include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h - llvm - Git at Google

 //===-- llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h -----*- 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 some helper functions which try to cleanup artifacts
 // such as G_TRUNCs/G_[ZSA]EXTENDS that were created during legalization to make
 // the types match. This file also contains some combines of merges that happens
 // at the end of the legalization.
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/GlobalISel/Legalizer.h"
 #include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
 #include "llvm/CodeGen/GlobalISel/MIPatternMatch.h"
 #include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
 #include "llvm/CodeGen/GlobalISel/Utils.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Support/Debug.h"

 #define DEBUG_TYPE "legalizer"
 using namespace llvm::MIPatternMatch;

 namespace llvm {
 class LegalizationArtifactCombiner {
   MachineIRBuilder &Builder;
   MachineRegisterInfo &MRI;
   const LegalizerInfo &LI;

 public:
   LegalizationArtifactCombiner(MachineIRBuilder &B, MachineRegisterInfo &MRI,
                     const LegalizerInfo &LI)
       : Builder(B), MRI(MRI), LI(LI) {}

   bool tryCombineAnyExt(MachineInstr &MI,
                         SmallVectorImpl<MachineInstr *> &DeadInsts) {
     if (MI.getOpcode() != TargetOpcode::G_ANYEXT)
       return false;

     Builder.setInstr(MI);
     unsigned DstReg = MI.getOperand(0).getReg();
     unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

     // aext(trunc x) - > aext/copy/trunc x
     unsigned TruncSrc;
     if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
       LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
       Builder.buildAnyExtOrTrunc(DstReg, TruncSrc);
       markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
       return true;
     }

     // aext([asz]ext x) -> [asz]ext x
     unsigned ExtSrc;
     MachineInstr *ExtMI;
     if (mi_match(SrcReg, MRI,
                  m_all_of(m_MInstr(ExtMI), m_any_of(m_GAnyExt(m_Reg(ExtSrc)),
                                                     m_GSExt(m_Reg(ExtSrc)),
                                                     m_GZExt(m_Reg(ExtSrc)))))) {
       Builder.buildInstr(ExtMI->getOpcode(), {DstReg}, {ExtSrc});
       markInstAndDefDead(MI, *ExtMI, DeadInsts);
       return true;
     }
     return tryFoldImplicitDef(MI, DeadInsts);
   }

   bool tryCombineZExt(MachineInstr &MI,
                       SmallVectorImpl<MachineInstr *> &DeadInsts) {

     if (MI.getOpcode() != TargetOpcode::G_ZEXT)
       return false;

     Builder.setInstr(MI);
     unsigned DstReg = MI.getOperand(0).getReg();
     unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

     // zext(trunc x) - > and (aext/copy/trunc x), mask
     unsigned TruncSrc;
     if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
       LLT DstTy = MRI.getType(DstReg);
       if (isInstUnsupported({TargetOpcode::G_AND, {DstTy}}) ||
           isConstantUnsupported(DstTy))
         return false;
       LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
       LLT SrcTy = MRI.getType(SrcReg);
       APInt Mask = APInt::getAllOnesValue(SrcTy.getScalarSizeInBits());
       auto MIBMask = Builder.buildConstant(DstTy, Mask.getZExtValue());
       Builder.buildAnd(DstReg, Builder.buildAnyExtOrTrunc(DstTy, TruncSrc),
                        MIBMask);
       markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
       return true;
     }
     return tryFoldImplicitDef(MI, DeadInsts);
   }

   bool tryCombineSExt(MachineInstr &MI,
                       SmallVectorImpl<MachineInstr *> &DeadInsts) {

     if (MI.getOpcode() != TargetOpcode::G_SEXT)
       return false;

     Builder.setInstr(MI);
     unsigned DstReg = MI.getOperand(0).getReg();
     unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

     // sext(trunc x) - > ashr (shl (aext/copy/trunc x), c), c
     unsigned TruncSrc;
     if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
       LLT DstTy = MRI.getType(DstReg);
       // Guess on the RHS shift amount type, which should be re-legalized if
       // applicable.
       if (isInstUnsupported({TargetOpcode::G_SHL, {DstTy, DstTy}}) ||
           isInstUnsupported({TargetOpcode::G_ASHR, {DstTy, DstTy}}) ||
           isConstantUnsupported(DstTy))
         return false;
       LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
       LLT SrcTy = MRI.getType(SrcReg);
       unsigned ShAmt = DstTy.getScalarSizeInBits() - SrcTy.getScalarSizeInBits();
       auto MIBShAmt = Builder.buildConstant(DstTy, ShAmt);
       auto MIBShl = Builder.buildInstr(
           TargetOpcode::G_SHL, {DstTy},
           {Builder.buildAnyExtOrTrunc(DstTy, TruncSrc), MIBShAmt});
       Builder.buildInstr(TargetOpcode::G_ASHR, {DstReg}, {MIBShl, MIBShAmt});
       markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
       return true;
     }
     return tryFoldImplicitDef(MI, DeadInsts);
   }

   /// Try to fold G_[ASZ]EXT (G_IMPLICIT_DEF).
   bool tryFoldImplicitDef(MachineInstr &MI,
                           SmallVectorImpl<MachineInstr *> &DeadInsts) {
     unsigned Opcode = MI.getOpcode();
     if (Opcode != TargetOpcode::G_ANYEXT && Opcode != TargetOpcode::G_ZEXT &&
         Opcode != TargetOpcode::G_SEXT)
       return false;

     if (MachineInstr *DefMI = getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF,
                                            MI.getOperand(1).getReg(), MRI)) {
       Builder.setInstr(MI);
       unsigned DstReg = MI.getOperand(0).getReg();
       LLT DstTy = MRI.getType(DstReg);

       if (Opcode == TargetOpcode::G_ANYEXT) {
         // G_ANYEXT (G_IMPLICIT_DEF) -> G_IMPLICIT_DEF
         if (isInstUnsupported({TargetOpcode::G_IMPLICIT_DEF, {DstTy}}))
           return false;
         LLVM_DEBUG(dbgs() << ".. Combine G_ANYEXT(G_IMPLICIT_DEF): " << MI;);
         Builder.buildInstr(TargetOpcode::G_IMPLICIT_DEF, {DstReg}, {});
       } else {
         // G_[SZ]EXT (G_IMPLICIT_DEF) -> G_CONSTANT 0 because the top
         // bits will be 0 for G_ZEXT and 0/1 for the G_SEXT.
         if (isConstantUnsupported(DstTy))
           return false;
         LLVM_DEBUG(dbgs() << ".. Combine G_[SZ]EXT(G_IMPLICIT_DEF): " << MI;);
         Builder.buildConstant(DstReg, 0);
       }

       markInstAndDefDead(MI, *DefMI, DeadInsts);
       return true;
     }
     return false;
   }

   static unsigned getMergeOpcode(LLT OpTy, LLT DestTy) {
     if (OpTy.isVector() && DestTy.isVector())
       return TargetOpcode::G_CONCAT_VECTORS;

     if (OpTy.isVector() && !DestTy.isVector())
       return TargetOpcode::G_BUILD_VECTOR;

     return TargetOpcode::G_MERGE_VALUES;
   }

   bool tryCombineMerges(MachineInstr &MI,
                         SmallVectorImpl<MachineInstr *> &DeadInsts) {

     if (MI.getOpcode() != TargetOpcode::G_UNMERGE_VALUES)
       return false;

     unsigned NumDefs = MI.getNumOperands() - 1;

     LLT OpTy = MRI.getType(MI.getOperand(NumDefs).getReg());
     LLT DestTy = MRI.getType(MI.getOperand(0).getReg());

     unsigned MergingOpcode = getMergeOpcode(OpTy, DestTy);
     MachineInstr *MergeI =
         getOpcodeDef(MergingOpcode, MI.getOperand(NumDefs).getReg(), MRI);

     if (!MergeI)
       return false;

     const unsigned NumMergeRegs = MergeI->getNumOperands() - 1;

     if (NumMergeRegs < NumDefs) {
       if (NumDefs % NumMergeRegs != 0)
         return false;

       Builder.setInstr(MI);
       // Transform to UNMERGEs, for example
       //   %1 = G_MERGE_VALUES %4, %5
       //   %9, %10, %11, %12 = G_UNMERGE_VALUES %1
       // to
       //   %9, %10 = G_UNMERGE_VALUES %4
       //   %11, %12 = G_UNMERGE_VALUES %5

       const unsigned NewNumDefs = NumDefs / NumMergeRegs;
       for (unsigned Idx = 0; Idx < NumMergeRegs; ++Idx) {
         SmallVector<unsigned, 2> DstRegs;
         for (unsigned j = 0, DefIdx = Idx * NewNumDefs; j < NewNumDefs;
              ++j, ++DefIdx)
           DstRegs.push_back(MI.getOperand(DefIdx).getReg());

         Builder.buildUnmerge(DstRegs, MergeI->getOperand(Idx + 1).getReg());
       }

     } else if (NumMergeRegs > NumDefs) {
       if (NumMergeRegs % NumDefs != 0)
         return false;

       Builder.setInstr(MI);
       // Transform to MERGEs
       //   %6 = G_MERGE_VALUES %17, %18, %19, %20
       //   %7, %8 = G_UNMERGE_VALUES %6
       // to
       //   %7 = G_MERGE_VALUES %17, %18
       //   %8 = G_MERGE_VALUES %19, %20

       const unsigned NumRegs = NumMergeRegs / NumDefs;
       for (unsigned DefIdx = 0; DefIdx < NumDefs; ++DefIdx) {
         SmallVector<unsigned, 2> Regs;
         for (unsigned j = 0, Idx = NumRegs * DefIdx + 1; j < NumRegs;
              ++j, ++Idx)
           Regs.push_back(MergeI->getOperand(Idx).getReg());

         Builder.buildMerge(MI.getOperand(DefIdx).getReg(), Regs);
       }

     } else {
       // FIXME: is a COPY appropriate if the types mismatch? We know both
       // registers are allocatable by now.
       if (MRI.getType(MI.getOperand(0).getReg()) !=
           MRI.getType(MergeI->getOperand(1).getReg()))
         return false;

       for (unsigned Idx = 0; Idx < NumDefs; ++Idx)
         MRI.replaceRegWith(MI.getOperand(Idx).getReg(),
                            MergeI->getOperand(Idx + 1).getReg());
     }

     markInstAndDefDead(MI, *MergeI, DeadInsts);
     return true;
   }

   static bool isMergeLikeOpcode(unsigned Opc) {
     switch (Opc) {
     case TargetOpcode::G_MERGE_VALUES:
     case TargetOpcode::G_BUILD_VECTOR:
     case TargetOpcode::G_CONCAT_VECTORS:
       return true;
     default:
       return false;
     }
   }

   bool tryCombineExtract(MachineInstr &MI,
                          SmallVectorImpl<MachineInstr *> &DeadInsts) {
     assert(MI.getOpcode() == TargetOpcode::G_EXTRACT);

     // Try to use the source registers from a G_MERGE_VALUES
     //
     // %2 = G_MERGE_VALUES %0, %1
     // %3 = G_EXTRACT %2, N
     // =>
     //
     // for N < %2.getSizeInBits() / 2
     //     %3 = G_EXTRACT %0, N
     //
     // for N >= %2.getSizeInBits() / 2
     //    %3 = G_EXTRACT %1, (N - %0.getSizeInBits()

     unsigned Src = lookThroughCopyInstrs(MI.getOperand(1).getReg());
     MachineInstr *MergeI = MRI.getVRegDef(Src);
     if (!MergeI || !isMergeLikeOpcode(MergeI->getOpcode()))
       return false;

     LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
     LLT SrcTy = MRI.getType(Src);

     // TODO: Do we need to check if the resulting extract is supported?
     unsigned ExtractDstSize = DstTy.getSizeInBits();
     unsigned Offset = MI.getOperand(2).getImm();
     unsigned NumMergeSrcs = MergeI->getNumOperands() - 1;
     unsigned MergeSrcSize = SrcTy.getSizeInBits() / NumMergeSrcs;
     unsigned MergeSrcIdx = Offset / MergeSrcSize;

     // Compute the offset of the last bit the extract needs.
     unsigned EndMergeSrcIdx = (Offset + ExtractDstSize - 1) / MergeSrcSize;

     // Can't handle the case where the extract spans multiple inputs.
     if (MergeSrcIdx != EndMergeSrcIdx)
       return false;

     // TODO: We could modify MI in place in most cases.
     Builder.setInstr(MI);
     Builder.buildExtract(
       MI.getOperand(0).getReg(),
       MergeI->getOperand(MergeSrcIdx + 1).getReg(),
       Offset - MergeSrcIdx * MergeSrcSize);
     markInstAndDefDead(MI, *MergeI, DeadInsts);
     return true;
   }

   /// Try to combine away MI.
   /// Returns true if it combined away the MI.
   /// Adds instructions that are dead as a result of the combine
   /// into DeadInsts, which can include MI.
   bool tryCombineInstruction(MachineInstr &MI,
                              SmallVectorImpl<MachineInstr *> &DeadInsts) {
     switch (MI.getOpcode()) {
     default:
       return false;
     case TargetOpcode::G_ANYEXT:
       return tryCombineAnyExt(MI, DeadInsts);
     case TargetOpcode::G_ZEXT:
       return tryCombineZExt(MI, DeadInsts);
     case TargetOpcode::G_SEXT:
       return tryCombineSExt(MI, DeadInsts);
     case TargetOpcode::G_UNMERGE_VALUES:
       return tryCombineMerges(MI, DeadInsts);
     case TargetOpcode::G_EXTRACT:
       return tryCombineExtract(MI, DeadInsts);
     case TargetOpcode::G_TRUNC: {
       bool Changed = false;
       for (auto &Use : MRI.use_instructions(MI.getOperand(0).getReg()))
         Changed |= tryCombineInstruction(Use, DeadInsts);
       return Changed;
     }
     }
   }

 private:

   static unsigned getArtifactSrcReg(const MachineInstr &MI) {
     switch (MI.getOpcode()) {
     case TargetOpcode::COPY:
     case TargetOpcode::G_TRUNC:
     case TargetOpcode::G_ZEXT:
     case TargetOpcode::G_ANYEXT:
     case TargetOpcode::G_SEXT:
     case TargetOpcode::G_UNMERGE_VALUES:
       return MI.getOperand(MI.getNumOperands() - 1).getReg();
     case TargetOpcode::G_EXTRACT:
       return MI.getOperand(1).getReg();
     default:
       llvm_unreachable("Not a legalization artifact happen");
     }
   }

   /// Mark MI as dead. If a def of one of MI's operands, DefMI, would also be
   /// dead due to MI being killed, then mark DefMI as dead too.
   /// Some of the combines (extends(trunc)), try to walk through redundant
   /// copies in between the extends and the truncs, and this attempts to collect
   /// the in between copies if they're dead.
   void markInstAndDefDead(MachineInstr &MI, MachineInstr &DefMI,
                           SmallVectorImpl<MachineInstr *> &DeadInsts) {
     DeadInsts.push_back(&MI);

     // Collect all the copy instructions that are made dead, due to deleting
     // this instruction. Collect all of them until the Trunc(DefMI).
     // Eg,
     // %1(s1) = G_TRUNC %0(s32)
     // %2(s1) = COPY %1(s1)
     // %3(s1) = COPY %2(s1)
     // %4(s32) = G_ANYEXT %3(s1)
     // In this case, we would have replaced %4 with a copy of %0,
     // and as a result, %3, %2, %1 are dead.
     MachineInstr *PrevMI = &MI;
     while (PrevMI != &DefMI) {
       unsigned PrevRegSrc = getArtifactSrcReg(*PrevMI);

       MachineInstr *TmpDef = MRI.getVRegDef(PrevRegSrc);
       if (MRI.hasOneUse(PrevRegSrc)) {
         if (TmpDef != &DefMI) {
           assert(TmpDef->getOpcode() == TargetOpcode::COPY &&
                  "Expecting copy here");
           DeadInsts.push_back(TmpDef);
         }
       } else
         break;
       PrevMI = TmpDef;
     }
     if (PrevMI == &DefMI && MRI.hasOneUse(DefMI.getOperand(0).getReg()))
       DeadInsts.push_back(&DefMI);
   }

   /// Checks if the target legalizer info has specified anything about the
   /// instruction, or if unsupported.
   bool isInstUnsupported(const LegalityQuery &Query) const {
     using namespace LegalizeActions;
     auto Step = LI.getAction(Query);
     return Step.Action == Unsupported || Step.Action == NotFound;
   }

   bool isConstantUnsupported(LLT Ty) const {
     if (!Ty.isVector())
       return isInstUnsupported({TargetOpcode::G_CONSTANT, {Ty}});

     LLT EltTy = Ty.getElementType();
     return isInstUnsupported({TargetOpcode::G_CONSTANT, {EltTy}}) ||
            isInstUnsupported({TargetOpcode::G_BUILD_VECTOR, {Ty, EltTy}});
   }

   /// Looks through copy instructions and returns the actual
   /// source register.
   unsigned lookThroughCopyInstrs(unsigned Reg) {
     unsigned TmpReg;
     while (mi_match(Reg, MRI, m_Copy(m_Reg(TmpReg)))) {
       if (MRI.getType(TmpReg).isValid())
         Reg = TmpReg;
       else
         break;
     }
     return Reg;
   }
 };

 } // namespace llvm
	//===-- llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h ------ 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 some helper functions which try to cleanup artifacts
	// such as G_TRUNCs/G_[ZSA]EXTENDS that were created during legalization to make
	// the types match. This file also contains some combines of merges that happens
	// at the end of the legalization.
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/GlobalISel/Legalizer.h"
	#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
	#include "llvm/CodeGen/GlobalISel/MIPatternMatch.h"
	#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
	#include "llvm/CodeGen/GlobalISel/Utils.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/Support/Debug.h"

	#define DEBUG_TYPE "legalizer"
	using namespace llvm::MIPatternMatch;

	namespace llvm {
	class LegalizationArtifactCombiner {
	MachineIRBuilder &Builder;
	MachineRegisterInfo &MRI;
	const LegalizerInfo &LI;

	public:
	LegalizationArtifactCombiner(MachineIRBuilder &B, MachineRegisterInfo &MRI,
	const LegalizerInfo &LI)
	: Builder(B), MRI(MRI), LI(LI) {}

	bool tryCombineAnyExt(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {
	if (MI.getOpcode() != TargetOpcode::G_ANYEXT)
	return false;

	Builder.setInstr(MI);
	unsigned DstReg = MI.getOperand(0).getReg();
	unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

	// aext(trunc x) - > aext/copy/trunc x
	unsigned TruncSrc;
	if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
	LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
	Builder.buildAnyExtOrTrunc(DstReg, TruncSrc);
	markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
	return true;
	}

	// aext([asz]ext x) -> [asz]ext x
	unsigned ExtSrc;
	MachineInstr *ExtMI;
	if (mi_match(SrcReg, MRI,
	m_all_of(m_MInstr(ExtMI), m_any_of(m_GAnyExt(m_Reg(ExtSrc)),
	m_GSExt(m_Reg(ExtSrc)),
	m_GZExt(m_Reg(ExtSrc)))))) {
	Builder.buildInstr(ExtMI->getOpcode(), {DstReg}, {ExtSrc});
	markInstAndDefDead(MI, *ExtMI, DeadInsts);
	return true;
	}
	return tryFoldImplicitDef(MI, DeadInsts);
	}

	bool tryCombineZExt(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {

	if (MI.getOpcode() != TargetOpcode::G_ZEXT)
	return false;

	Builder.setInstr(MI);
	unsigned DstReg = MI.getOperand(0).getReg();
	unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

	// zext(trunc x) - > and (aext/copy/trunc x), mask
	unsigned TruncSrc;
	if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
	LLT DstTy = MRI.getType(DstReg);
	if (isInstUnsupported({TargetOpcode::G_AND, {DstTy}}) \|\|
	isConstantUnsupported(DstTy))
	return false;
	LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
	LLT SrcTy = MRI.getType(SrcReg);
	APInt Mask = APInt::getAllOnesValue(SrcTy.getScalarSizeInBits());
	auto MIBMask = Builder.buildConstant(DstTy, Mask.getZExtValue());
	Builder.buildAnd(DstReg, Builder.buildAnyExtOrTrunc(DstTy, TruncSrc),
	MIBMask);
	markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
	return true;
	}
	return tryFoldImplicitDef(MI, DeadInsts);
	}

	bool tryCombineSExt(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {

	if (MI.getOpcode() != TargetOpcode::G_SEXT)
	return false;

	Builder.setInstr(MI);
	unsigned DstReg = MI.getOperand(0).getReg();
	unsigned SrcReg = lookThroughCopyInstrs(MI.getOperand(1).getReg());

	// sext(trunc x) - > ashr (shl (aext/copy/trunc x), c), c
	unsigned TruncSrc;
	if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc)))) {
	LLT DstTy = MRI.getType(DstReg);
	// Guess on the RHS shift amount type, which should be re-legalized if
	// applicable.
	if (isInstUnsupported({TargetOpcode::G_SHL, {DstTy, DstTy}}) \|\|
	isInstUnsupported({TargetOpcode::G_ASHR, {DstTy, DstTy}}) \|\|
	isConstantUnsupported(DstTy))
	return false;
	LLVM_DEBUG(dbgs() << ".. Combine MI: " << MI;);
	LLT SrcTy = MRI.getType(SrcReg);
	unsigned ShAmt = DstTy.getScalarSizeInBits() - SrcTy.getScalarSizeInBits();
	auto MIBShAmt = Builder.buildConstant(DstTy, ShAmt);
	auto MIBShl = Builder.buildInstr(
	TargetOpcode::G_SHL, {DstTy},
	{Builder.buildAnyExtOrTrunc(DstTy, TruncSrc), MIBShAmt});
	Builder.buildInstr(TargetOpcode::G_ASHR, {DstReg}, {MIBShl, MIBShAmt});
	markInstAndDefDead(MI, *MRI.getVRegDef(SrcReg), DeadInsts);
	return true;
	}
	return tryFoldImplicitDef(MI, DeadInsts);
	}

	/// Try to fold G_[ASZ]EXT (G_IMPLICIT_DEF).
	bool tryFoldImplicitDef(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {
	unsigned Opcode = MI.getOpcode();
	if (Opcode != TargetOpcode::G_ANYEXT && Opcode != TargetOpcode::G_ZEXT &&
	Opcode != TargetOpcode::G_SEXT)
	return false;

	if (MachineInstr *DefMI = getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF,
	MI.getOperand(1).getReg(), MRI)) {
	Builder.setInstr(MI);
	unsigned DstReg = MI.getOperand(0).getReg();
	LLT DstTy = MRI.getType(DstReg);

	if (Opcode == TargetOpcode::G_ANYEXT) {
	// G_ANYEXT (G_IMPLICIT_DEF) -> G_IMPLICIT_DEF
	if (isInstUnsupported({TargetOpcode::G_IMPLICIT_DEF, {DstTy}}))
	return false;
	LLVM_DEBUG(dbgs() << ".. Combine G_ANYEXT(G_IMPLICIT_DEF): " << MI;);
	Builder.buildInstr(TargetOpcode::G_IMPLICIT_DEF, {DstReg}, {});
	} else {
	// G_[SZ]EXT (G_IMPLICIT_DEF) -> G_CONSTANT 0 because the top
	// bits will be 0 for G_ZEXT and 0/1 for the G_SEXT.
	if (isConstantUnsupported(DstTy))
	return false;
	LLVM_DEBUG(dbgs() << ".. Combine G_[SZ]EXT(G_IMPLICIT_DEF): " << MI;);
	Builder.buildConstant(DstReg, 0);
	}

	markInstAndDefDead(MI, *DefMI, DeadInsts);
	return true;
	}
	return false;
	}

	static unsigned getMergeOpcode(LLT OpTy, LLT DestTy) {
	if (OpTy.isVector() && DestTy.isVector())
	return TargetOpcode::G_CONCAT_VECTORS;

	if (OpTy.isVector() && !DestTy.isVector())
	return TargetOpcode::G_BUILD_VECTOR;

	return TargetOpcode::G_MERGE_VALUES;
	}

	bool tryCombineMerges(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {

	if (MI.getOpcode() != TargetOpcode::G_UNMERGE_VALUES)
	return false;

	unsigned NumDefs = MI.getNumOperands() - 1;

	LLT OpTy = MRI.getType(MI.getOperand(NumDefs).getReg());
	LLT DestTy = MRI.getType(MI.getOperand(0).getReg());

	unsigned MergingOpcode = getMergeOpcode(OpTy, DestTy);
	MachineInstr *MergeI =
	getOpcodeDef(MergingOpcode, MI.getOperand(NumDefs).getReg(), MRI);

	if (!MergeI)
	return false;

	const unsigned NumMergeRegs = MergeI->getNumOperands() - 1;

	if (NumMergeRegs < NumDefs) {
	if (NumDefs % NumMergeRegs != 0)
	return false;

	Builder.setInstr(MI);
	// Transform to UNMERGEs, for example
	// %1 = G_MERGE_VALUES %4, %5
	// %9, %10, %11, %12 = G_UNMERGE_VALUES %1
	// to
	// %9, %10 = G_UNMERGE_VALUES %4
	// %11, %12 = G_UNMERGE_VALUES %5

	const unsigned NewNumDefs = NumDefs / NumMergeRegs;
	for (unsigned Idx = 0; Idx < NumMergeRegs; ++Idx) {
	SmallVector<unsigned, 2> DstRegs;
	for (unsigned j = 0, DefIdx = Idx * NewNumDefs; j < NewNumDefs;
	++j, ++DefIdx)
	DstRegs.push_back(MI.getOperand(DefIdx).getReg());

	Builder.buildUnmerge(DstRegs, MergeI->getOperand(Idx + 1).getReg());
	}

	} else if (NumMergeRegs > NumDefs) {
	if (NumMergeRegs % NumDefs != 0)
	return false;

	Builder.setInstr(MI);
	// Transform to MERGEs
	// %6 = G_MERGE_VALUES %17, %18, %19, %20
	// %7, %8 = G_UNMERGE_VALUES %6
	// to
	// %7 = G_MERGE_VALUES %17, %18
	// %8 = G_MERGE_VALUES %19, %20

	const unsigned NumRegs = NumMergeRegs / NumDefs;
	for (unsigned DefIdx = 0; DefIdx < NumDefs; ++DefIdx) {
	SmallVector<unsigned, 2> Regs;
	for (unsigned j = 0, Idx = NumRegs * DefIdx + 1; j < NumRegs;
	++j, ++Idx)
	Regs.push_back(MergeI->getOperand(Idx).getReg());

	Builder.buildMerge(MI.getOperand(DefIdx).getReg(), Regs);
	}

	} else {
	// FIXME: is a COPY appropriate if the types mismatch? We know both
	// registers are allocatable by now.
	if (MRI.getType(MI.getOperand(0).getReg()) !=
	MRI.getType(MergeI->getOperand(1).getReg()))
	return false;

	for (unsigned Idx = 0; Idx < NumDefs; ++Idx)
	MRI.replaceRegWith(MI.getOperand(Idx).getReg(),
	MergeI->getOperand(Idx + 1).getReg());
	}

	markInstAndDefDead(MI, *MergeI, DeadInsts);
	return true;
	}

	static bool isMergeLikeOpcode(unsigned Opc) {
	switch (Opc) {
	case TargetOpcode::G_MERGE_VALUES:
	case TargetOpcode::G_BUILD_VECTOR:
	case TargetOpcode::G_CONCAT_VECTORS:
	return true;
	default:
	return false;
	}
	}

	bool tryCombineExtract(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {
	assert(MI.getOpcode() == TargetOpcode::G_EXTRACT);

	// Try to use the source registers from a G_MERGE_VALUES
	//
	// %2 = G_MERGE_VALUES %0, %1
	// %3 = G_EXTRACT %2, N
	// =>
	//
	// for N < %2.getSizeInBits() / 2
	// %3 = G_EXTRACT %0, N
	//
	// for N >= %2.getSizeInBits() / 2
	// %3 = G_EXTRACT %1, (N - %0.getSizeInBits()

	unsigned Src = lookThroughCopyInstrs(MI.getOperand(1).getReg());
	MachineInstr *MergeI = MRI.getVRegDef(Src);
	if (!MergeI \|\| !isMergeLikeOpcode(MergeI->getOpcode()))
	return false;

	LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
	LLT SrcTy = MRI.getType(Src);

	// TODO: Do we need to check if the resulting extract is supported?
	unsigned ExtractDstSize = DstTy.getSizeInBits();
	unsigned Offset = MI.getOperand(2).getImm();
	unsigned NumMergeSrcs = MergeI->getNumOperands() - 1;
	unsigned MergeSrcSize = SrcTy.getSizeInBits() / NumMergeSrcs;
	unsigned MergeSrcIdx = Offset / MergeSrcSize;

	// Compute the offset of the last bit the extract needs.
	unsigned EndMergeSrcIdx = (Offset + ExtractDstSize - 1) / MergeSrcSize;

	// Can't handle the case where the extract spans multiple inputs.
	if (MergeSrcIdx != EndMergeSrcIdx)
	return false;

	// TODO: We could modify MI in place in most cases.
	Builder.setInstr(MI);
	Builder.buildExtract(
	MI.getOperand(0).getReg(),
	MergeI->getOperand(MergeSrcIdx + 1).getReg(),
	Offset - MergeSrcIdx * MergeSrcSize);
	markInstAndDefDead(MI, *MergeI, DeadInsts);
	return true;
	}

	/// Try to combine away MI.
	/// Returns true if it combined away the MI.
	/// Adds instructions that are dead as a result of the combine
	/// into DeadInsts, which can include MI.
	bool tryCombineInstruction(MachineInstr &MI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {
	switch (MI.getOpcode()) {
	default:
	return false;
	case TargetOpcode::G_ANYEXT:
	return tryCombineAnyExt(MI, DeadInsts);
	case TargetOpcode::G_ZEXT:
	return tryCombineZExt(MI, DeadInsts);
	case TargetOpcode::G_SEXT:
	return tryCombineSExt(MI, DeadInsts);
	case TargetOpcode::G_UNMERGE_VALUES:
	return tryCombineMerges(MI, DeadInsts);
	case TargetOpcode::G_EXTRACT:
	return tryCombineExtract(MI, DeadInsts);
	case TargetOpcode::G_TRUNC: {
	bool Changed = false;
	for (auto &Use : MRI.use_instructions(MI.getOperand(0).getReg()))
	Changed \|= tryCombineInstruction(Use, DeadInsts);
	return Changed;
	}
	}
	}

	private:

	static unsigned getArtifactSrcReg(const MachineInstr &MI) {
	switch (MI.getOpcode()) {
	case TargetOpcode::COPY:
	case TargetOpcode::G_TRUNC:
	case TargetOpcode::G_ZEXT:
	case TargetOpcode::G_ANYEXT:
	case TargetOpcode::G_SEXT:
	case TargetOpcode::G_UNMERGE_VALUES:
	return MI.getOperand(MI.getNumOperands() - 1).getReg();
	case TargetOpcode::G_EXTRACT:
	return MI.getOperand(1).getReg();
	default:
	llvm_unreachable("Not a legalization artifact happen");
	}
	}

	/// Mark MI as dead. If a def of one of MI's operands, DefMI, would also be
	/// dead due to MI being killed, then mark DefMI as dead too.
	/// Some of the combines (extends(trunc)), try to walk through redundant
	/// copies in between the extends and the truncs, and this attempts to collect
	/// the in between copies if they're dead.
	void markInstAndDefDead(MachineInstr &MI, MachineInstr &DefMI,
	SmallVectorImpl<MachineInstr *> &DeadInsts) {
	DeadInsts.push_back(&MI);

	// Collect all the copy instructions that are made dead, due to deleting
	// this instruction. Collect all of them until the Trunc(DefMI).
	// Eg,
	// %1(s1) = G_TRUNC %0(s32)
	// %2(s1) = COPY %1(s1)
	// %3(s1) = COPY %2(s1)
	// %4(s32) = G_ANYEXT %3(s1)
	// In this case, we would have replaced %4 with a copy of %0,
	// and as a result, %3, %2, %1 are dead.
	MachineInstr *PrevMI = &MI;
	while (PrevMI != &DefMI) {
	unsigned PrevRegSrc = getArtifactSrcReg(*PrevMI);

	MachineInstr *TmpDef = MRI.getVRegDef(PrevRegSrc);
	if (MRI.hasOneUse(PrevRegSrc)) {
	if (TmpDef != &DefMI) {
	assert(TmpDef->getOpcode() == TargetOpcode::COPY &&
	"Expecting copy here");
	DeadInsts.push_back(TmpDef);
	}
	} else
	break;
	PrevMI = TmpDef;
	}
	if (PrevMI == &DefMI && MRI.hasOneUse(DefMI.getOperand(0).getReg()))
	DeadInsts.push_back(&DefMI);
	}

	/// Checks if the target legalizer info has specified anything about the
	/// instruction, or if unsupported.
	bool isInstUnsupported(const LegalityQuery &Query) const {
	using namespace LegalizeActions;
	auto Step = LI.getAction(Query);
	return Step.Action == Unsupported \|\| Step.Action == NotFound;
	}

	bool isConstantUnsupported(LLT Ty) const {
	if (!Ty.isVector())
	return isInstUnsupported({TargetOpcode::G_CONSTANT, {Ty}});

	LLT EltTy = Ty.getElementType();
	return isInstUnsupported({TargetOpcode::G_CONSTANT, {EltTy}}) \|\|
	isInstUnsupported({TargetOpcode::G_BUILD_VECTOR, {Ty, EltTy}});
	}

	/// Looks through copy instructions and returns the actual
	/// source register.
	unsigned lookThroughCopyInstrs(unsigned Reg) {
	unsigned TmpReg;
	while (mi_match(Reg, MRI, m_Copy(m_Reg(TmpReg)))) {
	if (MRI.getType(TmpReg).isValid())
	Reg = TmpReg;
	else
	break;
	}
	return Reg;
	}
	};

	} // namespace llvm