lib/Transforms/Scalar/DivRemPairs.cpp - llvm - Git at Google

 //===- DivRemPairs.cpp - Hoist/[dr]ecompose division and remainder --------===//
 //
 // 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 pass hoists and/or decomposes/recomposes integer division and remainder
 // instructions to enable CFG improvements and better codegen.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/Scalar/DivRemPairs.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/DebugCounter.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/BypassSlowDivision.h"

 using namespace llvm;
 using namespace llvm::PatternMatch;

 #define DEBUG_TYPE "div-rem-pairs"
 STATISTIC(NumPairs, "Number of div/rem pairs");
 STATISTIC(NumRecomposed, "Number of instructions recomposed");
 STATISTIC(NumHoisted, "Number of instructions hoisted");
 STATISTIC(NumDecomposed, "Number of instructions decomposed");
 DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform",
               "Controls transformations in div-rem-pairs pass");

 namespace {
 struct ExpandedMatch {
   DivRemMapKey Key;
   Instruction *Value;
 };
 } // namespace

 /// See if we can match: (which is the form we expand into)
 ///   X - ((X ?/ Y) * Y)
 /// which is equivalent to:
 ///   X ?% Y
 static llvm::Optional<ExpandedMatch> matchExpandedRem(Instruction &I) {
   Value *Dividend, *XroundedDownToMultipleOfY;
   if (!match(&I, m_Sub(m_Value(Dividend), m_Value(XroundedDownToMultipleOfY))))
     return llvm::None;

   Value *Divisor;
   Instruction *Div;
   // Look for  ((X / Y) * Y)
   if (!match(
           XroundedDownToMultipleOfY,
           m_c_Mul(m_CombineAnd(m_IDiv(m_Specific(Dividend), m_Value(Divisor)),
                                m_Instruction(Div)),
                   m_Deferred(Divisor))))
     return llvm::None;

   ExpandedMatch M;
   M.Key.SignedOp = Div->getOpcode() == Instruction::SDiv;
   M.Key.Dividend = Dividend;
   M.Key.Divisor = Divisor;
   M.Value = &I;
   return M;
 }

 /// A thin wrapper to store two values that we matched as div-rem pair.
 /// We want this extra indirection to avoid dealing with RAUW'ing the map keys.
 struct DivRemPairWorklistEntry {
   /// The actual udiv/sdiv instruction. Source of truth.
   AssertingVH<Instruction> DivInst;

   /// The instruction that we have matched as a remainder instruction.
   /// Should only be used as Value, don't introspect it.
   AssertingVH<Instruction> RemInst;

   DivRemPairWorklistEntry(Instruction *DivInst_, Instruction *RemInst_)
       : DivInst(DivInst_), RemInst(RemInst_) {
     assert((DivInst->getOpcode() == Instruction::UDiv ||
             DivInst->getOpcode() == Instruction::SDiv) &&
            "Not a division.");
     assert(DivInst->getType() == RemInst->getType() && "Types should match.");
     // We can't check anything else about remainder instruction,
     // it's not strictly required to be a urem/srem.
   }

   /// The type for this pair, identical for both the div and rem.
   Type *getType() const { return DivInst->getType(); }

   /// Is this pair signed or unsigned?
   bool isSigned() const { return DivInst->getOpcode() == Instruction::SDiv; }

   /// In this pair, what are the divident and divisor?
   Value *getDividend() const { return DivInst->getOperand(0); }
   Value *getDivisor() const { return DivInst->getOperand(1); }

   bool isRemExpanded() const {
     switch (RemInst->getOpcode()) {
     case Instruction::SRem:
     case Instruction::URem:
       return false; // single 'rem' instruction - unexpanded form.
     default:
       return true; // anything else means we have remainder in expanded form.
     }
   }
 };
 using DivRemWorklistTy = SmallVector<DivRemPairWorklistEntry, 4>;

 /// Find matching pairs of integer div/rem ops (they have the same numerator,
 /// denominator, and signedness). Place those pairs into a worklist for further
 /// processing. This indirection is needed because we have to use TrackingVH<>
 /// because we will be doing RAUW, and if one of the rem instructions we change
 /// happens to be an input to another div/rem in the maps, we'd have problems.
 static DivRemWorklistTy getWorklist(Function &F) {
   // Insert all divide and remainder instructions into maps keyed by their
   // operands and opcode (signed or unsigned).
   DenseMap<DivRemMapKey, Instruction *> DivMap;
   // Use a MapVector for RemMap so that instructions are moved/inserted in a
   // deterministic order.
   MapVector<DivRemMapKey, Instruction *> RemMap;
   for (auto &BB : F) {
     for (auto &I : BB) {
       if (I.getOpcode() == Instruction::SDiv)
         DivMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
       else if (I.getOpcode() == Instruction::UDiv)
         DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
       else if (I.getOpcode() == Instruction::SRem)
         RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
       else if (I.getOpcode() == Instruction::URem)
         RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
       else if (auto Match = matchExpandedRem(I))
         RemMap[Match->Key] = Match->Value;
     }
   }

   // We'll accumulate the matching pairs of div-rem instructions here.
   DivRemWorklistTy Worklist;

   // We can iterate over either map because we are only looking for matched
   // pairs. Choose remainders for efficiency because they are usually even more
   // rare than division.
   for (auto &RemPair : RemMap) {
     // Find the matching division instruction from the division map.
     Instruction *DivInst = DivMap[RemPair.first];
     if (!DivInst)
       continue;

     // We have a matching pair of div/rem instructions.
     NumPairs++;
     Instruction *RemInst = RemPair.second;

     // Place it in the worklist.
     Worklist.emplace_back(DivInst, RemInst);
   }

   return Worklist;
 }

 /// Find matching pairs of integer div/rem ops (they have the same numerator,
 /// denominator, and signedness). If they exist in different basic blocks, bring
 /// them together by hoisting or replace the common division operation that is
 /// implicit in the remainder:
 /// X % Y <--> X - ((X / Y) * Y).
 ///
 /// We can largely ignore the normal safety and cost constraints on speculation
 /// of these ops when we find a matching pair. This is because we are already
 /// guaranteed that any exceptions and most cost are already incurred by the
 /// first member of the pair.
 ///
 /// Note: This transform could be an oddball enhancement to EarlyCSE, GVN, or
 /// SimplifyCFG, but it's split off on its own because it's different enough
 /// that it doesn't quite match the stated objectives of those passes.
 static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
                            const DominatorTree &DT) {
   bool Changed = false;

   // Get the matching pairs of div-rem instructions. We want this extra
   // indirection to avoid dealing with having to RAUW the keys of the maps.
   DivRemWorklistTy Worklist = getWorklist(F);

   // Process each entry in the worklist.
   for (DivRemPairWorklistEntry &E : Worklist) {
     if (!DebugCounter::shouldExecute(DRPCounter))
       continue;

     bool HasDivRemOp = TTI.hasDivRemOp(E.getType(), E.isSigned());

     auto &DivInst = E.DivInst;
     auto &RemInst = E.RemInst;

     const bool RemOriginallyWasInExpandedForm = E.isRemExpanded();
     (void)RemOriginallyWasInExpandedForm; // suppress unused variable warning

     if (HasDivRemOp && E.isRemExpanded()) {
       // The target supports div+rem but the rem is expanded.
       // We should recompose it first.
       Value *X = E.getDividend();
       Value *Y = E.getDivisor();
       Instruction *RealRem = E.isSigned() ? BinaryOperator::CreateSRem(X, Y)
                                           : BinaryOperator::CreateURem(X, Y);
       // Note that we place it right next to the original expanded instruction,
       // and letting further handling to move it if needed.
       RealRem->setName(RemInst->getName() + ".recomposed");
       RealRem->insertAfter(RemInst);
       Instruction *OrigRemInst = RemInst;
       // Update AssertingVH<> with new instruction so it doesn't assert.
       RemInst = RealRem;
       // And replace the original instruction with the new one.
       OrigRemInst->replaceAllUsesWith(RealRem);
       OrigRemInst->eraseFromParent();
       NumRecomposed++;
       // Note that we have left ((X / Y) * Y) around.
       // If it had other uses we could rewrite it as X - X % Y
     }

     assert((!E.isRemExpanded() || !HasDivRemOp) &&
            "*If* the target supports div-rem, then by now the RemInst *is* "
            "Instruction::[US]Rem.");

     // If the target supports div+rem and the instructions are in the same block
     // already, there's nothing to do. The backend should handle this. If the
     // target does not support div+rem, then we will decompose the rem.
     if (HasDivRemOp && RemInst->getParent() == DivInst->getParent())
       continue;

     bool DivDominates = DT.dominates(DivInst, RemInst);
     if (!DivDominates && !DT.dominates(RemInst, DivInst)) {
       // We have matching div-rem pair, but they are in two different blocks,
       // neither of which dominates one another.
       // FIXME: We could hoist both ops to the common predecessor block?
       continue;
     }

     // The target does not have a single div/rem operation,
     // and the rem is already in expanded form. Nothing to do.
     if (!HasDivRemOp && E.isRemExpanded())
       continue;

     if (HasDivRemOp) {
       // The target has a single div/rem operation. Hoist the lower instruction
       // to make the matched pair visible to the backend.
       if (DivDominates)
         RemInst->moveAfter(DivInst);
       else
         DivInst->moveAfter(RemInst);
       NumHoisted++;
     } else {
       // The target does not have a single div/rem operation,
       // and the rem is *not* in a already-expanded form.
       // Decompose the remainder calculation as:
       // X % Y --> X - ((X / Y) * Y).

       assert(!RemOriginallyWasInExpandedForm &&
              "We should not be expanding if the rem was in expanded form to "
              "begin with.");

       Value *X = E.getDividend();
       Value *Y = E.getDivisor();
       Instruction *Mul = BinaryOperator::CreateMul(DivInst, Y);
       Instruction *Sub = BinaryOperator::CreateSub(X, Mul);

       // If the remainder dominates, then hoist the division up to that block:
       //
       // bb1:
       //   %rem = srem %x, %y
       // bb2:
       //   %div = sdiv %x, %y
       // -->
       // bb1:
       //   %div = sdiv %x, %y
       //   %mul = mul %div, %y
       //   %rem = sub %x, %mul
       //
       // If the division dominates, it's already in the right place. The mul+sub
       // will be in a different block because we don't assume that they are
       // cheap to speculatively execute:
       //
       // bb1:
       //   %div = sdiv %x, %y
       // bb2:
       //   %rem = srem %x, %y
       // -->
       // bb1:
       //   %div = sdiv %x, %y
       // bb2:
       //   %mul = mul %div, %y
       //   %rem = sub %x, %mul
       //
       // If the div and rem are in the same block, we do the same transform,
       // but any code movement would be within the same block.

       if (!DivDominates)
         DivInst->moveBefore(RemInst);
       Mul->insertAfter(RemInst);
       Sub->insertAfter(Mul);

       // Now kill the explicit remainder. We have replaced it with:
       // (sub X, (mul (div X, Y), Y)
       Sub->setName(RemInst->getName() + ".decomposed");
       Instruction *OrigRemInst = RemInst;
       // Update AssertingVH<> with new instruction so it doesn't assert.
       RemInst = Sub;
       // And replace the original instruction with the new one.
       OrigRemInst->replaceAllUsesWith(Sub);
       OrigRemInst->eraseFromParent();
       NumDecomposed++;
     }
     Changed = true;
   }

   return Changed;
 }

 // Pass manager boilerplate below here.

 namespace {
 struct DivRemPairsLegacyPass : public FunctionPass {
   static char ID;
   DivRemPairsLegacyPass() : FunctionPass(ID) {
     initializeDivRemPairsLegacyPassPass(*PassRegistry::getPassRegistry());
   }

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
     AU.setPreservesCFG();
     AU.addPreserved<DominatorTreeWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
     FunctionPass::getAnalysisUsage(AU);
   }

   bool runOnFunction(Function &F) override {
     if (skipFunction(F))
       return false;
     auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     return optimizeDivRem(F, TTI, DT);
   }
 };
 } // namespace

 char DivRemPairsLegacyPass::ID = 0;
 INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs",
                       "Hoist/decompose integer division and remainder", false,
                       false)
 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_END(DivRemPairsLegacyPass, "div-rem-pairs",
                     "Hoist/decompose integer division and remainder", false,
                     false)
 FunctionPass *llvm::createDivRemPairsPass() {
   return new DivRemPairsLegacyPass();
 }

 PreservedAnalyses DivRemPairsPass::run(Function &F,
                                        FunctionAnalysisManager &FAM) {
   TargetTransformInfo &TTI = FAM.getResult<TargetIRAnalysis>(F);
   DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
   if (!optimizeDivRem(F, TTI, DT))
     return PreservedAnalyses::all();
   // TODO: This pass just hoists/replaces math ops - all analyses are preserved?
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   PA.preserve<GlobalsAA>();
   return PA;
 }
	//===- DivRemPairs.cpp - Hoist/[dr]ecompose division and remainder --------===//
	//
	// 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 pass hoists and/or decomposes/recomposes integer division and remainder
	// instructions to enable CFG improvements and better codegen.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/Scalar/DivRemPairs.h"
	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/MapVector.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/GlobalsModRef.h"
	#include "llvm/Analysis/TargetTransformInfo.h"
	#include "llvm/IR/Dominators.h"
	#include "llvm/IR/Function.h"
	#include "llvm/IR/PatternMatch.h"
	#include "llvm/Pass.h"
	#include "llvm/Support/DebugCounter.h"
	#include "llvm/Transforms/Scalar.h"
	#include "llvm/Transforms/Utils/BypassSlowDivision.h"

	using namespace llvm;
	using namespace llvm::PatternMatch;

	#define DEBUG_TYPE "div-rem-pairs"
	STATISTIC(NumPairs, "Number of div/rem pairs");
	STATISTIC(NumRecomposed, "Number of instructions recomposed");
	STATISTIC(NumHoisted, "Number of instructions hoisted");
	STATISTIC(NumDecomposed, "Number of instructions decomposed");
	DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform",
	"Controls transformations in div-rem-pairs pass");

	namespace {
	struct ExpandedMatch {
	DivRemMapKey Key;
	Instruction *Value;
	};
	} // namespace

	/// See if we can match: (which is the form we expand into)
	/// X - ((X ?/ Y) * Y)
	/// which is equivalent to:
	/// X ?% Y
	static llvm::Optional<ExpandedMatch> matchExpandedRem(Instruction &I) {
	Value Dividend, XroundedDownToMultipleOfY;
	if (!match(&I, m_Sub(m_Value(Dividend), m_Value(XroundedDownToMultipleOfY))))
	return llvm::None;

	Value *Divisor;
	Instruction *Div;
	// Look for ((X / Y) * Y)
	if (!match(
	XroundedDownToMultipleOfY,
	m_c_Mul(m_CombineAnd(m_IDiv(m_Specific(Dividend), m_Value(Divisor)),
	m_Instruction(Div)),
	m_Deferred(Divisor))))
	return llvm::None;

	ExpandedMatch M;
	M.Key.SignedOp = Div->getOpcode() == Instruction::SDiv;
	M.Key.Dividend = Dividend;
	M.Key.Divisor = Divisor;
	M.Value = &I;
	return M;
	}

	/// A thin wrapper to store two values that we matched as div-rem pair.
	/// We want this extra indirection to avoid dealing with RAUW'ing the map keys.
	struct DivRemPairWorklistEntry {
	/// The actual udiv/sdiv instruction. Source of truth.
	AssertingVH<Instruction> DivInst;

	/// The instruction that we have matched as a remainder instruction.
	/// Should only be used as Value, don't introspect it.
	AssertingVH<Instruction> RemInst;

	DivRemPairWorklistEntry(Instruction DivInst_, Instruction RemInst_)
	: DivInst(DivInst_), RemInst(RemInst_) {
	assert((DivInst->getOpcode() == Instruction::UDiv \|\|
	DivInst->getOpcode() == Instruction::SDiv) &&
	"Not a division.");
	assert(DivInst->getType() == RemInst->getType() && "Types should match.");
	// We can't check anything else about remainder instruction,
	// it's not strictly required to be a urem/srem.
	}

	/// The type for this pair, identical for both the div and rem.
	Type *getType() const { return DivInst->getType(); }

	/// Is this pair signed or unsigned?
	bool isSigned() const { return DivInst->getOpcode() == Instruction::SDiv; }

	/// In this pair, what are the divident and divisor?
	Value *getDividend() const { return DivInst->getOperand(0); }
	Value *getDivisor() const { return DivInst->getOperand(1); }

	bool isRemExpanded() const {
	switch (RemInst->getOpcode()) {
	case Instruction::SRem:
	case Instruction::URem:
	return false; // single 'rem' instruction - unexpanded form.
	default:
	return true; // anything else means we have remainder in expanded form.
	}
	}
	};
	using DivRemWorklistTy = SmallVector<DivRemPairWorklistEntry, 4>;

	/// Find matching pairs of integer div/rem ops (they have the same numerator,
	/// denominator, and signedness). Place those pairs into a worklist for further
	/// processing. This indirection is needed because we have to use TrackingVH<>
	/// because we will be doing RAUW, and if one of the rem instructions we change
	/// happens to be an input to another div/rem in the maps, we'd have problems.
	static DivRemWorklistTy getWorklist(Function &F) {
	// Insert all divide and remainder instructions into maps keyed by their
	// operands and opcode (signed or unsigned).
	DenseMap<DivRemMapKey, Instruction *> DivMap;
	// Use a MapVector for RemMap so that instructions are moved/inserted in a
	// deterministic order.
	MapVector<DivRemMapKey, Instruction *> RemMap;
	for (auto &BB : F) {
	for (auto &I : BB) {
	if (I.getOpcode() == Instruction::SDiv)
	DivMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
	else if (I.getOpcode() == Instruction::UDiv)
	DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
	else if (I.getOpcode() == Instruction::SRem)
	RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
	else if (I.getOpcode() == Instruction::URem)
	RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
	else if (auto Match = matchExpandedRem(I))
	RemMap[Match->Key] = Match->Value;
	}
	}

	// We'll accumulate the matching pairs of div-rem instructions here.
	DivRemWorklistTy Worklist;

	// We can iterate over either map because we are only looking for matched
	// pairs. Choose remainders for efficiency because they are usually even more
	// rare than division.
	for (auto &RemPair : RemMap) {
	// Find the matching division instruction from the division map.
	Instruction *DivInst = DivMap[RemPair.first];
	if (!DivInst)
	continue;

	// We have a matching pair of div/rem instructions.
	NumPairs++;
	Instruction *RemInst = RemPair.second;

	// Place it in the worklist.
	Worklist.emplace_back(DivInst, RemInst);
	}

	return Worklist;
	}

	/// Find matching pairs of integer div/rem ops (they have the same numerator,
	/// denominator, and signedness). If they exist in different basic blocks, bring
	/// them together by hoisting or replace the common division operation that is
	/// implicit in the remainder:
	/// X % Y <--> X - ((X / Y) * Y).
	///
	/// We can largely ignore the normal safety and cost constraints on speculation
	/// of these ops when we find a matching pair. This is because we are already
	/// guaranteed that any exceptions and most cost are already incurred by the
	/// first member of the pair.
	///
	/// Note: This transform could be an oddball enhancement to EarlyCSE, GVN, or
	/// SimplifyCFG, but it's split off on its own because it's different enough
	/// that it doesn't quite match the stated objectives of those passes.
	static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
	const DominatorTree &DT) {
	bool Changed = false;

	// Get the matching pairs of div-rem instructions. We want this extra
	// indirection to avoid dealing with having to RAUW the keys of the maps.
	DivRemWorklistTy Worklist = getWorklist(F);

	// Process each entry in the worklist.
	for (DivRemPairWorklistEntry &E : Worklist) {
	if (!DebugCounter::shouldExecute(DRPCounter))
	continue;

	bool HasDivRemOp = TTI.hasDivRemOp(E.getType(), E.isSigned());

	auto &DivInst = E.DivInst;
	auto &RemInst = E.RemInst;

	const bool RemOriginallyWasInExpandedForm = E.isRemExpanded();
	(void)RemOriginallyWasInExpandedForm; // suppress unused variable warning

	if (HasDivRemOp && E.isRemExpanded()) {
	// The target supports div+rem but the rem is expanded.
	// We should recompose it first.
	Value *X = E.getDividend();
	Value *Y = E.getDivisor();
	Instruction *RealRem = E.isSigned() ? BinaryOperator::CreateSRem(X, Y)
	: BinaryOperator::CreateURem(X, Y);
	// Note that we place it right next to the original expanded instruction,
	// and letting further handling to move it if needed.
	RealRem->setName(RemInst->getName() + ".recomposed");
	RealRem->insertAfter(RemInst);
	Instruction *OrigRemInst = RemInst;
	// Update AssertingVH<> with new instruction so it doesn't assert.
	RemInst = RealRem;
	// And replace the original instruction with the new one.
	OrigRemInst->replaceAllUsesWith(RealRem);
	OrigRemInst->eraseFromParent();
	NumRecomposed++;
	// Note that we have left ((X / Y) * Y) around.
	// If it had other uses we could rewrite it as X - X % Y
	}

	assert((!E.isRemExpanded() \|\| !HasDivRemOp) &&
	"If the target supports div-rem, then by now the RemInst is "
	"Instruction::[US]Rem.");

	// If the target supports div+rem and the instructions are in the same block
	// already, there's nothing to do. The backend should handle this. If the
	// target does not support div+rem, then we will decompose the rem.
	if (HasDivRemOp && RemInst->getParent() == DivInst->getParent())
	continue;

	bool DivDominates = DT.dominates(DivInst, RemInst);
	if (!DivDominates && !DT.dominates(RemInst, DivInst)) {
	// We have matching div-rem pair, but they are in two different blocks,
	// neither of which dominates one another.
	// FIXME: We could hoist both ops to the common predecessor block?
	continue;
	}

	// The target does not have a single div/rem operation,
	// and the rem is already in expanded form. Nothing to do.
	if (!HasDivRemOp && E.isRemExpanded())
	continue;

	if (HasDivRemOp) {
	// The target has a single div/rem operation. Hoist the lower instruction
	// to make the matched pair visible to the backend.
	if (DivDominates)
	RemInst->moveAfter(DivInst);
	else
	DivInst->moveAfter(RemInst);
	NumHoisted++;
	} else {
	// The target does not have a single div/rem operation,
	// and the rem is not in a already-expanded form.
	// Decompose the remainder calculation as:
	// X % Y --> X - ((X / Y) * Y).

	assert(!RemOriginallyWasInExpandedForm &&
	"We should not be expanding if the rem was in expanded form to "
	"begin with.");

	Value *X = E.getDividend();
	Value *Y = E.getDivisor();
	Instruction *Mul = BinaryOperator::CreateMul(DivInst, Y);
	Instruction *Sub = BinaryOperator::CreateSub(X, Mul);

	// If the remainder dominates, then hoist the division up to that block:
	//
	// bb1:
	// %rem = srem %x, %y
	// bb2:
	// %div = sdiv %x, %y
	// -->
	// bb1:
	// %div = sdiv %x, %y
	// %mul = mul %div, %y
	// %rem = sub %x, %mul
	//
	// If the division dominates, it's already in the right place. The mul+sub
	// will be in a different block because we don't assume that they are
	// cheap to speculatively execute:
	//
	// bb1:
	// %div = sdiv %x, %y
	// bb2:
	// %rem = srem %x, %y
	// -->
	// bb1:
	// %div = sdiv %x, %y
	// bb2:
	// %mul = mul %div, %y
	// %rem = sub %x, %mul
	//
	// If the div and rem are in the same block, we do the same transform,
	// but any code movement would be within the same block.

	if (!DivDominates)
	DivInst->moveBefore(RemInst);
	Mul->insertAfter(RemInst);
	Sub->insertAfter(Mul);

	// Now kill the explicit remainder. We have replaced it with:
	// (sub X, (mul (div X, Y), Y)
	Sub->setName(RemInst->getName() + ".decomposed");
	Instruction *OrigRemInst = RemInst;
	// Update AssertingVH<> with new instruction so it doesn't assert.
	RemInst = Sub;
	// And replace the original instruction with the new one.
	OrigRemInst->replaceAllUsesWith(Sub);
	OrigRemInst->eraseFromParent();
	NumDecomposed++;
	}
	Changed = true;
	}

	return Changed;
	}

	// Pass manager boilerplate below here.

	namespace {
	struct DivRemPairsLegacyPass : public FunctionPass {
	static char ID;
	DivRemPairsLegacyPass() : FunctionPass(ID) {
	initializeDivRemPairsLegacyPassPass(*PassRegistry::getPassRegistry());
	}

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.addRequired<DominatorTreeWrapperPass>();
	AU.addRequired<TargetTransformInfoWrapperPass>();
	AU.setPreservesCFG();
	AU.addPreserved<DominatorTreeWrapperPass>();
	AU.addPreserved<GlobalsAAWrapperPass>();
	FunctionPass::getAnalysisUsage(AU);
	}

	bool runOnFunction(Function &F) override {
	if (skipFunction(F))
	return false;
	auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
	auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
	return optimizeDivRem(F, TTI, DT);
	}
	};
	} // namespace

	char DivRemPairsLegacyPass::ID = 0;
	INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs",
	"Hoist/decompose integer division and remainder", false,
	false)
	INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
	INITIALIZE_PASS_END(DivRemPairsLegacyPass, "div-rem-pairs",
	"Hoist/decompose integer division and remainder", false,
	false)
	FunctionPass *llvm::createDivRemPairsPass() {
	return new DivRemPairsLegacyPass();
	}

	PreservedAnalyses DivRemPairsPass::run(Function &F,
	FunctionAnalysisManager &FAM) {
	TargetTransformInfo &TTI = FAM.getResult<TargetIRAnalysis>(F);
	DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
	if (!optimizeDivRem(F, TTI, DT))
	return PreservedAnalyses::all();
	// TODO: This pass just hoists/replaces math ops - all analyses are preserved?
	PreservedAnalyses PA;
	PA.preserveSet<CFGAnalyses>();
	PA.preserve<GlobalsAA>();
	return PA;
	}