lib/Target/ARM/MVETailPredication.cpp - llvm-project/llvm - Git at Google

 //===- MVETailPredication.cpp - MVE Tail Predication ------------*- 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
 //
 //===----------------------------------------------------------------------===//
 //
 /// \file
 /// Armv8.1m introduced MVE, M-Profile Vector Extension, and low-overhead
 /// branches to help accelerate DSP applications. These two extensions,
 /// combined with a new form of predication called tail-predication, can be used
 /// to provide implicit vector predication within a low-overhead loop.
 /// This is implicit because the predicate of active/inactive lanes is
 /// calculated by hardware, and thus does not need to be explicitly passed
 /// to vector instructions. The instructions responsible for this are the
 /// DLSTP and WLSTP instructions, which setup a tail-predicated loop and the
 /// the total number of data elements processed by the loop. The loop-end
 /// LETP instruction is responsible for decrementing and setting the remaining
 /// elements to be processed and generating the mask of active lanes.
 ///
 /// The HardwareLoops pass inserts intrinsics identifying loops that the
 /// backend will attempt to convert into a low-overhead loop. The vectorizer is
 /// responsible for generating a vectorized loop in which the lanes are
 /// predicated upon an get.active.lane.mask intrinsic. This pass looks at these
 /// get.active.lane.mask intrinsic and attempts to convert them to VCTP
 /// instructions. This will be picked up by the ARM Low-overhead loop pass later
 /// in the backend, which performs the final transformation to a DLSTP or WLSTP
 /// tail-predicated loop.
 //
 //===----------------------------------------------------------------------===//

 #include "ARM.h"
 #include "ARMSubtarget.h"
 #include "ARMTargetTransformInfo.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicsARM.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"

 using namespace llvm;

 #define DEBUG_TYPE "mve-tail-predication"
 #define DESC "Transform predicated vector loops to use MVE tail predication"

 cl::opt<TailPredication::Mode> EnableTailPredication(
    "tail-predication", cl::desc("MVE tail-predication pass options"),
    cl::init(TailPredication::Enabled),
    cl::values(clEnumValN(TailPredication::Disabled, "disabled",
                          "Don't tail-predicate loops"),
               clEnumValN(TailPredication::EnabledNoReductions,
                          "enabled-no-reductions",
                          "Enable tail-predication, but not for reduction loops"),
               clEnumValN(TailPredication::Enabled,
                          "enabled",
                          "Enable tail-predication, including reduction loops"),
               clEnumValN(TailPredication::ForceEnabledNoReductions,
                          "force-enabled-no-reductions",
                          "Enable tail-predication, but not for reduction loops, "
                          "and force this which might be unsafe"),
               clEnumValN(TailPredication::ForceEnabled,
                          "force-enabled",
                          "Enable tail-predication, including reduction loops, "
                          "and force this which might be unsafe")));


 namespace {

 class MVETailPredication : public LoopPass {
   SmallVector<IntrinsicInst*, 4> MaskedInsts;
   Loop *L = nullptr;
   ScalarEvolution *SE = nullptr;
   TargetTransformInfo *TTI = nullptr;
   const ARMSubtarget *ST = nullptr;

 public:
   static char ID;

   MVETailPredication() : LoopPass(ID) { }

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<ScalarEvolutionWrapperPass>();
     AU.addRequired<LoopInfoWrapperPass>();
     AU.addRequired<TargetPassConfig>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
     AU.addPreserved<LoopInfoWrapperPass>();
     AU.setPreservesCFG();
   }

   bool runOnLoop(Loop *L, LPPassManager&) override;

 private:
   /// Perform the relevant checks on the loop and convert active lane masks if
   /// possible.
   bool TryConvertActiveLaneMask(Value *TripCount);

   /// Perform several checks on the arguments of @llvm.get.active.lane.mask
   /// intrinsic. E.g., check that the loop induction variable and the element
   /// count are of the form we expect, and also perform overflow checks for
   /// the new expressions that are created.
   bool IsSafeActiveMask(IntrinsicInst *ActiveLaneMask, Value *TripCount);

   /// Insert the intrinsic to represent the effect of tail predication.
   void InsertVCTPIntrinsic(IntrinsicInst *ActiveLaneMask, Value *TripCount);

   /// Rematerialize the iteration count in exit blocks, which enables
   /// ARMLowOverheadLoops to better optimise away loop update statements inside
   /// hardware-loops.
   void RematerializeIterCount();
 };

 } // end namespace

 bool MVETailPredication::runOnLoop(Loop *L, LPPassManager&) {
   if (skipLoop(L) || !EnableTailPredication)
     return false;

   MaskedInsts.clear();
   Function &F = *L->getHeader()->getParent();
   auto &TPC = getAnalysis<TargetPassConfig>();
   auto &TM = TPC.getTM<TargetMachine>();
   ST = &TM.getSubtarget<ARMSubtarget>(F);
   TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
   SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
   this->L = L;

   // The MVE and LOB extensions are combined to enable tail-predication, but
   // there's nothing preventing us from generating VCTP instructions for v8.1m.
   if (!ST->hasMVEIntegerOps() || !ST->hasV8_1MMainlineOps()) {
     LLVM_DEBUG(dbgs() << "ARM TP: Not a v8.1m.main+mve target.\n");
     return false;
   }

   BasicBlock *Preheader = L->getLoopPreheader();
   if (!Preheader)
     return false;

   auto FindLoopIterations = [](BasicBlock *BB) -> IntrinsicInst* {
     for (auto &I : *BB) {
       auto *Call = dyn_cast<IntrinsicInst>(&I);
       if (!Call)
         continue;

       Intrinsic::ID ID = Call->getIntrinsicID();
       if (ID == Intrinsic::start_loop_iterations ||
           ID == Intrinsic::test_start_loop_iterations)
         return cast<IntrinsicInst>(&I);
     }
     return nullptr;
   };

   // Look for the hardware loop intrinsic that sets the iteration count.
   IntrinsicInst *Setup = FindLoopIterations(Preheader);

   // The test.set iteration could live in the pre-preheader.
   if (!Setup) {
     if (!Preheader->getSinglePredecessor())
       return false;
     Setup = FindLoopIterations(Preheader->getSinglePredecessor());
     if (!Setup)
       return false;
   }

   LLVM_DEBUG(dbgs() << "ARM TP: Running on Loop: " << *L << *Setup << "\n");

   bool Changed = TryConvertActiveLaneMask(Setup->getArgOperand(0));

   return Changed;
 }

 // The active lane intrinsic has this form:
 //
 //    @llvm.get.active.lane.mask(IV, TC)
 //
 // Here we perform checks that this intrinsic behaves as expected,
 // which means:
 //
 // 1) Check that the TripCount (TC) belongs to this loop (originally).
 // 2) The element count (TC) needs to be sufficiently large that the decrement
 //    of element counter doesn't overflow, which means that we need to prove:
 //        ceil(ElementCount / VectorWidth) >= TripCount
 //    by rounding up ElementCount up:
 //        ((ElementCount + (VectorWidth - 1)) / VectorWidth
 //    and evaluate if expression isKnownNonNegative:
 //        (((ElementCount + (VectorWidth - 1)) / VectorWidth) - TripCount
 // 3) The IV must be an induction phi with an increment equal to the
 //    vector width.
 bool MVETailPredication::IsSafeActiveMask(IntrinsicInst *ActiveLaneMask,
                                           Value *TripCount) {
   bool ForceTailPredication =
     EnableTailPredication == TailPredication::ForceEnabledNoReductions ||
     EnableTailPredication == TailPredication::ForceEnabled;

   Value *ElemCount = ActiveLaneMask->getOperand(1);
   auto *EC= SE->getSCEV(ElemCount);
   auto *TC = SE->getSCEV(TripCount);
   int VectorWidth =
       cast<FixedVectorType>(ActiveLaneMask->getType())->getNumElements();
   if (VectorWidth != 4 && VectorWidth != 8 && VectorWidth != 16)
     return false;
   ConstantInt *ConstElemCount = nullptr;

   // 1) Smoke tests that the original scalar loop TripCount (TC) belongs to
   // this loop.  The scalar tripcount corresponds the number of elements
   // processed by the loop, so we will refer to that from this point on.
   if (!SE->isLoopInvariant(EC, L)) {
     LLVM_DEBUG(dbgs() << "ARM TP: element count must be loop invariant.\n");
     return false;
   }

   if ((ConstElemCount = dyn_cast<ConstantInt>(ElemCount))) {
     ConstantInt *TC = dyn_cast<ConstantInt>(TripCount);
     if (!TC) {
       LLVM_DEBUG(dbgs() << "ARM TP: Constant tripcount expected in "
                            "set.loop.iterations\n");
       return false;
     }

     // Calculate 2 tripcount values and check that they are consistent with
     // each other. The TripCount for a predicated vector loop body is
     // ceil(ElementCount/Width), or floor((ElementCount+Width-1)/Width) as we
     // work it out here.
     uint64_t TC1 = TC->getZExtValue();
     uint64_t TC2 =
         (ConstElemCount->getZExtValue() + VectorWidth - 1) / VectorWidth;

     // If the tripcount values are inconsistent, we can't insert the VCTP and
     // trigger tail-predication; keep the intrinsic as a get.active.lane.mask
     // and legalize this.
     if (TC1 != TC2) {
       LLVM_DEBUG(dbgs() << "ARM TP: inconsistent constant tripcount values: "
                  << TC1 << " from set.loop.iterations, and "
                  << TC2 << " from get.active.lane.mask\n");
       return false;
     }
   } else if (!ForceTailPredication) {
     // 2) We need to prove that the sub expression that we create in the
     // tail-predicated loop body, which calculates the remaining elements to be
     // processed, is non-negative, i.e. it doesn't overflow:
     //
     //   ((ElementCount + VectorWidth - 1) / VectorWidth) - TripCount >= 0
     //
     // This is true if:
     //
     //    TripCount == (ElementCount + VectorWidth - 1) / VectorWidth
     //
     // which what we will be using here.
     //
     auto *VW = SE->getSCEV(ConstantInt::get(TripCount->getType(), VectorWidth));
     // ElementCount + (VW-1):
     auto *ECPlusVWMinus1 = SE->getAddExpr(EC,
         SE->getSCEV(ConstantInt::get(TripCount->getType(), VectorWidth - 1)));

     // Ceil = ElementCount + (VW-1) / VW
     auto *Ceil = SE->getUDivExpr(ECPlusVWMinus1, VW);

     // Prevent unused variable warnings with TC
     (void)TC;
     LLVM_DEBUG(
       dbgs() << "ARM TP: Analysing overflow behaviour for:\n";
       dbgs() << "ARM TP: - TripCount = "; TC->dump();
       dbgs() << "ARM TP: - ElemCount = "; EC->dump();
       dbgs() << "ARM TP: - VecWidth =  " << VectorWidth << "\n";
       dbgs() << "ARM TP: - (ElemCount+VW-1) / VW = "; Ceil->dump();
     );

     // As an example, almost all the tripcount expressions (produced by the
     // vectoriser) look like this:
     //
     //   TC = ((-4 + (4 * ((3 + %N) /u 4))<nuw>) /u 4)
     //
     // and "ElementCount + (VW-1) / VW":
     //
     //   Ceil = ((3 + %N) /u 4)
     //
     // Check for equality of TC and Ceil by calculating SCEV expression
     // TC - Ceil and test it for zero.
     //
     bool Zero = SE->getMinusSCEV(
                       SE->getBackedgeTakenCount(L),
                       SE->getUDivExpr(SE->getAddExpr(SE->getMulExpr(Ceil, VW),
                                                      SE->getNegativeSCEV(VW)),
                                       VW))
                     ->isZero();

     if (!Zero) {
       LLVM_DEBUG(dbgs() << "ARM TP: possible overflow in sub expression.\n");
       return false;
     }
   }

   // 3) Find out if IV is an induction phi. Note that we can't use Loop
   // helpers here to get the induction variable, because the hardware loop is
   // no longer in loopsimplify form, and also the hwloop intrinsic uses a
   // different counter. Using SCEV, we check that the induction is of the
   // form i = i + 4, where the increment must be equal to the VectorWidth.
   auto *IV = ActiveLaneMask->getOperand(0);
   auto *IVExpr = SE->getSCEV(IV);
   auto *AddExpr = dyn_cast<SCEVAddRecExpr>(IVExpr);

   if (!AddExpr) {
     LLVM_DEBUG(dbgs() << "ARM TP: induction not an add expr: "; IVExpr->dump());
     return false;
   }
   // Check that this AddRec is associated with this loop.
   if (AddExpr->getLoop() != L) {
     LLVM_DEBUG(dbgs() << "ARM TP: phi not part of this loop\n");
     return false;
   }
   auto *Base = dyn_cast<SCEVConstant>(AddExpr->getOperand(0));
   if (!Base || !Base->isZero()) {
     LLVM_DEBUG(dbgs() << "ARM TP: induction base is not 0\n");
     return false;
   }
   auto *Step = dyn_cast<SCEVConstant>(AddExpr->getOperand(1));
   if (!Step) {
     LLVM_DEBUG(dbgs() << "ARM TP: induction step is not a constant: ";
                AddExpr->getOperand(1)->dump());
     return false;
   }
   auto StepValue = Step->getValue()->getSExtValue();
   if (VectorWidth == StepValue)
     return true;

   LLVM_DEBUG(dbgs() << "ARM TP: Step value " << StepValue
                     << " doesn't match vector width " << VectorWidth << "\n");

   return false;
 }

 void MVETailPredication::InsertVCTPIntrinsic(IntrinsicInst *ActiveLaneMask,
                                              Value *TripCount) {
   IRBuilder<> Builder(L->getLoopPreheader()->getTerminator());
   Module *M = L->getHeader()->getModule();
   Type *Ty = IntegerType::get(M->getContext(), 32);
   unsigned VectorWidth =
       cast<FixedVectorType>(ActiveLaneMask->getType())->getNumElements();

   // Insert a phi to count the number of elements processed by the loop.
   Builder.SetInsertPoint(L->getHeader()->getFirstNonPHI());
   PHINode *Processed = Builder.CreatePHI(Ty, 2);
   Processed->addIncoming(ActiveLaneMask->getOperand(1), L->getLoopPreheader());

   // Replace @llvm.get.active.mask() with the ARM specific VCTP intrinic, and
   // thus represent the effect of tail predication.
   Builder.SetInsertPoint(ActiveLaneMask);
   ConstantInt *Factor = ConstantInt::get(cast<IntegerType>(Ty), VectorWidth);

   Intrinsic::ID VCTPID;
   switch (VectorWidth) {
   default:
     llvm_unreachable("unexpected number of lanes");
   case 4:  VCTPID = Intrinsic::arm_mve_vctp32; break;
   case 8:  VCTPID = Intrinsic::arm_mve_vctp16; break;
   case 16: VCTPID = Intrinsic::arm_mve_vctp8; break;

     // FIXME: vctp64 currently not supported because the predicate
     // vector wants to be <2 x i1>, but v2i1 is not a legal MVE
     // type, so problems happen at isel time.
     // Intrinsic::arm_mve_vctp64 exists for ACLE intrinsics
     // purposes, but takes a v4i1 instead of a v2i1.
   }
   Function *VCTP = Intrinsic::getDeclaration(M, VCTPID);
   Value *VCTPCall = Builder.CreateCall(VCTP, Processed);
   ActiveLaneMask->replaceAllUsesWith(VCTPCall);

   // Add the incoming value to the new phi.
   // TODO: This add likely already exists in the loop.
   Value *Remaining = Builder.CreateSub(Processed, Factor);
   Processed->addIncoming(Remaining, L->getLoopLatch());
   LLVM_DEBUG(dbgs() << "ARM TP: Insert processed elements phi: "
              << *Processed << "\n"
              << "ARM TP: Inserted VCTP: " << *VCTPCall << "\n");
 }

 bool MVETailPredication::TryConvertActiveLaneMask(Value *TripCount) {
   SmallVector<IntrinsicInst *, 4> ActiveLaneMasks;
   for (auto *BB : L->getBlocks())
     for (auto &I : *BB)
       if (auto *Int = dyn_cast<IntrinsicInst>(&I))
         if (Int->getIntrinsicID() == Intrinsic::get_active_lane_mask)
           ActiveLaneMasks.push_back(Int);

   if (ActiveLaneMasks.empty())
     return false;

   LLVM_DEBUG(dbgs() << "ARM TP: Found predicated vector loop.\n");

   for (auto *ActiveLaneMask : ActiveLaneMasks) {
     LLVM_DEBUG(dbgs() << "ARM TP: Found active lane mask: "
                       << *ActiveLaneMask << "\n");

     if (!IsSafeActiveMask(ActiveLaneMask, TripCount)) {
       LLVM_DEBUG(dbgs() << "ARM TP: Not safe to insert VCTP.\n");
       return false;
     }
     LLVM_DEBUG(dbgs() << "ARM TP: Safe to insert VCTP.\n");
     InsertVCTPIntrinsic(ActiveLaneMask, TripCount);
   }

   // Remove dead instructions and now dead phis.
   for (auto *II : ActiveLaneMasks)
     RecursivelyDeleteTriviallyDeadInstructions(II);
   for (auto I : L->blocks())
     DeleteDeadPHIs(I);
   return true;
 }

 Pass *llvm::createMVETailPredicationPass() {
   return new MVETailPredication();
 }

 char MVETailPredication::ID = 0;

 INITIALIZE_PASS_BEGIN(MVETailPredication, DEBUG_TYPE, DESC, false, false)
 INITIALIZE_PASS_END(MVETailPredication, DEBUG_TYPE, DESC, false, false)
	//===- MVETailPredication.cpp - MVE Tail Predication ------------- 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
	//
	//===----------------------------------------------------------------------===//
	//
	/// \file
	/// Armv8.1m introduced MVE, M-Profile Vector Extension, and low-overhead
	/// branches to help accelerate DSP applications. These two extensions,
	/// combined with a new form of predication called tail-predication, can be used
	/// to provide implicit vector predication within a low-overhead loop.
	/// This is implicit because the predicate of active/inactive lanes is
	/// calculated by hardware, and thus does not need to be explicitly passed
	/// to vector instructions. The instructions responsible for this are the
	/// DLSTP and WLSTP instructions, which setup a tail-predicated loop and the
	/// the total number of data elements processed by the loop. The loop-end
	/// LETP instruction is responsible for decrementing and setting the remaining
	/// elements to be processed and generating the mask of active lanes.
	///
	/// The HardwareLoops pass inserts intrinsics identifying loops that the
	/// backend will attempt to convert into a low-overhead loop. The vectorizer is
	/// responsible for generating a vectorized loop in which the lanes are
	/// predicated upon an get.active.lane.mask intrinsic. This pass looks at these
	/// get.active.lane.mask intrinsic and attempts to convert them to VCTP
	/// instructions. This will be picked up by the ARM Low-overhead loop pass later
	/// in the backend, which performs the final transformation to a DLSTP or WLSTP
	/// tail-predicated loop.
	//
	//===----------------------------------------------------------------------===//

	#include "ARM.h"
	#include "ARMSubtarget.h"
	#include "ARMTargetTransformInfo.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/Analysis/LoopPass.h"
	#include "llvm/Analysis/ScalarEvolution.h"
	#include "llvm/Analysis/ScalarEvolutionExpressions.h"
	#include "llvm/Analysis/TargetLibraryInfo.h"
	#include "llvm/Analysis/TargetTransformInfo.h"
	#include "llvm/CodeGen/TargetPassConfig.h"
	#include "llvm/IR/IRBuilder.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/IR/IntrinsicsARM.h"
	#include "llvm/IR/PatternMatch.h"
	#include "llvm/InitializePasses.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Transforms/Utils/BasicBlockUtils.h"
	#include "llvm/Transforms/Utils/Local.h"
	#include "llvm/Transforms/Utils/LoopUtils.h"
	#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"

	using namespace llvm;

	#define DEBUG_TYPE "mve-tail-predication"
	#define DESC "Transform predicated vector loops to use MVE tail predication"

	cl::opt<TailPredication::Mode> EnableTailPredication(
	"tail-predication", cl::desc("MVE tail-predication pass options"),
	cl::init(TailPredication::Enabled),
	cl::values(clEnumValN(TailPredication::Disabled, "disabled",
	"Don't tail-predicate loops"),
	clEnumValN(TailPredication::EnabledNoReductions,
	"enabled-no-reductions",
	"Enable tail-predication, but not for reduction loops"),
	clEnumValN(TailPredication::Enabled,
	"enabled",
	"Enable tail-predication, including reduction loops"),
	clEnumValN(TailPredication::ForceEnabledNoReductions,
	"force-enabled-no-reductions",
	"Enable tail-predication, but not for reduction loops, "
	"and force this which might be unsafe"),
	clEnumValN(TailPredication::ForceEnabled,
	"force-enabled",
	"Enable tail-predication, including reduction loops, "
	"and force this which might be unsafe")));


	namespace {

	class MVETailPredication : public LoopPass {
	SmallVector<IntrinsicInst*, 4> MaskedInsts;
	Loop *L = nullptr;
	ScalarEvolution *SE = nullptr;
	TargetTransformInfo *TTI = nullptr;
	const ARMSubtarget *ST = nullptr;

	public:
	static char ID;

	MVETailPredication() : LoopPass(ID) { }

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.addRequired<ScalarEvolutionWrapperPass>();
	AU.addRequired<LoopInfoWrapperPass>();
	AU.addRequired<TargetPassConfig>();
	AU.addRequired<TargetTransformInfoWrapperPass>();
	AU.addPreserved<LoopInfoWrapperPass>();
	AU.setPreservesCFG();
	}

	bool runOnLoop(Loop *L, LPPassManager&) override;

	private:
	/// Perform the relevant checks on the loop and convert active lane masks if
	/// possible.
	bool TryConvertActiveLaneMask(Value *TripCount);

	/// Perform several checks on the arguments of @llvm.get.active.lane.mask
	/// intrinsic. E.g., check that the loop induction variable and the element
	/// count are of the form we expect, and also perform overflow checks for
	/// the new expressions that are created.
	bool IsSafeActiveMask(IntrinsicInst ActiveLaneMask, Value TripCount);

	/// Insert the intrinsic to represent the effect of tail predication.
	void InsertVCTPIntrinsic(IntrinsicInst ActiveLaneMask, Value TripCount);

	/// Rematerialize the iteration count in exit blocks, which enables
	/// ARMLowOverheadLoops to better optimise away loop update statements inside
	/// hardware-loops.
	void RematerializeIterCount();
	};

	} // end namespace

	bool MVETailPredication::runOnLoop(Loop *L, LPPassManager&) {
	if (skipLoop(L) \|\| !EnableTailPredication)
	return false;

	MaskedInsts.clear();
	Function &F = *L->getHeader()->getParent();
	auto &TPC = getAnalysis<TargetPassConfig>();
	auto &TM = TPC.getTM<TargetMachine>();
	ST = &TM.getSubtarget<ARMSubtarget>(F);
	TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
	SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
	this->L = L;

	// The MVE and LOB extensions are combined to enable tail-predication, but
	// there's nothing preventing us from generating VCTP instructions for v8.1m.
	if (!ST->hasMVEIntegerOps() \|\| !ST->hasV8_1MMainlineOps()) {
	LLVM_DEBUG(dbgs() << "ARM TP: Not a v8.1m.main+mve target.\n");
	return false;
	}

	BasicBlock *Preheader = L->getLoopPreheader();
	if (!Preheader)
	return false;

	auto FindLoopIterations = [](BasicBlock BB) -> IntrinsicInst {
	for (auto &I : *BB) {
	auto *Call = dyn_cast<IntrinsicInst>(&I);
	if (!Call)
	continue;

	Intrinsic::ID ID = Call->getIntrinsicID();
	if (ID == Intrinsic::start_loop_iterations \|\|
	ID == Intrinsic::test_start_loop_iterations)
	return cast<IntrinsicInst>(&I);
	}
	return nullptr;
	};

	// Look for the hardware loop intrinsic that sets the iteration count.
	IntrinsicInst *Setup = FindLoopIterations(Preheader);

	// The test.set iteration could live in the pre-preheader.
	if (!Setup) {
	if (!Preheader->getSinglePredecessor())
	return false;
	Setup = FindLoopIterations(Preheader->getSinglePredecessor());
	if (!Setup)
	return false;
	}

	LLVM_DEBUG(dbgs() << "ARM TP: Running on Loop: " << L << Setup << "\n");

	bool Changed = TryConvertActiveLaneMask(Setup->getArgOperand(0));

	return Changed;
	}

	// The active lane intrinsic has this form:
	//
	// @llvm.get.active.lane.mask(IV, TC)
	//
	// Here we perform checks that this intrinsic behaves as expected,
	// which means:
	//
	// 1) Check that the TripCount (TC) belongs to this loop (originally).
	// 2) The element count (TC) needs to be sufficiently large that the decrement
	// of element counter doesn't overflow, which means that we need to prove:
	// ceil(ElementCount / VectorWidth) >= TripCount
	// by rounding up ElementCount up:
	// ((ElementCount + (VectorWidth - 1)) / VectorWidth
	// and evaluate if expression isKnownNonNegative:
	// (((ElementCount + (VectorWidth - 1)) / VectorWidth) - TripCount
	// 3) The IV must be an induction phi with an increment equal to the
	// vector width.
	bool MVETailPredication::IsSafeActiveMask(IntrinsicInst *ActiveLaneMask,
	Value *TripCount) {
	bool ForceTailPredication =
	EnableTailPredication == TailPredication::ForceEnabledNoReductions \|\|
	EnableTailPredication == TailPredication::ForceEnabled;

	Value *ElemCount = ActiveLaneMask->getOperand(1);
	auto *EC= SE->getSCEV(ElemCount);
	auto *TC = SE->getSCEV(TripCount);
	int VectorWidth =
	cast<FixedVectorType>(ActiveLaneMask->getType())->getNumElements();
	if (VectorWidth != 4 && VectorWidth != 8 && VectorWidth != 16)
	return false;
	ConstantInt *ConstElemCount = nullptr;

	// 1) Smoke tests that the original scalar loop TripCount (TC) belongs to
	// this loop. The scalar tripcount corresponds the number of elements
	// processed by the loop, so we will refer to that from this point on.
	if (!SE->isLoopInvariant(EC, L)) {
	LLVM_DEBUG(dbgs() << "ARM TP: element count must be loop invariant.\n");
	return false;
	}

	if ((ConstElemCount = dyn_cast<ConstantInt>(ElemCount))) {
	ConstantInt *TC = dyn_cast<ConstantInt>(TripCount);
	if (!TC) {
	LLVM_DEBUG(dbgs() << "ARM TP: Constant tripcount expected in "
	"set.loop.iterations\n");
	return false;
	}

	// Calculate 2 tripcount values and check that they are consistent with
	// each other. The TripCount for a predicated vector loop body is
	// ceil(ElementCount/Width), or floor((ElementCount+Width-1)/Width) as we
	// work it out here.
	uint64_t TC1 = TC->getZExtValue();
	uint64_t TC2 =
	(ConstElemCount->getZExtValue() + VectorWidth - 1) / VectorWidth;

	// If the tripcount values are inconsistent, we can't insert the VCTP and
	// trigger tail-predication; keep the intrinsic as a get.active.lane.mask
	// and legalize this.
	if (TC1 != TC2) {
	LLVM_DEBUG(dbgs() << "ARM TP: inconsistent constant tripcount values: "
	<< TC1 << " from set.loop.iterations, and "
	<< TC2 << " from get.active.lane.mask\n");
	return false;
	}
	} else if (!ForceTailPredication) {
	// 2) We need to prove that the sub expression that we create in the
	// tail-predicated loop body, which calculates the remaining elements to be
	// processed, is non-negative, i.e. it doesn't overflow:
	//
	// ((ElementCount + VectorWidth - 1) / VectorWidth) - TripCount >= 0
	//
	// This is true if:
	//
	// TripCount == (ElementCount + VectorWidth - 1) / VectorWidth
	//
	// which what we will be using here.
	//
	auto *VW = SE->getSCEV(ConstantInt::get(TripCount->getType(), VectorWidth));
	// ElementCount + (VW-1):
	auto *ECPlusVWMinus1 = SE->getAddExpr(EC,
	SE->getSCEV(ConstantInt::get(TripCount->getType(), VectorWidth - 1)));

	// Ceil = ElementCount + (VW-1) / VW
	auto *Ceil = SE->getUDivExpr(ECPlusVWMinus1, VW);

	// Prevent unused variable warnings with TC
	(void)TC;
	LLVM_DEBUG(
	dbgs() << "ARM TP: Analysing overflow behaviour for:\n";
	dbgs() << "ARM TP: - TripCount = "; TC->dump();
	dbgs() << "ARM TP: - ElemCount = "; EC->dump();
	dbgs() << "ARM TP: - VecWidth = " << VectorWidth << "\n";
	dbgs() << "ARM TP: - (ElemCount+VW-1) / VW = "; Ceil->dump();
	);

	// As an example, almost all the tripcount expressions (produced by the
	// vectoriser) look like this:
	//
	// TC = ((-4 + (4 * ((3 + %N) /u 4))<nuw>) /u 4)
	//
	// and "ElementCount + (VW-1) / VW":
	//
	// Ceil = ((3 + %N) /u 4)
	//
	// Check for equality of TC and Ceil by calculating SCEV expression
	// TC - Ceil and test it for zero.
	//
	bool Zero = SE->getMinusSCEV(
	SE->getBackedgeTakenCount(L),
	SE->getUDivExpr(SE->getAddExpr(SE->getMulExpr(Ceil, VW),
	SE->getNegativeSCEV(VW)),
	VW))
	->isZero();

	if (!Zero) {
	LLVM_DEBUG(dbgs() << "ARM TP: possible overflow in sub expression.\n");
	return false;
	}
	}

	// 3) Find out if IV is an induction phi. Note that we can't use Loop
	// helpers here to get the induction variable, because the hardware loop is
	// no longer in loopsimplify form, and also the hwloop intrinsic uses a
	// different counter. Using SCEV, we check that the induction is of the
	// form i = i + 4, where the increment must be equal to the VectorWidth.
	auto *IV = ActiveLaneMask->getOperand(0);
	auto *IVExpr = SE->getSCEV(IV);
	auto *AddExpr = dyn_cast<SCEVAddRecExpr>(IVExpr);

	if (!AddExpr) {
	LLVM_DEBUG(dbgs() << "ARM TP: induction not an add expr: "; IVExpr->dump());
	return false;
	}
	// Check that this AddRec is associated with this loop.
	if (AddExpr->getLoop() != L) {
	LLVM_DEBUG(dbgs() << "ARM TP: phi not part of this loop\n");
	return false;
	}
	auto *Base = dyn_cast<SCEVConstant>(AddExpr->getOperand(0));
	if (!Base \|\| !Base->isZero()) {
	LLVM_DEBUG(dbgs() << "ARM TP: induction base is not 0\n");
	return false;
	}
	auto *Step = dyn_cast<SCEVConstant>(AddExpr->getOperand(1));
	if (!Step) {
	LLVM_DEBUG(dbgs() << "ARM TP: induction step is not a constant: ";
	AddExpr->getOperand(1)->dump());
	return false;
	}
	auto StepValue = Step->getValue()->getSExtValue();
	if (VectorWidth == StepValue)
	return true;

	LLVM_DEBUG(dbgs() << "ARM TP: Step value " << StepValue
	<< " doesn't match vector width " << VectorWidth << "\n");

	return false;
	}

	void MVETailPredication::InsertVCTPIntrinsic(IntrinsicInst *ActiveLaneMask,
	Value *TripCount) {
	IRBuilder<> Builder(L->getLoopPreheader()->getTerminator());
	Module *M = L->getHeader()->getModule();
	Type *Ty = IntegerType::get(M->getContext(), 32);
	unsigned VectorWidth =
	cast<FixedVectorType>(ActiveLaneMask->getType())->getNumElements();

	// Insert a phi to count the number of elements processed by the loop.
	Builder.SetInsertPoint(L->getHeader()->getFirstNonPHI());
	PHINode *Processed = Builder.CreatePHI(Ty, 2);
	Processed->addIncoming(ActiveLaneMask->getOperand(1), L->getLoopPreheader());

	// Replace @llvm.get.active.mask() with the ARM specific VCTP intrinic, and
	// thus represent the effect of tail predication.
	Builder.SetInsertPoint(ActiveLaneMask);
	ConstantInt *Factor = ConstantInt::get(cast<IntegerType>(Ty), VectorWidth);

	Intrinsic::ID VCTPID;
	switch (VectorWidth) {
	default:
	llvm_unreachable("unexpected number of lanes");
	case 4: VCTPID = Intrinsic::arm_mve_vctp32; break;
	case 8: VCTPID = Intrinsic::arm_mve_vctp16; break;
	case 16: VCTPID = Intrinsic::arm_mve_vctp8; break;

	// FIXME: vctp64 currently not supported because the predicate
	// vector wants to be <2 x i1>, but v2i1 is not a legal MVE
	// type, so problems happen at isel time.
	// Intrinsic::arm_mve_vctp64 exists for ACLE intrinsics
	// purposes, but takes a v4i1 instead of a v2i1.
	}
	Function *VCTP = Intrinsic::getDeclaration(M, VCTPID);
	Value *VCTPCall = Builder.CreateCall(VCTP, Processed);
	ActiveLaneMask->replaceAllUsesWith(VCTPCall);

	// Add the incoming value to the new phi.
	// TODO: This add likely already exists in the loop.
	Value *Remaining = Builder.CreateSub(Processed, Factor);
	Processed->addIncoming(Remaining, L->getLoopLatch());
	LLVM_DEBUG(dbgs() << "ARM TP: Insert processed elements phi: "
	<< *Processed << "\n"
	<< "ARM TP: Inserted VCTP: " << *VCTPCall << "\n");
	}

	bool MVETailPredication::TryConvertActiveLaneMask(Value *TripCount) {
	SmallVector<IntrinsicInst *, 4> ActiveLaneMasks;
	for (auto *BB : L->getBlocks())
	for (auto &I : *BB)
	if (auto *Int = dyn_cast<IntrinsicInst>(&I))
	if (Int->getIntrinsicID() == Intrinsic::get_active_lane_mask)
	ActiveLaneMasks.push_back(Int);

	if (ActiveLaneMasks.empty())
	return false;

	LLVM_DEBUG(dbgs() << "ARM TP: Found predicated vector loop.\n");

	for (auto *ActiveLaneMask : ActiveLaneMasks) {
	LLVM_DEBUG(dbgs() << "ARM TP: Found active lane mask: "
	<< *ActiveLaneMask << "\n");

	if (!IsSafeActiveMask(ActiveLaneMask, TripCount)) {
	LLVM_DEBUG(dbgs() << "ARM TP: Not safe to insert VCTP.\n");
	return false;
	}
	LLVM_DEBUG(dbgs() << "ARM TP: Safe to insert VCTP.\n");
	InsertVCTPIntrinsic(ActiveLaneMask, TripCount);
	}

	// Remove dead instructions and now dead phis.
	for (auto *II : ActiveLaneMasks)
	RecursivelyDeleteTriviallyDeadInstructions(II);
	for (auto I : L->blocks())
	DeleteDeadPHIs(I);
	return true;
	}

	Pass *llvm::createMVETailPredicationPass() {
	return new MVETailPredication();
	}

	char MVETailPredication::ID = 0;

	INITIALIZE_PASS_BEGIN(MVETailPredication, DEBUG_TYPE, DESC, false, false)
	INITIALIZE_PASS_END(MVETailPredication, DEBUG_TYPE, DESC, false, false)