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+//===- IVUsers.cpp - Induction Variable Users -------------------*- 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 implements bookkeeping for "interesting" users of expressions
+// computed from induction variables.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/IVUsers.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/LoopAnalysisManager.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Type.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+using namespace llvm;
+
+#define DEBUG_TYPE "iv-users"
+
+AnalysisKey IVUsersAnalysis::Key;
+
+IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
+ LoopStandardAnalysisResults &AR) {
+ return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
+}
+
+char IVUsersWrapperPass::ID = 0;
+INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
+ "Induction Variable Users", false, true)
+INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
+ false, true)
+
+Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }
+
+/// isInteresting - Test whether the given expression is "interesting" when
+/// used by the given expression, within the context of analyzing the
+/// given loop.
+static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
+ ScalarEvolution *SE, LoopInfo *LI) {
+ // An addrec is interesting if it's affine or if it has an interesting start.
+ if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
+ // Keep things simple. Don't touch loop-variant strides unless they're
+ // only used outside the loop and we can simplify them.
+ if (AR->getLoop() == L)
+ return AR->isAffine() ||
+ (!L->contains(I) &&
+ SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
+ // Otherwise recurse to see if the start value is interesting, and that
+ // the step value is not interesting, since we don't yet know how to
+ // do effective SCEV expansions for addrecs with interesting steps.
+ return isInteresting(AR->getStart(), I, L, SE, LI) &&
+ !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
+ }
+
+ // An add is interesting if exactly one of its operands is interesting.
+ if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
+ bool AnyInterestingYet = false;
+ for (const auto *Op : Add->operands())
+ if (isInteresting(Op, I, L, SE, LI)) {
+ if (AnyInterestingYet)
+ return false;
+ AnyInterestingYet = true;
+ }
+ return AnyInterestingYet;
+ }
+
+ // Nothing else is interesting here.
+ return false;
+}
+
+/// Return true if all loop headers that dominate this block are in simplified
+/// form.
+static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
+ const LoopInfo *LI,
+ SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
+ Loop *NearestLoop = nullptr;
+ for (DomTreeNode *Rung = DT->getNode(BB);
+ Rung; Rung = Rung->getIDom()) {
+ BasicBlock *DomBB = Rung->getBlock();
+ Loop *DomLoop = LI->getLoopFor(DomBB);
+ if (DomLoop && DomLoop->getHeader() == DomBB) {
+ // If the domtree walk reaches a loop with no preheader, return false.
+ if (!DomLoop->isLoopSimplifyForm())
+ return false;
+ // If we have already checked this loop nest, stop checking.
+ if (SimpleLoopNests.count(DomLoop))
+ break;
+ // If we have not already checked this loop nest, remember the loop
+ // header nearest to BB. The nearest loop may not contain BB.
+ if (!NearestLoop)
+ NearestLoop = DomLoop;
+ }
+ }
+ if (NearestLoop)
+ SimpleLoopNests.insert(NearestLoop);
+ return true;
+}
+
+/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
+/// and now we need to decide whether the user should use the preinc or post-inc
+/// value. If this user should use the post-inc version of the IV, return true.
+///
+/// Choosing wrong here can break dominance properties (if we choose to use the
+/// post-inc value when we cannot) or it can end up adding extra live-ranges to
+/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
+/// should use the post-inc value).
+static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
+ const Loop *L, DominatorTree *DT) {
+ // If the user is in the loop, use the preinc value.
+ if (L->contains(User))
+ return false;
+
+ BasicBlock *LatchBlock = L->getLoopLatch();
+ if (!LatchBlock)
+ return false;
+
+ // Ok, the user is outside of the loop. If it is dominated by the latch
+ // block, use the post-inc value.
+ if (DT->dominates(LatchBlock, User->getParent()))
+ return true;
+
+ // There is one case we have to be careful of: PHI nodes. These little guys
+ // can live in blocks that are not dominated by the latch block, but (since
+ // their uses occur in the predecessor block, not the block the PHI lives in)
+ // should still use the post-inc value. Check for this case now.
+ PHINode *PN = dyn_cast<PHINode>(User);
+ if (!PN || !Operand)
+ return false; // not a phi, not dominated by latch block.
+
+ // Look at all of the uses of Operand by the PHI node. If any use corresponds
+ // to a block that is not dominated by the latch block, give up and use the
+ // preincremented value.
+ for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ if (PN->getIncomingValue(i) == Operand &&
+ !DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
+ return false;
+
+ // Okay, all uses of Operand by PN are in predecessor blocks that really are
+ // dominated by the latch block. Use the post-incremented value.
+ return true;
+}
+
+/// AddUsersImpl - Inspect the specified instruction. If it is a
+/// reducible SCEV, recursively add its users to the IVUsesByStride set and
+/// return true. Otherwise, return false.
+bool IVUsers::AddUsersImpl(Instruction *I,
+ SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
+ const DataLayout &DL = I->getModule()->getDataLayout();
+
+ // Add this IV user to the Processed set before returning false to ensure that
+ // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
+ if (!Processed.insert(I).second)
+ return true; // Instruction already handled.
+
+ if (!SE->isSCEVable(I->getType()))
+ return false; // Void and FP expressions cannot be reduced.
+
+ // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
+ // pass to SCEVExpander. Expressions are not safe to expand if they represent
+ // operations that are not safe to speculate, namely integer division.
+ if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
+ return false;
+
+ // LSR is not APInt clean, do not touch integers bigger than 64-bits.
+ // Also avoid creating IVs of non-native types. For example, we don't want a
+ // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
+ uint64_t Width = SE->getTypeSizeInBits(I->getType());
+ if (Width > 64 || !DL.isLegalInteger(Width))
+ return false;
+
+ // Don't attempt to promote ephemeral values to indvars. They will be removed
+ // later anyway.
+ if (EphValues.count(I))
+ return false;
+
+ // Get the symbolic expression for this instruction.
+ const SCEV *ISE = SE->getSCEV(I);
+
+ // If we've come to an uninteresting expression, stop the traversal and
+ // call this a user.
+ if (!isInteresting(ISE, I, L, SE, LI))
+ return false;
+
+ SmallPtrSet<Instruction *, 4> UniqueUsers;
+ for (Use &U : I->uses()) {
+ Instruction *User = cast<Instruction>(U.getUser());
+ if (!UniqueUsers.insert(User).second)
+ continue;
+
+ // Do not infinitely recurse on PHI nodes.
+ if (isa<PHINode>(User) && Processed.count(User))
+ continue;
+
+ // Only consider IVUsers that are dominated by simplified loop
+ // headers. Otherwise, SCEVExpander will crash.
+ BasicBlock *UseBB = User->getParent();
+ // A phi's use is live out of its predecessor block.
+ if (PHINode *PHI = dyn_cast<PHINode>(User)) {
+ unsigned OperandNo = U.getOperandNo();
+ unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
+ UseBB = PHI->getIncomingBlock(ValNo);
+ }
+ if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
+ return false;
+
+ // Descend recursively, but not into PHI nodes outside the current loop.
+ // It's important to see the entire expression outside the loop to get
+ // choices that depend on addressing mode use right, although we won't
+ // consider references outside the loop in all cases.
+ // If User is already in Processed, we don't want to recurse into it again,
+ // but do want to record a second reference in the same instruction.
+ bool AddUserToIVUsers = false;
+ if (LI->getLoopFor(User->getParent()) != L) {
+ if (isa<PHINode>(User) || Processed.count(User) ||
+ !AddUsersImpl(User, SimpleLoopNests)) {
+ LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
+ << " OF SCEV: " << *ISE << '\n');
+ AddUserToIVUsers = true;
+ }
+ } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
+ LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
+ << " OF SCEV: " << *ISE << '\n');
+ AddUserToIVUsers = true;
+ }
+
+ if (AddUserToIVUsers) {
+ // Okay, we found a user that we cannot reduce.
+ IVStrideUse &NewUse = AddUser(User, I);
+ // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
+ // The regular return value here is discarded; instead of recording
+ // it, we just recompute it when we need it.
+ const SCEV *OriginalISE = ISE;
+
+ auto NormalizePred = [&](const SCEVAddRecExpr *AR) {
+ auto *L = AR->getLoop();
+ bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);
+ if (Result)
+ NewUse.PostIncLoops.insert(L);
+ return Result;
+ };
+
+ ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);
+
+ // PostIncNormalization effectively simplifies the expression under
+ // pre-increment assumptions. Those assumptions (no wrapping) might not
+ // hold for the post-inc value. Catch such cases by making sure the
+ // transformation is invertible.
+ if (OriginalISE != ISE) {
+ const SCEV *DenormalizedISE =
+ denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);
+
+ // If we normalized the expression, but denormalization doesn't give the
+ // original one, discard this user.
+ if (OriginalISE != DenormalizedISE) {
+ LLVM_DEBUG(dbgs()
+ << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
+ << *ISE << '\n');
+ IVUses.pop_back();
+ return false;
+ }
+ }
+ LLVM_DEBUG(if (SE->getSCEV(I) != ISE) dbgs()
+ << " NORMALIZED TO: " << *ISE << '\n');
+ }
+ }
+ return true;
+}
+
+bool IVUsers::AddUsersIfInteresting(Instruction *I) {
+ // SCEVExpander can only handle users that are dominated by simplified loop
+ // entries. Keep track of all loops that are only dominated by other simple
+ // loops so we don't traverse the domtree for each user.
+ SmallPtrSet<Loop*,16> SimpleLoopNests;
+
+ return AddUsersImpl(I, SimpleLoopNests);
+}
+
+IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
+ IVUses.push_back(new IVStrideUse(this, User, Operand));
+ return IVUses.back();
+}
+
+IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
+ ScalarEvolution *SE)
+ : L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
+ // Collect ephemeral values so that AddUsersIfInteresting skips them.
+ EphValues.clear();
+ CodeMetrics::collectEphemeralValues(L, AC, EphValues);
+
+ // Find all uses of induction variables in this loop, and categorize
+ // them by stride. Start by finding all of the PHI nodes in the header for
+ // this loop. If they are induction variables, inspect their uses.
+ for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
+ (void)AddUsersIfInteresting(&*I);
+}
+
+void IVUsers::print(raw_ostream &OS, const Module *M) const {
+ OS << "IV Users for loop ";
+ L->getHeader()->printAsOperand(OS, false);
+ if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
+ OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
+ }
+ OS << ":\n";
+
+ for (const IVStrideUse &IVUse : IVUses) {
+ OS << " ";
+ IVUse.getOperandValToReplace()->printAsOperand(OS, false);
+ OS << " = " << *getReplacementExpr(IVUse);
+ for (auto PostIncLoop : IVUse.PostIncLoops) {
+ OS << " (post-inc with loop ";
+ PostIncLoop->getHeader()->printAsOperand(OS, false);
+ OS << ")";
+ }
+ OS << " in ";
+ if (IVUse.getUser())
+ IVUse.getUser()->print(OS);
+ else
+ OS << "Printing <null> User";
+ OS << '\n';
+ }
+}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
+#endif
+
+void IVUsers::releaseMemory() {
+ Processed.clear();
+ IVUses.clear();
+}
+
+IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
+ initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());
+}
+
+void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<AssumptionCacheTracker>();
+ AU.addRequired<LoopInfoWrapperPass>();
+ AU.addRequired<DominatorTreeWrapperPass>();
+ AU.addRequired<ScalarEvolutionWrapperPass>();
+ AU.setPreservesAll();
+}
+
+bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
+ auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
+ *L->getHeader()->getParent());
+ auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+
+ IU.reset(new IVUsers(L, AC, LI, DT, SE));
+ return false;
+}
+
+void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
+ IU->print(OS, M);
+}
+
+void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
+
+/// getReplacementExpr - Return a SCEV expression which computes the
+/// value of the OperandValToReplace.
+const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
+ return SE->getSCEV(IU.getOperandValToReplace());
+}
+
+/// getExpr - Return the expression for the use.
+const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
+ return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
+ *SE);
+}
+
+static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
+ if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
+ if (AR->getLoop() == L)
+ return AR;
+ return findAddRecForLoop(AR->getStart(), L);
+ }
+
+ if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
+ for (const auto *Op : Add->operands())
+ if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
+ return AR;
+ return nullptr;
+ }
+
+ return nullptr;
+}
+
+const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
+ if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
+ return AR->getStepRecurrence(*SE);
+ return nullptr;
+}
+
+void IVStrideUse::transformToPostInc(const Loop *L) {
+ PostIncLoops.insert(L);
+}
+
+void IVStrideUse::deleted() {
+ // Remove this user from the list.
+ Parent->Processed.erase(this->getUser());
+ Parent->IVUses.erase(this);
+ // this now dangles!
+}
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