SplitKit.h

Go to the documentation of this file.

//===- SplitKit.h - Toolkit for splitting live ranges -----------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the SplitAnalysis class as well as mutator functions for
// live range splitting.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_LIB_CODEGEN_SPLITKIT_H
#define LLVM_LIB_CODEGEN_SPLITKIT_H
 
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/IntervalMap.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/LiveIntervalCalc.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/Support/Compiler.h"
#include <utility>
 
namespace llvm {
 
class LiveInterval;
class LiveRange;
class LiveIntervals;
class LiveRangeEdit;
class MachineBlockFrequencyInfo;
class MachineDominatorTree;
class MachineLoopInfo;
class MachineRegisterInfo;
class TargetInstrInfo;
class TargetRegisterInfo;
class VirtRegMap;
class VirtRegAuxInfo;
 
/// Determines the latest safe point in a block in which we can insert a split,
/// spill or other instruction related with CurLI.
class LLVM_LIBRARY_VISIBILITY InsertPointAnalysis {
private:
  const LiveIntervals &LIS;
 
  /// Last legal insert point in each basic block in the current function.
  /// The first entry is the first terminator, the second entry is the
  /// last valid point to insert a split or spill for a variable that is
  /// live into a landing pad or inlineasm_br successor.
  SmallVector<std::pair<SlotIndex, SlotIndex>, 8> LastInsertPoint;
 
  SlotIndex computeLastInsertPoint(const LiveInterval &CurLI,
                                   const MachineBasicBlock &MBB);
 
public:
  InsertPointAnalysis(const LiveIntervals &lis, unsigned BBNum);
 
  /// Return the base index of the last valid insert point for \pCurLI in \pMBB.
  SlotIndex getLastInsertPoint(const LiveInterval &CurLI,
                               const MachineBasicBlock &MBB) {
    unsigned Num = MBB.getNumber();
    // Inline the common simple case.
    if (LastInsertPoint[Num].first.isValid() &&
        !LastInsertPoint[Num].second.isValid())
      return LastInsertPoint[Num].first;
    return computeLastInsertPoint(CurLI, MBB);
  }
 
  /// Returns the last insert point as an iterator for \pCurLI in \pMBB.
  MachineBasicBlock::iterator getLastInsertPointIter(const LiveInterval &CurLI,
                                                     MachineBasicBlock &MBB);
 
  /// Return the base index of the first insert point in \pMBB.
  SlotIndex getFirstInsertPoint(MachineBasicBlock &MBB) {
    SlotIndex Res = LIS.getMBBStartIdx(&MBB);
    if (!MBB.empty()) {
      MachineBasicBlock::iterator MII = MBB.SkipPHIsLabelsAndDebug(MBB.begin());
      if (MII != MBB.end())
        Res = LIS.getInstructionIndex(*MII);
    }
    return Res;
  }
 
};
 
/// SplitAnalysis - Analyze a LiveInterval, looking for live range splitting
/// opportunities.
class LLVM_LIBRARY_VISIBILITY SplitAnalysis {
public:
  const MachineFunction &MF;
  const VirtRegMap &VRM;
  const LiveIntervals &LIS;
  const MachineLoopInfo &Loops;
  const TargetInstrInfo &TII;
 
  /// Additional information about basic blocks where the current variable is
  /// live. Such a block will look like one of these templates:
  ///
  ///  1. |   o---x   | Internal to block. Variable is only live in this block.
  ///  2. |---x       | Live-in, kill.
  ///  3. |       o---| Def, live-out.
  ///  4. |---x   o---| Live-in, kill, def, live-out. Counted by NumGapBlocks.
  ///  5. |---o---o---| Live-through with uses or defs.
  ///  6. |-----------| Live-through without uses. Counted by NumThroughBlocks.
  ///
  /// Two BlockInfo entries are created for template 4. One for the live-in
  /// segment, and one for the live-out segment. These entries look as if the
  /// block were split in the middle where the live range isn't live.
  ///
  /// Live-through blocks without any uses don't get BlockInfo entries. They
  /// are simply listed in ThroughBlocks instead.
  ///
  struct BlockInfo {
    MachineBasicBlock *MBB;
    SlotIndex FirstInstr; ///< First instr accessing current reg.
    SlotIndex LastInstr;  ///< Last instr accessing current reg.
    SlotIndex FirstDef;   ///< First non-phi valno->def, or SlotIndex().
    bool LiveIn;          ///< Current reg is live in.
    bool LiveOut;         ///< Current reg is live out.
 
    /// isOneInstr - Returns true when this BlockInfo describes a single
    /// instruction.
    bool isOneInstr() const {
      return SlotIndex::isSameInstr(FirstInstr, LastInstr);
    }
 
    void print(raw_ostream &OS) const;
    void dump() const;
  };
 
private:
  // Current live interval.
  const LiveInterval *CurLI = nullptr;
 
  /// Insert Point Analysis.
  InsertPointAnalysis IPA;
 
  // Sorted slot indexes of using instructions.
  SmallVector<SlotIndex, 8> UseSlots;
 
  /// UseBlocks - Blocks where CurLI has uses.
  SmallVector<BlockInfo, 8> UseBlocks;
 
  /// NumGapBlocks - Number of duplicate entries in UseBlocks for blocks where
  /// the live range has a gap.
  unsigned NumGapBlocks = 0u;
 
  /// ThroughBlocks - Block numbers where CurLI is live through without uses.
  BitVector ThroughBlocks;
 
  /// NumThroughBlocks - Number of live-through blocks.
  unsigned NumThroughBlocks = 0u;
 
  /// LooksLikeLoopIV - The variable defines what looks like it could be a loop
  /// IV, where it defs a variable in the latch.
  bool LooksLikeLoopIV = false;
 
  // Sumarize statistics by counting instructions using CurLI.
  void analyzeUses();
 
  /// calcLiveBlockInfo - Compute per-block information about CurLI.
  void calcLiveBlockInfo();
 
public:
  SplitAnalysis(const VirtRegMap &vrm, const LiveIntervals &lis,
                const MachineLoopInfo &mli);
 
  /// analyze - set CurLI to the specified interval, and analyze how it may be
  /// split.
  void analyze(const LiveInterval *li);
 
  /// clear - clear all data structures so SplitAnalysis is ready to analyze a
  /// new interval.
  void clear();
 
  /// getParent - Return the last analyzed interval.
  const LiveInterval &getParent() const { return *CurLI; }
 
  /// isOriginalEndpoint - Return true if the original live range was killed or
  /// (re-)defined at Idx. Idx should be the 'def' slot for a normal kill/def,
  /// and 'use' for an early-clobber def.
  /// This can be used to recognize code inserted by earlier live range
  /// splitting.
  bool isOriginalEndpoint(SlotIndex Idx) const;
 
  /// getUseSlots - Return an array of SlotIndexes of instructions using CurLI.
  /// This include both use and def operands, at most one entry per instruction.
  ArrayRef<SlotIndex> getUseSlots() const { return UseSlots; }
 
  /// getUseBlocks - Return an array of BlockInfo objects for the basic blocks
  /// where CurLI has uses.
  ArrayRef<BlockInfo> getUseBlocks() const { return UseBlocks; }
 
  /// getNumThroughBlocks - Return the number of through blocks.
  unsigned getNumThroughBlocks() const { return NumThroughBlocks; }
 
  /// isThroughBlock - Return true if CurLI is live through MBB without uses.
  bool isThroughBlock(unsigned MBB) const { return ThroughBlocks.test(MBB); }
 
  /// getThroughBlocks - Return the set of through blocks.
  const BitVector &getThroughBlocks() const { return ThroughBlocks; }
 
  /// getNumLiveBlocks - Return the number of blocks where CurLI is live.
  unsigned getNumLiveBlocks() const {
    return getUseBlocks().size() - NumGapBlocks + getNumThroughBlocks();
  }
 
  bool looksLikeLoopIV() const { return LooksLikeLoopIV; }
 
  /// countLiveBlocks - Return the number of blocks where li is live. This is
  /// guaranteed to return the same number as getNumLiveBlocks() after calling
  /// analyze(li).
  unsigned countLiveBlocks(const LiveInterval *li) const;
 
  using BlockPtrSet = SmallPtrSet<const MachineBasicBlock *, 16>;
 
  /// shouldSplitSingleBlock - Returns true if it would help to create a local
  /// live range for the instructions in BI. There is normally no benefit to
  /// creating a live range for a single instruction, but it does enable
  /// register class inflation if the instruction has a restricted register
  /// class.
  ///
  /// @param BI           The block to be isolated.
  /// @param SingleInstrs True when single instructions should be isolated.
  bool shouldSplitSingleBlock(const BlockInfo &BI, bool SingleInstrs) const;
 
  SlotIndex getLastSplitPoint(unsigned Num) {
    return IPA.getLastInsertPoint(*CurLI, *MF.getBlockNumbered(Num));
  }
 
  SlotIndex getLastSplitPoint(MachineBasicBlock *BB) {
    return IPA.getLastInsertPoint(*CurLI, *BB);
  }
 
  MachineBasicBlock::iterator getLastSplitPointIter(MachineBasicBlock *BB) {
    return IPA.getLastInsertPointIter(*CurLI, *BB);
  }
 
  SlotIndex getFirstSplitPoint(unsigned Num) {
    return IPA.getFirstInsertPoint(*MF.getBlockNumbered(Num));
  }
};
 
/// SplitEditor - Edit machine code and LiveIntervals for live range
/// splitting.
///
/// - Create a SplitEditor from a SplitAnalysis.
/// - Start a new live interval with openIntv.
/// - Mark the places where the new interval is entered using enterIntv*
/// - Mark the ranges where the new interval is used with useIntv*
/// - Mark the places where the interval is exited with exitIntv*.
/// - Finish the current interval with closeIntv and repeat from 2.
/// - Rewrite instructions with finish().
///
class LLVM_LIBRARY_VISIBILITY SplitEditor {
  SplitAnalysis &SA;
  LiveIntervals &LIS;
  VirtRegMap &VRM;
  MachineRegisterInfo &MRI;
  MachineDominatorTree &MDT;
  const TargetInstrInfo &TII;
  const TargetRegisterInfo &TRI;
  const MachineBlockFrequencyInfo &MBFI;
  VirtRegAuxInfo &VRAI;
 
public:
  /// ComplementSpillMode - Select how the complement live range should be
  /// created.  SplitEditor automatically creates interval 0 to contain
  /// anything that isn't added to another interval.  This complement interval
  /// can get quite complicated, and it can sometimes be an advantage to allow
  /// it to overlap the other intervals.  If it is going to spill anyway, no
  /// registers are wasted by keeping a value in two places at the same time.
  enum ComplementSpillMode {
    /// SM_Partition(Default) - Try to create the complement interval so it
    /// doesn't overlap any other intervals, and the original interval is
    /// partitioned.  This may require a large number of back copies and extra
    /// PHI-defs.  Only segments marked with overlapIntv will be overlapping.
    SM_Partition,
 
    /// SM_Size - Overlap intervals to minimize the number of inserted COPY
    /// instructions.  Copies to the complement interval are hoisted to their
    /// common dominator, so only one COPY is required per value in the
    /// complement interval.  This also means that no extra PHI-defs need to be
    /// inserted in the complement interval.
    SM_Size,
 
    /// SM_Speed - Overlap intervals to minimize the expected execution
    /// frequency of the inserted copies.  This is very similar to SM_Size, but
    /// the complement interval may get some extra PHI-defs.
    SM_Speed
  };
 
private:
  /// Edit - The current parent register and new intervals created.
  LiveRangeEdit *Edit = nullptr;
 
  /// Index into Edit of the currently open interval.
  /// The index 0 is used for the complement, so the first interval started by
  /// openIntv will be 1.
  unsigned OpenIdx = 0;
 
  /// The current spill mode, selected by reset().
  ComplementSpillMode SpillMode = SM_Partition;
 
  using RegAssignMap = IntervalMap<SlotIndex, unsigned>;
 
  /// Allocator for the interval map. This will eventually be shared with
  /// SlotIndexes and LiveIntervals.
  RegAssignMap::Allocator Allocator;
 
  /// RegAssign - Map of the assigned register indexes.
  /// Edit.get(RegAssign.lookup(Idx)) is the register that should be live at
  /// Idx.
  RegAssignMap RegAssign;
 
  using ValueForcePair = PointerIntPair<VNInfo *, 1>;
  using ValueMap = DenseMap<std::pair<unsigned, unsigned>, ValueForcePair>;
 
  /// Values - keep track of the mapping from parent values to values in the new
  /// intervals. Given a pair (RegIdx, ParentVNI->id), Values contains:
  ///
  /// 1. No entry - the value is not mapped to Edit.get(RegIdx).
  /// 2. (Null, false) - the value is mapped to multiple values in
  ///    Edit.get(RegIdx).  Each value is represented by a minimal live range at
  ///    its def.  The full live range can be inferred exactly from the range
  ///    of RegIdx in RegAssign.
  /// 3. (Null, true).  As above, but the ranges in RegAssign are too large, and
  ///    the live range must be recomputed using ::extend().
  /// 4. (VNI, false) The value is mapped to a single new value.
  ///    The new value has no live ranges anywhere.
  ValueMap Values;
 
  /// LICalc - Cache for computing live ranges and SSA update.  Each instance
  /// can only handle non-overlapping live ranges, so use a separate
  /// LiveIntervalCalc instance for the complement interval when in spill mode.
  LiveIntervalCalc LICalc[2];
 
  /// getLICalc - Return the LICalc to use for RegIdx.  In spill mode, the
  /// complement interval can overlap the other intervals, so it gets its own
  /// LICalc instance.  When not in spill mode, all intervals can share one.
  LiveIntervalCalc &getLICalc(unsigned RegIdx) {
    return LICalc[SpillMode != SM_Partition && RegIdx != 0];
  }
 
  /// Add a segment to the interval LI for the value number VNI. If LI has
  /// subranges, corresponding segments will be added to them as well, but
  /// with newly created value numbers. If Original is true, dead def will
  /// only be added a subrange of LI if the corresponding subrange of the
  /// original interval has a def at this index. Otherwise, all subranges
  /// of LI will be updated.
  void addDeadDef(LiveInterval &LI, VNInfo *VNI, bool Original);
 
  /// defValue - define a value in RegIdx from ParentVNI at Idx.
  /// Idx does not have to be ParentVNI->def, but it must be contained within
  /// ParentVNI's live range in ParentLI. The new value is added to the value
  /// map. The value being defined may either come from rematerialization
  /// (or an inserted copy), or it may be coming from the original interval.
  /// The parameter Original should be true in the latter case, otherwise
  /// it should be false.
  /// Return the new LI value.
  VNInfo *defValue(unsigned RegIdx, const VNInfo *ParentVNI, SlotIndex Idx,
                   bool Original);
 
  /// forceRecompute - Force the live range of ParentVNI in RegIdx to be
  /// recomputed by LiveRangeCalc::extend regardless of the number of defs.
  /// This is used for values whose live range doesn't match RegAssign exactly.
  /// They could have rematerialized, or back-copies may have been moved.
  void forceRecompute(unsigned RegIdx, const VNInfo &ParentVNI);
 
  /// Calls forceRecompute() on any affected regidx and on ParentVNI
  /// predecessors in case of a phi definition.
  void forceRecomputeVNI(const VNInfo &ParentVNI);
 
  /// defFromParent - Define Reg from ParentVNI at UseIdx using either
  /// rematerialization or a COPY from parent. Return the new value.
  VNInfo *defFromParent(unsigned RegIdx, const VNInfo *ParentVNI,
                        SlotIndex UseIdx, MachineBasicBlock &MBB,
                        MachineBasicBlock::iterator I);
 
  /// removeBackCopies - Remove the copy instructions that defines the values
  /// in the vector in the complement interval.
  void removeBackCopies(SmallVectorImpl<VNInfo*> &Copies);
 
  /// getShallowDominator - Returns the least busy dominator of MBB that is
  /// also dominated by DefMBB.  Busy is measured by loop depth.
  MachineBasicBlock *findShallowDominator(MachineBasicBlock *MBB,
                                          MachineBasicBlock *DefMBB);
 
  /// Find out all the backCopies dominated by others.
  void computeRedundantBackCopies(DenseSet<unsigned> &NotToHoistSet,
                                  SmallVectorImpl<VNInfo *> &BackCopies);
 
  /// Hoist back-copies to the complement interval. It tries to hoist all
  /// the back-copies to one BB if it is beneficial, or else simply remove
  /// redundant backcopies dominated by others.
  void hoistCopies();
 
  /// transferValues - Transfer values to the new ranges.
  /// Return true if any ranges were skipped.
  bool transferValues();
 
  /// Live range @p LR corresponding to the lane Mask @p LM has a live
  /// PHI def at the beginning of block @p B. Extend the range @p LR of
  /// all predecessor values that reach this def. If @p LR is a subrange,
  /// the array @p Undefs is the set of all locations where it is undefined
  /// via <def,read-undef> in other subranges for the same register.
  void extendPHIRange(MachineBasicBlock &B, LiveIntervalCalc &LIC,
                      LiveRange &LR, LaneBitmask LM,
                      ArrayRef<SlotIndex> Undefs);
 
  /// extendPHIKillRanges - Extend the ranges of all values killed by original
  /// parent PHIDefs.
  void extendPHIKillRanges();
 
  /// rewriteAssigned - Rewrite all uses of Edit.getReg() to assigned registers.
  void rewriteAssigned(bool ExtendRanges);
 
  /// deleteRematVictims - Delete defs that are dead after rematerializing.
  void deleteRematVictims();
 
  /// Add a copy instruction copying \p FromReg to \p ToReg before
  /// \p InsertBefore. This can be invoked with a \p LaneMask which may make it
  /// necessary to construct a sequence of copies to cover it exactly.
  SlotIndex buildCopy(Register FromReg, Register ToReg, LaneBitmask LaneMask,
      MachineBasicBlock &MBB, MachineBasicBlock::iterator InsertBefore,
      bool Late, unsigned RegIdx);
 
  SlotIndex buildSingleSubRegCopy(Register FromReg, Register ToReg,
                                  MachineBasicBlock &MB,
                                  MachineBasicBlock::iterator InsertBefore,
                                  unsigned SubIdx, LiveInterval &DestLI,
                                  bool Late, SlotIndex Def,
                                  const MCInstrDesc &Desc);
 
public:
  /// Create a new SplitEditor for editing the LiveInterval analyzed by SA.
  /// Newly created intervals will be appended to newIntervals.
  SplitEditor(SplitAnalysis &SA, LiveIntervals &LIS, VirtRegMap &VRM,
              MachineDominatorTree &MDT, MachineBlockFrequencyInfo &MBFI,
              VirtRegAuxInfo &VRAI);
 
  /// reset - Prepare for a new split.
  void reset(LiveRangeEdit&, ComplementSpillMode = SM_Partition);
 
  /// Create a new virtual register and live interval.
  /// Return the interval index, starting from 1. Interval index 0 is the
  /// implicit complement interval.
  unsigned openIntv();
 
  /// currentIntv - Return the current interval index.
  unsigned currentIntv() const { return OpenIdx; }
 
  /// selectIntv - Select a previously opened interval index.
  void selectIntv(unsigned Idx);
 
  /// enterIntvBefore - Enter the open interval before the instruction at Idx.
  /// If the parent interval is not live before Idx, a COPY is not inserted.
  /// Return the beginning of the new live range.
  SlotIndex enterIntvBefore(SlotIndex Idx);
 
  /// enterIntvAfter - Enter the open interval after the instruction at Idx.
  /// Return the beginning of the new live range.
  SlotIndex enterIntvAfter(SlotIndex Idx);
 
  /// enterIntvAtEnd - Enter the open interval at the end of MBB.
  /// Use the open interval from the inserted copy to the MBB end.
  /// Return the beginning of the new live range.
  SlotIndex enterIntvAtEnd(MachineBasicBlock &MBB);
 
  /// useIntv - indicate that all instructions in MBB should use OpenLI.
  void useIntv(const MachineBasicBlock &MBB);
 
  /// useIntv - indicate that all instructions in range should use OpenLI.
  void useIntv(SlotIndex Start, SlotIndex End);
 
  /// leaveIntvAfter - Leave the open interval after the instruction at Idx.
  /// Return the end of the live range.
  SlotIndex leaveIntvAfter(SlotIndex Idx);
 
  /// leaveIntvBefore - Leave the open interval before the instruction at Idx.
  /// Return the end of the live range.
  SlotIndex leaveIntvBefore(SlotIndex Idx);
 
  /// leaveIntvAtTop - Leave the interval at the top of MBB.
  /// Add liveness from the MBB top to the copy.
  /// Return the end of the live range.
  SlotIndex leaveIntvAtTop(MachineBasicBlock &MBB);
 
  /// overlapIntv - Indicate that all instructions in range should use the open
  /// interval if End does not have tied-def usage of the register and in this
  /// case complement interval is used. Let the complement interval be live.
  ///
  /// This doubles the register pressure, but is sometimes required to deal with
  /// register uses after the last valid split point.
  ///
  /// The Start index should be a return value from a leaveIntv* call, and End
  /// should be in the same basic block. The parent interval must have the same
  /// value across the range.
  ///
  void overlapIntv(SlotIndex Start, SlotIndex End);
 
  /// finish - after all the new live ranges have been created, compute the
  /// remaining live range, and rewrite instructions to use the new registers.
  /// @param LRMap When not null, this vector will map each live range in Edit
  ///              back to the indices returned by openIntv.
  ///              There may be extra indices created by dead code elimination.
  void finish(SmallVectorImpl<unsigned> *LRMap = nullptr);
 
  /// dump - print the current interval mapping to dbgs().
  void dump() const;
 
  // ===--- High level methods ---===
 
  /// splitSingleBlock - Split CurLI into a separate live interval around the
  /// uses in a single block. This is intended to be used as part of a larger
  /// split, and doesn't call finish().
  void splitSingleBlock(const SplitAnalysis::BlockInfo &BI);
 
  /// splitLiveThroughBlock - Split CurLI in the given block such that it
  /// enters the block in IntvIn and leaves it in IntvOut. There may be uses in
  /// the block, but they will be ignored when placing split points.
  ///
  /// @param MBBNum      Block number.
  /// @param IntvIn      Interval index entering the block.
  /// @param LeaveBefore When set, leave IntvIn before this point.
  /// @param IntvOut     Interval index leaving the block.
  /// @param EnterAfter  When set, enter IntvOut after this point.
  void splitLiveThroughBlock(unsigned MBBNum,
                             unsigned IntvIn, SlotIndex LeaveBefore,
                             unsigned IntvOut, SlotIndex EnterAfter);
 
  /// splitRegInBlock - Split CurLI in the given block such that it enters the
  /// block in IntvIn and leaves it on the stack (or not at all). Split points
  /// are placed in a way that avoids putting uses in the stack interval. This
  /// may require creating a local interval when there is interference.
  ///
  /// @param BI          Block descriptor.
  /// @param IntvIn      Interval index entering the block. Not 0.
  /// @param LeaveBefore When set, leave IntvIn before this point.
  void splitRegInBlock(const SplitAnalysis::BlockInfo &BI,
                       unsigned IntvIn, SlotIndex LeaveBefore);
 
  /// splitRegOutBlock - Split CurLI in the given block such that it enters the
  /// block on the stack (or isn't live-in at all) and leaves it in IntvOut.
  /// Split points are placed to avoid interference and such that the uses are
  /// not in the stack interval. This may require creating a local interval
  /// when there is interference.
  ///
  /// @param BI          Block descriptor.
  /// @param IntvOut     Interval index leaving the block.
  /// @param EnterAfter  When set, enter IntvOut after this point.
  void splitRegOutBlock(const SplitAnalysis::BlockInfo &BI,
                        unsigned IntvOut, SlotIndex EnterAfter);
};
 
} // end namespace llvm
 
#endif // LLVM_LIB_CODEGEN_SPLITKIT_H