[llvm] [LV] Introduce the EVLIVSimplify Pass for EVL-vectorized loops (PR #91796)
Min-Yih Hsu via llvm-commits
llvm-commits at lists.llvm.org
Mon Dec 16 16:26:35 PST 2024
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@@ -0,0 +1,271 @@
+//===------ EVLIndVarSimplify.cpp - Optimize vectorized loops w/ EVL IV----===//
+//
+// 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 optimizes a vectorized loop with canonical IV to using EVL-based
+// IV if it was tail-folded by predicated EVL.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/EVLIndVarSimplify.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/IVDescriptors.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Scalar/LoopPassManager.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
+
+#define DEBUG_TYPE "evl-iv-simplify"
+
+using namespace llvm;
+
+STATISTIC(NumEliminatedCanonicalIV, "Number of canonical IVs we eliminated");
+
+static cl::opt<bool> EnableEVLIndVarSimplify(
+ "enable-evl-indvar-simplify",
+ cl::desc("Enable EVL-based induction variable simplify Pass"), cl::Hidden,
+ cl::init(true));
+
+namespace {
+struct EVLIndVarSimplifyImpl {
+ ScalarEvolution &SE;
+
+ explicit EVLIndVarSimplifyImpl(LoopStandardAnalysisResults &LAR)
+ : SE(LAR.SE) {}
+
+ explicit EVLIndVarSimplifyImpl(ScalarEvolution &SE) : SE(SE) {}
+
+ // Returns true if modify the loop.
+ bool run(Loop &L);
+};
+
+struct EVLIndVarSimplify : public LoopPass {
+ static char ID;
+
+ EVLIndVarSimplify() : LoopPass(ID) {
+ initializeEVLIndVarSimplifyPass(*PassRegistry::getPassRegistry());
+ }
+
+ bool runOnLoop(Loop *L, LPPassManager &LPM) override;
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addRequired<ScalarEvolutionWrapperPass>();
+ AU.setPreservesCFG();
+ }
+};
+} // anonymous namespace
+
+static uint32_t getVFFromIndVar(const SCEV *Step, const Function &F) {
+ if (!Step)
+ return 0U;
+
+ // Looking for loops with IV step value in the form of `(<constant VF> x
+ // vscale)`.
+ if (auto *Mul = dyn_cast<SCEVMulExpr>(Step)) {
+ if (Mul->getNumOperands() == 2) {
+ const SCEV *LHS = Mul->getOperand(0);
+ const SCEV *RHS = Mul->getOperand(1);
+ if (auto *Const = dyn_cast<SCEVConstant>(LHS)) {
+ uint64_t V = Const->getAPInt().getLimitedValue();
+ if (isa<SCEVVScale>(RHS) && llvm::isUInt<32>(V))
+ return static_cast<uint32_t>(V);
+ }
+ }
+ }
+
+ // If not, see if the vscale_range of the parent function is a fixed value,
+ // which makes the step value to be replaced by a constant.
+ if (F.hasFnAttribute(Attribute::VScaleRange))
+ if (auto *ConstStep = dyn_cast<SCEVConstant>(Step)) {
+ APInt V = ConstStep->getAPInt().abs();
+ ConstantRange CR = llvm::getVScaleRange(&F, 64);
+ if (const APInt *Fixed = CR.getSingleElement()) {
+ V = V.zextOrTrunc(Fixed->getBitWidth());
+ uint64_t VF = V.udiv(*Fixed).getLimitedValue();
+ if (VF && llvm::isUInt<32>(VF) &&
+ // Make sure step is divisible by vscale.
+ V.urem(*Fixed).isZero())
+ return static_cast<uint32_t>(VF);
+ }
+ }
+
+ return 0U;
+}
+
+bool EVLIndVarSimplifyImpl::run(Loop &L) {
+ if (!EnableEVLIndVarSimplify)
+ return false;
+
+ InductionDescriptor IVD;
+ PHINode *IndVar = L.getInductionVariable(SE);
+ if (!IndVar || !L.getInductionDescriptor(SE, IVD)) {
+ LLVM_DEBUG(dbgs() << "Cannot retrieve IV from loop " << L.getName()
+ << "\n");
+ return false;
+ }
+
+ BasicBlock *InitBlock, *BackEdgeBlock;
+ if (!L.getIncomingAndBackEdge(InitBlock, BackEdgeBlock)) {
+ LLVM_DEBUG(dbgs() << "Expect unique incoming and backedge in "
+ << L.getName() << "\n");
+ return false;
+ }
+
+ // Retrieve the loop bounds.
+ std::optional<Loop::LoopBounds> Bounds = L.getBounds(SE);
+ if (!Bounds) {
+ LLVM_DEBUG(dbgs() << "Could not obtain the bounds for loop " << L.getName()
+ << "\n");
+ return false;
+ }
+ Value *CanonicalIVInit = &Bounds->getInitialIVValue();
+ Value *CanonicalIVFinal = &Bounds->getFinalIVValue();
+
+ const SCEV *StepV = IVD.getStep();
+ uint32_t VF = getVFFromIndVar(StepV, *L.getHeader()->getParent());
+ if (!VF) {
+ LLVM_DEBUG(dbgs() << "Could not infer VF from IndVar step '" << *StepV
+ << "'\n");
+ return false;
+ }
+ LLVM_DEBUG(dbgs() << "Using VF=" << VF << " for loop " << L.getName()
+ << "\n");
+
+ // Try to find the EVL-based induction variable.
+ using namespace PatternMatch;
+ BasicBlock *BB = IndVar->getParent();
+
+ Value *EVLIndVar = nullptr;
+ Value *RemTC = nullptr;
+ auto IntrinsicMatch = m_Intrinsic<Intrinsic::experimental_get_vector_length>(
+ m_Value(RemTC), m_SpecificInt(VF),
+ /*Scalable=*/m_SpecificInt(1));
+ for (auto &PN : BB->phis()) {
+ if (&PN == IndVar)
+ continue;
+
+ // Check 1: it has to contain both incoming (init) & backedge blocks
+ // from IndVar.
+ if (PN.getBasicBlockIndex(InitBlock) < 0 ||
+ PN.getBasicBlockIndex(BackEdgeBlock) < 0)
+ continue;
+ // Check 2: EVL index is always increasing, thus its inital value has to be
+ // equal to either the initial IV value (when the canonical IV is also
+ // increasing) or the last IV value (when canonical IV is decreasing).
+ Value *Init = PN.getIncomingValueForBlock(InitBlock);
+ using Direction = Loop::LoopBounds::Direction;
+ switch (Bounds->getDirection()) {
+ case Direction::Increasing:
+ if (Init != CanonicalIVInit)
+ continue;
+ break;
+ case Direction::Decreasing:
+ if (Init != CanonicalIVFinal)
+ continue;
+ break;
+ case Direction::Unknown:
+ // To be more permissive and see if either the initial or final IV value
+ // matches PN's init value.
+ if (Init != CanonicalIVInit && Init != CanonicalIVFinal)
+ continue;
+ break;
+ }
+ Value *RecValue = PN.getIncomingValueForBlock(BackEdgeBlock);
+ assert(RecValue);
+
+ LLVM_DEBUG(dbgs() << "Found candidate PN of EVL-based IndVar: " << PN
+ << "\n");
+
+ // Check 3: Pattern match to find the EVL-based index.
+ if (match(RecValue,
+ m_c_Add(m_ZExtOrSelf(IntrinsicMatch), m_Specific(&PN))) &&
+ match(RemTC, m_Sub(m_Value(), m_Specific(&PN)))) {
----------------
mshockwave wrote:
> Consider an EVL IV where the sub is M, and the canonical IV where BTC is in terms of N. N != M. In this case, the original loop would run a different number of iterations than the modified loop unless you can prove N and M are the same
To my best understandings, due to how LoopVectorizer derives N and M from the original trip count, N and M will be the same in the code produced by LV. So with my alternative solution which run this Pass right after LV, I think we can assure that N and M as seen by this Pass will be the same.
https://github.com/llvm/llvm-project/pull/91796
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