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llvm-project/llvm/lib/Target/ARM/MVETailPredication.cpp

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//===- MVETailPredication.cpp - MVE Tail Predication ----------------------===//
//
// 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 can be
/// combined to provide implicit vector predication within a low-overhead loop.
/// 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 the iteration counter. This pass looks at these predicated
/// vector loops, that are targets for low-overhead loops, and prepares it for
/// code generation. Once the vectorizer has produced a masked loop, there's a
/// couple of final forms:
/// - A tail-predicated loop, with implicit predication.
/// - A loop containing multiple VCPT instructions, predicating multiple VPT
/// blocks of instructions operating on different vector types.
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "ARM.h"
#include "ARMSubtarget.h"
using namespace llvm;
#define DEBUG_TYPE "mve-tail-predication"
#define DESC "Transform predicated vector loops to use MVE tail predication"
cl::opt<bool>
DisableTailPredication("disable-mve-tail-predication", cl::Hidden,
cl::init(true),
cl::desc("Disable MVE Tail Predication"));
namespace {
class MVETailPredication : public LoopPass {
SmallVector<IntrinsicInst*, 4> MaskedInsts;
Loop *L = nullptr;
ScalarEvolution *SE = nullptr;
TargetTransformInfo *TTI = 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 if possible.
bool TryConvert(Value *TripCount);
/// Return whether this is a vectorized loop, that contains masked
/// load/stores.
bool IsPredicatedVectorLoop();
/// Compute a value for the total number of elements that the predicated
/// loop will process.
Value *ComputeElements(Value *TripCount, VectorType *VecTy);
/// Is the icmp that generates an i1 vector, based upon a loop counter
/// and a limit that is defined outside the loop.
bool isTailPredicate(Instruction *Predicate, Value *NumElements);
};
} // end namespace
static bool IsDecrement(Instruction &I) {
auto *Call = dyn_cast<IntrinsicInst>(&I);
if (!Call)
return false;
Intrinsic::ID ID = Call->getIntrinsicID();
return ID == Intrinsic::loop_decrement_reg;
}
static bool IsMasked(Instruction *I) {
auto *Call = dyn_cast<IntrinsicInst>(I);
if (!Call)
return false;
Intrinsic::ID ID = Call->getIntrinsicID();
// TODO: Support gather/scatter expand/compress operations.
return ID == Intrinsic::masked_store || ID == Intrinsic::masked_load;
}
bool MVETailPredication::runOnLoop(Loop *L, LPPassManager&) {
if (skipLoop(L) || DisableTailPredication)
return false;
Function &F = *L->getHeader()->getParent();
auto &TPC = getAnalysis<TargetPassConfig>();
auto &TM = TPC.getTM<TargetMachine>();
auto *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() << "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::set_loop_iterations ||
ID == Intrinsic::test_set_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;
}
// Search for the hardware loop intrinic that decrements the loop counter.
IntrinsicInst *Decrement = nullptr;
for (auto *BB : L->getBlocks()) {
for (auto &I : *BB) {
if (IsDecrement(I)) {
Decrement = cast<IntrinsicInst>(&I);
break;
}
}
}
if (!Decrement)
return false;
LLVM_DEBUG(dbgs() << "TP: Running on Loop: " << *L
<< *Setup << "\n"
<< *Decrement << "\n");
bool Changed = TryConvert(Setup->getArgOperand(0));
return Changed;
}
bool MVETailPredication::isTailPredicate(Instruction *I, Value *NumElements) {
// Look for the following:
// %trip.count.minus.1 = add i32 %N, -1
// %broadcast.splatinsert10 = insertelement <4 x i32> undef,
// i32 %trip.count.minus.1, i32 0
// %broadcast.splat11 = shufflevector <4 x i32> %broadcast.splatinsert10,
// <4 x i32> undef,
// <4 x i32> zeroinitializer
// ...
// ...
// %index = phi i32
// %broadcast.splatinsert = insertelement <4 x i32> undef, i32 %index, i32 0
// %broadcast.splat = shufflevector <4 x i32> %broadcast.splatinsert,
// <4 x i32> undef,
// <4 x i32> zeroinitializer
// %induction = add <4 x i32> %broadcast.splat, <i32 0, i32 1, i32 2, i32 3>
// %pred = icmp ule <4 x i32> %induction, %broadcast.splat11
// And return whether V == %pred.
using namespace PatternMatch;
CmpInst::Predicate Pred;
Instruction *Shuffle = nullptr;
Instruction *Induction = nullptr;
// The vector icmp
if (!match(I, m_ICmp(Pred, m_Instruction(Induction),
m_Instruction(Shuffle))) ||
Pred != ICmpInst::ICMP_ULE || !L->isLoopInvariant(Shuffle))
return false;
// First find the stuff outside the loop which is setting up the limit
// vector....
// The invariant shuffle that broadcast the limit into a vector.
Instruction *Insert = nullptr;
if (!match(Shuffle, m_ShuffleVector(m_Instruction(Insert), m_Undef(),
m_Zero())))
return false;
// Insert the limit into a vector.
Instruction *BECount = nullptr;
if (!match(Insert, m_InsertElement(m_Undef(), m_Instruction(BECount),
m_Zero())))
return false;
// The limit calculation, backedge count.
Value *TripCount = nullptr;
if (!match(BECount, m_Add(m_Value(TripCount), m_AllOnes())))
return false;
if (TripCount != NumElements)
return false;
// Now back to searching inside the loop body...
// Find the add with takes the index iv and adds a constant vector to it.
Instruction *BroadcastSplat = nullptr;
Constant *Const = nullptr;
if (!match(Induction, m_Add(m_Instruction(BroadcastSplat),
m_Constant(Const))))
return false;
// Check that we're adding <0, 1, 2, 3...
if (auto *CDS = dyn_cast<ConstantDataSequential>(Const)) {
for (unsigned i = 0; i < CDS->getNumElements(); ++i) {
if (CDS->getElementAsInteger(i) != i)
return false;
}
} else
return false;
// The shuffle which broadcasts the index iv into a vector.
if (!match(BroadcastSplat, m_ShuffleVector(m_Instruction(Insert), m_Undef(),
m_Zero())))
return false;
// The insert element which initialises a vector with the index iv.
Instruction *IV = nullptr;
if (!match(Insert, m_InsertElement(m_Undef(), m_Instruction(IV), m_Zero())))
return false;
// The index iv.
auto *Phi = dyn_cast<PHINode>(IV);
if (!Phi)
return false;
// TODO: Don't think we need to check the entry value.
Value *OnEntry = Phi->getIncomingValueForBlock(L->getLoopPreheader());
if (!match(OnEntry, m_Zero()))
return false;
Value *InLoop = Phi->getIncomingValueForBlock(L->getLoopLatch());
unsigned Lanes = cast<VectorType>(Insert->getType())->getNumElements();
Instruction *LHS = nullptr;
if (!match(InLoop, m_Add(m_Instruction(LHS), m_SpecificInt(Lanes))))
return false;
return LHS == Phi;
}
static VectorType* getVectorType(IntrinsicInst *I) {
unsigned TypeOp = I->getIntrinsicID() == Intrinsic::masked_load ? 0 : 1;
auto *PtrTy = cast<PointerType>(I->getOperand(TypeOp)->getType());
return cast<VectorType>(PtrTy->getElementType());
}
bool MVETailPredication::IsPredicatedVectorLoop() {
// Check that the loop contains at least one masked load/store intrinsic.
// We only support 'normal' vector instructions - other than masked
// load/stores.
for (auto *BB : L->getBlocks()) {
for (auto &I : *BB) {
if (IsMasked(&I)) {
VectorType *VecTy = getVectorType(cast<IntrinsicInst>(&I));
unsigned Lanes = VecTy->getNumElements();
unsigned ElementWidth = VecTy->getScalarSizeInBits();
// MVE vectors are 128-bit, but don't support 128 x i1.
// TODO: Can we support vectors larger than 128-bits?
unsigned MaxWidth = TTI->getRegisterBitWidth(true);
if (Lanes * ElementWidth > MaxWidth || Lanes == MaxWidth)
return false;
MaskedInsts.push_back(cast<IntrinsicInst>(&I));
} else if (auto *Int = dyn_cast<IntrinsicInst>(&I)) {
for (auto &U : Int->args()) {
if (isa<VectorType>(U->getType()))
return false;
}
}
}
}
return !MaskedInsts.empty();
}
Value* MVETailPredication::ComputeElements(Value *TripCount,
VectorType *VecTy) {
const SCEV *TripCountSE = SE->getSCEV(TripCount);
ConstantInt *VF = ConstantInt::get(cast<IntegerType>(TripCount->getType()),
VecTy->getNumElements());
if (VF->equalsInt(1))
return nullptr;
// TODO: Support constant trip counts.
auto VisitAdd = [&](const SCEVAddExpr *S) -> const SCEVMulExpr* {
if (auto *Const = dyn_cast<SCEVConstant>(S->getOperand(0))) {
if (Const->getAPInt() != -VF->getValue())
return nullptr;
} else
return nullptr;
return dyn_cast<SCEVMulExpr>(S->getOperand(1));
};
auto VisitMul = [&](const SCEVMulExpr *S) -> const SCEVUDivExpr* {
if (auto *Const = dyn_cast<SCEVConstant>(S->getOperand(0))) {
if (Const->getValue() != VF)
return nullptr;
} else
return nullptr;
return dyn_cast<SCEVUDivExpr>(S->getOperand(1));
};
auto VisitDiv = [&](const SCEVUDivExpr *S) -> const SCEV* {
if (auto *Const = dyn_cast<SCEVConstant>(S->getRHS())) {
if (Const->getValue() != VF)
return nullptr;
} else
return nullptr;
if (auto *RoundUp = dyn_cast<SCEVAddExpr>(S->getLHS())) {
if (auto *Const = dyn_cast<SCEVConstant>(RoundUp->getOperand(0))) {
if (Const->getAPInt() != (VF->getValue() - 1))
return nullptr;
} else
return nullptr;
return RoundUp->getOperand(1);
}
return nullptr;
};
// TODO: Can we use SCEV helpers, such as findArrayDimensions, and friends to
// determine the numbers of elements instead? Looks like this is what is used
// for delinearization, but I'm not sure if it can be applied to the
// vectorized form - at least not without a bit more work than I feel
// comfortable with.
// Search for Elems in the following SCEV:
// (1 + ((-VF + (VF * (((VF - 1) + %Elems) /u VF))<nuw>) /u VF))<nuw><nsw>
const SCEV *Elems = nullptr;
if (auto *TC = dyn_cast<SCEVAddExpr>(TripCountSE))
if (auto *Div = dyn_cast<SCEVUDivExpr>(TC->getOperand(1)))
if (auto *Add = dyn_cast<SCEVAddExpr>(Div->getLHS()))
if (auto *Mul = VisitAdd(Add))
if (auto *Div = VisitMul(Mul))
if (auto *Res = VisitDiv(Div))
Elems = Res;
if (!Elems)
return nullptr;
Instruction *InsertPt = L->getLoopPreheader()->getTerminator();
if (!isSafeToExpandAt(Elems, InsertPt, *SE))
return nullptr;
auto DL = L->getHeader()->getModule()->getDataLayout();
SCEVExpander Expander(*SE, DL, "elements");
return Expander.expandCodeFor(Elems, Elems->getType(), InsertPt);
}
// Look through the exit block to see whether there's a duplicate predicate
// instruction. This can happen when we need to perform a select on values
// from the last and previous iteration. Instead of doing a straight
// replacement of that predicate with the vctp, clone the vctp and place it
// in the block. This means that the VPR doesn't have to be live into the
// exit block which should make it easier to convert this loop into a proper
// tail predicated loop.
static void Cleanup(DenseMap<Instruction*, Instruction*> &NewPredicates,
SetVector<Instruction*> &MaybeDead, Loop *L) {
if (BasicBlock *Exit = L->getUniqueExitBlock()) {
for (auto &Pair : NewPredicates) {
Instruction *OldPred = Pair.first;
Instruction *NewPred = Pair.second;
for (auto &I : *Exit) {
if (I.isSameOperationAs(OldPred)) {
Instruction *PredClone = NewPred->clone();
PredClone->insertBefore(&I);
I.replaceAllUsesWith(PredClone);
MaybeDead.insert(&I);
break;
}
}
}
}
// Drop references and add operands to check for dead.
SmallPtrSet<Instruction*, 4> Dead;
while (!MaybeDead.empty()) {
auto *I = MaybeDead.front();
MaybeDead.remove(I);
if (I->hasNUsesOrMore(1))
continue;
for (auto &U : I->operands()) {
if (auto *OpI = dyn_cast<Instruction>(U))
MaybeDead.insert(OpI);
}
I->dropAllReferences();
Dead.insert(I);
}
for (auto *I : Dead)
I->eraseFromParent();
for (auto I : L->blocks())
DeleteDeadPHIs(I);
}
bool MVETailPredication::TryConvert(Value *TripCount) {
if (!IsPredicatedVectorLoop())
return false;
LLVM_DEBUG(dbgs() << "TP: Found predicated vector loop.\n");
// Walk through the masked intrinsics and try to find whether the predicate
// operand is generated from an induction variable.
Module *M = L->getHeader()->getModule();
Type *Ty = IntegerType::get(M->getContext(), 32);
SetVector<Instruction*> Predicates;
DenseMap<Instruction*, Instruction*> NewPredicates;
for (auto *I : MaskedInsts) {
Intrinsic::ID ID = I->getIntrinsicID();
unsigned PredOp = ID == Intrinsic::masked_load ? 2 : 3;
auto *Predicate = dyn_cast<Instruction>(I->getArgOperand(PredOp));
if (!Predicate || Predicates.count(Predicate))
continue;
VectorType *VecTy = getVectorType(I);
Value *NumElements = ComputeElements(TripCount, VecTy);
if (!NumElements)
continue;
if (!isTailPredicate(Predicate, NumElements)) {
LLVM_DEBUG(dbgs() << "TP: Not tail predicate: " << *Predicate << "\n");
continue;
}
LLVM_DEBUG(dbgs() << "TP: Found tail predicate: " << *Predicate << "\n");
Predicates.insert(Predicate);
// Insert a phi to count the number of elements processed by the loop.
IRBuilder<> Builder(L->getHeader()->getFirstNonPHI());
PHINode *Processed = Builder.CreatePHI(Ty, 2);
Processed->addIncoming(NumElements, L->getLoopPreheader());
// Insert the intrinsic to represent the effect of tail predication.
Builder.SetInsertPoint(cast<Instruction>(Predicate));
ConstantInt *Factor =
ConstantInt::get(cast<IntegerType>(Ty), VecTy->getNumElements());
Intrinsic::ID VCTPID;
switch (VecTy->getNumElements()) {
default:
llvm_unreachable("unexpected number of lanes");
case 2: VCTPID = Intrinsic::arm_vctp64; break;
case 4: VCTPID = Intrinsic::arm_vctp32; break;
case 8: VCTPID = Intrinsic::arm_vctp16; break;
case 16: VCTPID = Intrinsic::arm_vctp8; break;
}
Function *VCTP = Intrinsic::getDeclaration(M, VCTPID);
Value *TailPredicate = Builder.CreateCall(VCTP, Processed);
Predicate->replaceAllUsesWith(TailPredicate);
NewPredicates[Predicate] = cast<Instruction>(TailPredicate);
// 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() << "TP: Insert processed elements phi: "
<< *Processed << "\n"
<< "TP: Inserted VCTP: " << *TailPredicate << "\n");
}
// Now clean up.
Cleanup(NewPredicates, Predicates, L);
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)
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