string-builder-optimizer.h

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// Copyright 2022 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_COMPILER_STRING_BUILDER_OPTIMIZER_H_
#define V8_COMPILER_STRING_BUILDER_OPTIMIZER_H_
 
#include <cstdint>
#include <optional>
#include <unordered_map>
#include <vector>
 
#include "src/base/macros.h"
#include "src/compiler/graph-assembler.h"
#include "src/compiler/graph-reducer.h"
#include "src/compiler/js-graph.h"
#include "src/compiler/machine-operator.h"
#include "src/compiler/node-marker.h"
#include "src/compiler/node.h"
#include "src/compiler/schedule.h"
#include "src/zone/zone-containers.h"
 
namespace v8 {
namespace internal {
 
class Zone;
 
namespace compiler {
 
class JSGraphAssembler;
class NodeOriginTable;
class Schedule;
class SourcePositionTable;
class JSHeapBroker;
 
// StringBuilderOptimizer aims at avoid ConsString for some loops that build
// strings, and instead update a mutable over-allocated backing store, while
// keeping a (mutable) SlicedString to the valid part of the backing store.
//
// StringBuilderOptimizer only does the analysis: it finds out which nodes could
// benefit from this optimization. Then, EffectControlLinearizer actually
// applies the optimization to the graph.
//
// A typical example of what the StringBuilderOptimizer can optimize is:
//
//    let s = "";
//    for (...) {
//        s += "...";
//    }
//
// In general, for a series of concatenations to be optimized, they need:
//   - To start on a single initial concatenation
//   - All the concatenations in the string builder must have constant strings
//     or String.FromCharCode on their right-hand side.
//   - At least one of the concatenation must be in a loop.
//
// Because everything is nicer with a picture, here is one of what kind of
// patterns the StringBuilderOptimizer tries to optimize:
//
//            +--------+
//            |kLiteral|
//            +--------+
//                |
//                |
//                v
//         +-------------+          +--------+
//         |kStringConcat| <------- |kLiteral|
//         +-------------+          +--------+
//                |
//                |
//                v
//           optionally,
//        more kStringConcat
//                |
//                |
//                v
//             +----+
//    -------->|kPhi|------------------------------------------
//    |        +----+                                         |
//    |           |                                           |
//    |           |                                           |
//    |           |                                           |
//    |           |                                           |
//    |           |                                           |
//    |           |                                           |
//    |           |                                           |
//    |           v                                           |
//    |    +-------------+          +--------+                |
//    |    |kStringConcat| <------- |kLiteral|                |
//    |    +-------------+          +--------+                |
//    |           |                                           |
//    |           |                                           |
//    |           v                                           |
//    |      optionally,                                      v
//    |   more kStringConcat                            optionally,
//    |           |                                   more kStringConcat
//    |           |                                   or more kPhi/loops
//    |           |                                           |
//    ------------|                                           |
//                                                            |
//                                                            |
//                                                            v
//
// Where "kLiteral" actually means "either a string literal (HeapConstant) or a
// StringFromSingleCharCode". And kStringConcat can also be kNewConsString (when
// the size of the concatenation is known to be more than 13 bytes, Turbofan's
// front-end generates kNewConsString opcodes rather than kStringConcat).
// The StringBuilder also supports merge phis. For instance:
//
//                                  +--------+
//                                  |kLiteral|
//                                  +--------+
//                                      |
//                                      |
//                                      v
//                               +-------------+          +--------+
//                               |kStringConcat| <------- |kLiteral|
//                               +-------------+          +--------+
//                                  |       |
//                                  |       |
//                                  |       |
//                  +---------------+       +---------------+
//                  |                                       |
//                  |                                       |
//                  v                                       v
//           +-------------+                         +-------------+
//           |kStringConcat|                         |kStringConcat|
//           +-------------+                         +-------------+
//                  |                                       |
//                  |                                       |
//                  |                                       |
//                  +---------------+       +---------------+
//                                  |       |
//                                  |       |
//                                  v       v
//                               +-------------+
//                               |    kPhi     |
//                               |   (merge)   |
//                               +-------------+
//                                      |
//                                      |
//                                      v
//
// (and, of course, loops and merge can be mixed).
 
class OneOrTwoByteAnalysis final {
  // The class OneOrTwoByteAnalysis is used to try to statically determine
  // whether a string constant or StringFromSingleCharCode is a 1-byte or a
  // 2-byte string.
  // If we succeed to do this analysis for all of the nodes in a string builder,
  // then we know statically whether this string builder is building a 1-byte or
  // a 2-byte string, and we can optimize the generated code to remove all
  // 1-byte/2-byte checks.
 public:
  OneOrTwoByteAnalysis(TFGraph* graph, Zone* zone, JSHeapBroker* broker)
      : states_(graph->NodeCount(), State::kUnknown, zone), broker_(broker) {}
 
  enum class State : uint8_t {
    kUnknown,  // Not yet determined if the string is 1 or 2-bytes
    kOneByte,  // Only 1-byte strings in the string builder
    kTwoByte,  // At least one 2-byte string in the string builder
    kCantKnow  // Cannot determine statically if the string will be 1 or 2-bytes
 
    // Lattice of possible transitions:
    //
    //      kUnknown
    //      /     | \
    //     /      |  \
    //    v       |   \
    //  kOneByte  |    |
    //  |   |     |    |
    //  |   |     |    |
    //  |   v     v    |
    //  |   kTwoByte   |
    //  |      |      /
    //   \     |     /
    //    v    v    v
    //     kCantKnow
    //
    // Which means that for instance it's possible to realize that a kUnknown
    // string builder will produce a 1-byte string, and we can later realize
    // that it will instead be a 2-byte string. Or, we could be in kOneByte
    // state, and then realize that the string may or may not end up being
    // 2-byte, so we'll move to kCantKnow state.
  };
 
  // Computes and returns a State reflecting whether {node} is a 1-byte or
  // 2-byte string.
  State OneOrTwoByte(Node* node);
 
  // Computes whether the string builder will be on 1-byte or 2-byte if it
  // contains two nodes that have states {a} and {b}. For instance, if both {a}
  // and {b} are kOneByte, ConcatResultIsOneOrTwoByte returns kOneByte.
  static State ConcatResultIsOneOrTwoByte(State a, State b);
 
 private:
  // Returns the positive integral range that {node} can take. If {node} can be
  // negative or is not a number, returns nullopt. If the range exceeds 2**32,
  // returns nullopt as well. The analysis of TryGetRange is not complete (some
  // operators are ignored), so if {node} isn't handled, then nullopt is
  // returned. If this function returns a range between 0 and 255, then we
  // assume that calling StringFromSingleCharCode on {node} will produce a
  // 1-byte string. The analysis is sound (it doesn't make mistake), but is not
  // complete (it bails out (returns nullopt) on operators that are not
  // handled).
  std::optional<std::pair<int64_t, int64_t>> TryGetRange(Node* node);
 
  JSHeapBroker* broker() { return broker_; }
 
  ZoneVector<State> states_;
  JSHeapBroker* broker_;
};
 
class V8_EXPORT_PRIVATE StringBuilderOptimizer final {
 public:
  StringBuilderOptimizer(JSGraph* jsgraph, Schedule* schedule, Zone* temp_zone,
                         JSHeapBroker* broker);
 
  // Returns true if some trimming code should be inserted at the beginning of
  // {block} to finalize some string builders.
  bool BlockShouldFinalizeStringBuilders(BasicBlock* block);
  // Returns which nodes should be trimmed at the beginning of {block} to
  // finalize some string builders.
  ZoneVector<Node*> GetStringBuildersToFinalize(BasicBlock* block);
 
  // Returns true if {node} is the last node of a StringBuilder (which means
  // that trimming code should be inserted after {node}).
  // Note that string builders cannot end in the middle of a loop (unless it was
  // started in the same loop). The way it's enforced is that when we first
  // visit a loop Phi that could be part of a String Builder, we set its status
  // to State::kPendingPhi. Only once we've visited the whole loop and the
  // backedge and that the use chain following the loop phi up to and including
  // the backedge are valid as part of a String Builder, then the loop phi
  // status is siwtched to State::kInStringBuilder. Then, in the final step
  // where we switch the status to State::kConfirmedInStringBuilder, we ignore
  // nodes that have a status that isn't kInStringBuilder, which means that we
  // ignore loop phis that still have the kPendingPhi status (and their
  // successors). The String Builders thus cannot end inside loops.
  bool IsStringBuilderEnd(Node* node);
  // Returns true if {node} is a the last node of a StringBuilder and is not a
  // loop phi. The "loop phi" distinction matters, because trimming for loop
  // phis is trickier (because we don't want to trim at every iteration of the
  // loop, but only once after the loop).
  bool IsNonLoopPhiStringBuilderEnd(Node* node);
  // Returns true if {node} is the input of a concatenation that is part of a
  // StringBuilder.
  bool IsStringBuilderConcatInput(Node* node);
  // Returns true if {node} is part of a StringBuilder.
  bool ConcatIsInStringBuilder(Node* node);
  // Returns true if {node} is the 1st node of a StringBuilder (which means that
  // when lowering {node}, we should allocate and initialize everything for this
  // particular StringBuilder).
  bool IsFirstConcatInStringBuilder(Node* node);
 
  // Returns an OneOrTwoByteAnalysis::State representing whether the
  // StringBuilder that contains {node} is building a 1-byte or a 2-byte.
  OneOrTwoByteAnalysis::State GetOneOrTwoByte(Node* node);
 
  void Run();
 
  JSGraph* jsgraph() const { return jsgraph_; }
  TFGraph* graph() const { return jsgraph_->graph(); }
  Schedule* schedule() const { return schedule_; }
  Zone* temp_zone() const { return temp_zone_; }
  JSHeapBroker* broker() const { return broker_; }
 
 private:
  enum class State : uint8_t {
    kUnvisited = 0,
    kBeginStringBuilder,        // A (potential) beginning of a StringBuilder
    kInStringBuilder,           // A node that could be in a StringBuilder
    kPendingPhi,                // A phi that could be in a StringBuilder
    kConfirmedInStringBuilder,  // A node that is definitely in a StringBuilder
    kEndStringBuilder,  // A node that ends definitely a StringBuilder, and that
                        // can be trimmed right away
    kEndStringBuilderLoopPhi,  // A phi that ends a StringBuilder, and whose
                               // trimming need to be done at the beginning of
                               // the following blocks.
    kInvalid,  // A node that we visited and that we can't optimize.
    kNumberOfState
  };
 
  struct Status {
    int id;       // The id of the StringBuilder that the node belongs to (or
                  // kInvalidId).
    State state;  // The state of the node.
  };
  static constexpr int kInvalidId = -1;
 
  Status GetStatus(Node* node) const {
    if (node->id() > status_.size()) {
      return Status{kInvalidId, State::kInvalid};
    } else {
      return status_[node->id()];
    }
  }
  void SetStatus(Node* node, State state, int id = kInvalidId) {
    DCHECK_NE(state, State::kUnvisited);
    DCHECK_IMPLIES(id != kInvalidId, state != State::kInvalid);
    if (node->id() >= status_.size()) {
      // We should really not allocate too many new nodes: the only new nodes we
      // allocate are constant inputs of nodes in the string builder that have
      // multiple uses. Thus, we use a slow exponential growth for {status_}.
      constexpr double growth_factor = 1.1;
      status_.resize(node->id() * growth_factor,
                     Status{kInvalidId, State::kUnvisited});
    }
    status_[node->id()] = Status{id, state};
  }
  void UpdateStatus(Node* node, State state) {
    int id = state == State::kInvalid ? kInvalidId : GetStatus(node).id;
    status_[node->id()] = Status{id, state};
  }
 
  struct StringBuilder {
    Node* start;
    int id;
    bool has_loop_phi;
    OneOrTwoByteAnalysis::State one_or_two_bytes;
  };
  const StringBuilder kInvalidStringBuilder = {
      nullptr, kInvalidId, false, OneOrTwoByteAnalysis::State::kUnknown};
 
#ifdef DEBUG
  bool StringBuilderIsValid(StringBuilder string_builder) {
    return string_builder.start != nullptr && string_builder.id != kInvalidId &&
           string_builder.has_loop_phi;
  }
#endif
 
  bool IsLoopPhi(Node* node) const {
    return node->opcode() == IrOpcode::kPhi &&
           schedule()->block(node)->IsLoopHeader();
  }
  bool LoopContains(Node* loop_phi, Node* node) {
    DCHECK(IsLoopPhi(loop_phi));
    return schedule()->block(loop_phi)->LoopContains(schedule()->block(node));
  }
 
  int GetStringBuilderIdForConcat(Node* node);
  void ReplaceConcatInputIfNeeded(Node* node, int input_idx);
  bool CheckNodeUses(Node* node, Node* concat_child, Status status);
  bool CheckPreviousNodeUses(Node* child, Status status,
                             int input_if_loop_phi = 0);
  int GetPhiPredecessorsCommonId(Node* node);
 
  void FinalizeStringBuilders();
  void VisitNode(Node* node, BasicBlock* block);
  void VisitGraph();
 
  static constexpr bool kAllowAnyStringOnTheRhs = false;
 
  JSGraph* jsgraph_;
  Schedule* schedule_;
  Zone* temp_zone_;
  JSHeapBroker* broker_;
  unsigned int string_builder_count_ = 0;
  // {blocks_to_trimmings_map_} is a map from block IDs to loop phi nodes that
  // end string builders. For each such node, a trimming should be inserted at
  // the beginning of the block (in EffectControlLinearizer) in order to
  // properly finish the string builder (well, most things will work if the
  // trimming is omitted, but adding this trimming save memory and removes the
  // SlicedString indirection; the only thing that would be an issue is that the
  // rest of the VM could have access to a SlicedString that is less than
  // SlicedString::kMinLength characters, which may or may not break things).
  ZoneVector<std::optional<ZoneVector<Node*>>> blocks_to_trimmings_map_;
  ZoneVector<Status> status_;
  ZoneVector<StringBuilder> string_builders_;
  // {loop_headers_} is used to keep track ot the start of each loop that the
  // block currently being visited is part of.
  ZoneVector<BasicBlock*> loop_headers_;
};
 
}  // namespace compiler
}  // namespace internal
}  // namespace v8
 
#endif  // V8_COMPILER_STRING_BUILDER_OPTIMIZER_H_