Args&&... args) {
+        stack.emplace(std::forward<Args>(args)...);
+    }
+
+    Frame& topFrame() {
+        invariant(!stack.empty());
+        return stack.top();
+    }
+
+    const Frame& topFrame() const {
+        invariant(!stack.empty());
+        return stack.top();
+    }
+
+    EvalExprStagePair popFrame() {
+        invariant(framesCount() > 0);
+        auto& frame = topFrame();
+
+        invariant(frame.exprsCount() == 1);
+        auto expr = frame.popExpr();
+        auto stage = frame.extractStage();
+
+        stack.pop();
+        return {std::move(expr), std::move(stage)};
+    }
+
+    size_t framesCount() const {
+        return stack.size();
+    }
+
+private:
+    std::stack<Frame> stack;
+};
+
+}  // namespace mongo::stage_builder