LiveRegMatrix.h

Go to the documentation of this file.

//===- LiveRegMatrix.h - Track register interference ----------*- C++ -*---===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The LiveRegMatrix analysis pass keeps track of virtual register interference
// along two dimensions: Slot indexes and register units. The matrix is used by
// register allocators to ensure that no interfering virtual registers get
// assigned to overlapping physical registers.
//
// Register units are defined in MCRegisterInfo.h, they represent the smallest
// unit of interference when dealing with overlapping physical registers. The
// LiveRegMatrix is represented as a LiveIntervalUnion per register unit. When
// a virtual register is assigned to a physical register, the live range for
// the virtual register is inserted into the LiveIntervalUnion for each regunit
// in the physreg.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_LIVEREGMATRIX_H
#define LLVM_CODEGEN_LIVEREGMATRIX_H

#include "llvm/ADT/BitVector.h"
#include "llvm/CodeGen/LiveIntervalUnion.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include <memory>

namespace llvm {

class AnalysisUsage;
class LiveInterval;
class LiveIntervals;
class MachineFunction;
class TargetRegisterInfo;
class VirtRegMap;

class LiveRegMatrix : public MachineFunctionPass {
  const TargetRegisterInfo *TRI;
  LiveIntervals *LIS;
  VirtRegMap *VRM;

  // UserTag changes whenever virtual registers have been modified.
  unsigned UserTag = 0;

  // The matrix is represented as a LiveIntervalUnion per register unit.
  LiveIntervalUnion::Allocator LIUAlloc;
  LiveIntervalUnion::Array Matrix;

  // Cached queries per register unit.
  std::unique_ptr<LiveIntervalUnion::Query[]> Queries;

  // Cached register mask interference info.
  unsigned RegMaskTag = 0;
  unsigned RegMaskVirtReg = 0;
  BitVector RegMaskUsable;

  // MachineFunctionPass boilerplate.
  void getAnalysisUsage(AnalysisUsage &) const override;
  bool runOnMachineFunction(MachineFunction &) override;
  void releaseMemory() override;

public:
  static char ID;

  LiveRegMatrix();

  //===--------------------------------------------------------------------===//
  // High-level interface.
  //===--------------------------------------------------------------------===//
  //
  // Check for interference before assigning virtual registers to physical
  // registers.
  //

  /// Invalidate cached interference queries after modifying virtual register
  /// live ranges. Interference checks may return stale information unless
  /// caches are invalidated.
  void invalidateVirtRegs() { ++UserTag; }

  enum InterferenceKind {
    /// No interference, go ahead and assign.
    IK_Free = 0,

    /// Virtual register interference. There are interfering virtual registers
    /// assigned to PhysReg or its aliases. This interference could be resolved
    /// by unassigning those other virtual registers.
    IK_VirtReg,

    /// Register unit interference. A fixed live range is in the way, typically
    /// argument registers for a call. This can't be resolved by unassigning
    /// other virtual registers.
    IK_RegUnit,

    /// RegMask interference. The live range is crossing an instruction with a
    /// regmask operand that doesn't preserve PhysReg. This typically means
    /// VirtReg is live across a call, and PhysReg isn't call-preserved.
    IK_RegMask
  };

  /// Check for interference before assigning VirtReg to PhysReg.
  /// If this function returns IK_Free, it is legal to assign(VirtReg, PhysReg).
  /// When there is more than one kind of interference, the InterferenceKind
  /// with the highest enum value is returned.
  InterferenceKind checkInterference(LiveInterval &VirtReg, unsigned PhysReg);

  /// Check for interference in the segment [Start, End) that may prevent
  /// assignment to PhysReg. If this function returns true, there is
  /// interference in the segment [Start, End) of some other interval already
  /// assigned to PhysReg. If this function returns false, PhysReg is free at
  /// the segment [Start, End).
  bool checkInterference(SlotIndex Start, SlotIndex End, unsigned PhysReg);

  /// Assign VirtReg to PhysReg.
  /// This will mark VirtReg's live range as occupied in the LiveRegMatrix and
  /// update VirtRegMap. The live range is expected to be available in PhysReg.
  void assign(LiveInterval &VirtReg, unsigned PhysReg);

  /// Unassign VirtReg from its PhysReg.
  /// Assuming that VirtReg was previously assigned to a PhysReg, this undoes
  /// the assignment and updates VirtRegMap accordingly.
  void unassign(LiveInterval &VirtReg);

  /// Returns true if the given \p PhysReg has any live intervals assigned.
  bool isPhysRegUsed(unsigned PhysReg) const;

  //===--------------------------------------------------------------------===//
  // Low-level interface.
  //===--------------------------------------------------------------------===//
  //
  // Provide access to the underlying LiveIntervalUnions.
  //

  /// Check for regmask interference only.
  /// Return true if VirtReg crosses a regmask operand that clobbers PhysReg.
  /// If PhysReg is null, check if VirtReg crosses any regmask operands.
  bool checkRegMaskInterference(LiveInterval &VirtReg, unsigned PhysReg = 0);

  /// Check for regunit interference only.
  /// Return true if VirtReg overlaps a fixed assignment of one of PhysRegs's
  /// register units.
  bool checkRegUnitInterference(LiveInterval &VirtReg, unsigned PhysReg);

  /// Query a line of the assigned virtual register matrix directly.
  /// Use MCRegUnitIterator to enumerate all regunits in the desired PhysReg.
  /// This returns a reference to an internal Query data structure that is only
  /// valid until the next query() call.
  LiveIntervalUnion::Query &query(const LiveRange &LR, unsigned RegUnit);

  /// Directly access the live interval unions per regunit.
  /// This returns an array indexed by the regunit number.
  LiveIntervalUnion *getLiveUnions() { return &Matrix[0]; }
};

} // end namespace llvm

#endif // LLVM_CODEGEN_LIVEREGMATRIX_H