Liveness analysis for environment slots in Hydrogen

src/hydrogen-environment-liveness.h

+// Copyright 2013 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
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+
+#ifndef V8_HYDROGEN_ENVIRONMENT_LIVENESS_H_
+#define V8_HYDROGEN_ENVIRONMENT_LIVENESS_H_
+
+
+#include "hydrogen.h"
+
+namespace v8 {
+namespace internal {
+
+
+// Trims live ranges of environment slots by doing explicit liveness analysis.
+// Values in the environment are kept alive by every subsequent LInstruction
+// that is assigned an LEnvironment, which creates register pressure and
+// unnecessary spill slot moves. Therefore it is beneficial to trim the
+// live ranges of environment slots by zapping them with a constant after
+// the last lookup that refers to them.
+// Slots are identified by their index and only affected if whitelisted in
+// HOptimizedGraphBuilder::IsEligibleForEnvironmentLivenessAnalysis().
+class EnvironmentSlotLivenessAnalyzer {
+ public:
+  explicit EnvironmentSlotLivenessAnalyzer(HGraph* graph);
+
+  void AnalyzeAndTrim();
+
+ private:
+  void ZapEnvironmentSlot(int index, HSimulate* simulate);
+  void ZapEnvironmentSlotsInSuccessors(HBasicBlock* block, BitVector* live);
+  void ZapEnvironmentSlotsForInstruction(HEnvironmentMarker* marker);
+  void UpdateLivenessAtBlockEnd(HBasicBlock* block, BitVector* live);
+  void UpdateLivenessAtInstruction(HInstruction* instr, BitVector* live);
+
+  Zone* zone() { return &zone_; }
+
+  HGraph* graph_;
+  // Use a dedicated Zone for this phase, with a ZoneScope to ensure it
+  // gets freed.
+  Zone zone_;
+  ZoneScope zone_scope_;
+
+  int block_count_;
+
+  // Largest number of local variables in any environment in the graph
+  // (including inlined environments).
+  int maximum_environment_size_;
+
+  // Per-block data. All these lists are indexed by block_id.
+  ZoneList<BitVector*>* live_at_block_start_;
+  ZoneList<HSimulate*>* first_simulate_;
+  ZoneList<BitVector*>* first_simulate_invalid_for_index_;
+
+  // List of all HEnvironmentMarker instructions for quick iteration/deletion.
+  // It is populated during the first pass over the graph, controlled by
+  // |collect_markers_|.
+  ZoneList<HEnvironmentMarker*>* markers_;
+  bool collect_markers_;
+
+  // Keeps track of the last simulate seen, as well as the environment slots
+  // for which a new live range has started since (so they must not be zapped
+  // in that simulate when the end of another live range of theirs is found).
+  HSimulate* last_simulate_;
+  BitVector* went_live_since_last_simulate_;
+};
+
+
+} }  // namespace v8::internal
+
+#endif /* V8_HYDROGEN_ENVIRONMENT_LIVENESS_H_ */