LLE 6/6: Add LoopLoadElimination pass

[anemet]

Summary

The goal of this pass is to perform store-to-load forwarding across the
backedge of a loop. E.g.:

for (i)
   A[i + 1] = A[i] + B[i]

=>

T = A[0]
for (i)
   T = T + B[i]
   A[i + 1] = T

The pass relies on loop dependence analysis via LoopAccessAnalisys to
find opportunities of loop-carried dependences with a distance of one
between a store and a load. Since it's using LoopAccessAnalysis, it was
easy to also add support for versioning away may-aliasing intervening
stores that would otherwise prevent this transformation.

This optimization is also performed by Load-PRE in GVN without the
option of multi-versioning. As was discussed with Daniel Berlin in
http://reviews.llvm.org/D9548, this is inferior to a more loop-aware
solution applied here. Hopefully, we will be able to remove some
complexity from GVN/MemorySSA as a consequence.

In the long run, we may want to extend this pass (or create a new one if
there is little overlap) to also eliminate loop-indepedent redundant
loads and store that *require* versioning due to may-aliasing
intervening stores/loads. I have some motivating cases for store
elimination. My plan right now is to wait for MemorySSA to come online
first rather than using memdep for this.

The main motiviation for this pass is the 456.hmmer loop in SPECint2006
where after distributing the original loop and vectorizing the top part,
we are left with the critical path exposed in the bottom loop. Being
able to promote the memory dependence into a register depedence (even
though the HW does perform store-to-load fowarding as well) results in a
major gain (~20%). This gain also transfers over to x86: it's
around 8-10%.

Right now the pass is off by default and can be enabled
with -enable-loop-load-elim. On the LNT testsuite, there are two
performance changes (negative number -> improvement):

1. -28% in Polybench/linear-algebra/solvers/dynprog: the length of the critical paths is reduced
2. +2% in Polybench/stencils/adi: Unfortunately, I couldn't reproduce this outside of LNT

The pass is scheduled after the loop vectorizer (which is after loop
distribution). The rational is to try to reuse LAA state, rather than
recomputing it. The order between LV and LLE is not critical because
normally LV does not touch scalar st->ld forwarding cases where
vectorizing would inhibit the CPU's st->ld forwarding to kick in.

LoopLoadElimination requires LAA to provide the full set of dependences
(including forward dependences). LAA is known to omit loop-independent
dependences in certain situations. The big comment before
removeDependencesFromMultipleStores explains why this should not occur
for the cases that we're interested in.

[• dberlin]

Thanks so much for taking this on.

Out of curiosity, does this actually catch the PRE load-forwarding cases that currently are tested?

That is, tests/Transform/GVN/pre-load.ll tests 9 and 10 i believe.

lib/Transforms/Scalar/LoopLoadElimination.cpp

65 ↗

(On Diff #36003)

This and the checks below it really look like checks to see if the load and store are really the same type, and not related to the dependence distance.
Can this be abstracted out somewhere?

IE it makes more sense to be calling Cand.isSameTypeAs(SE) && Cand.iisDependenceDistanceOfOne(SE)

100 ↗

(On Diff #36003)

This is an interesting restriction, and i understand why it is here.
I haven't thought too hard about it, but wonder if it could not be gotten rid of in a lot of cases with code or guard insertion, PRE style.

137 ↗

(On Diff #36003)

You also could do the type filtering you are doing in isDependenceDistanceOfOne here, no?

[anemet]

Thanks very much, Daniel, for looking at the patch.

Out of curiosity, does this actually catch the PRE load-forwarding cases that currently are tested?

That is, tests/Transform/GVN/pre-load.ll tests 9 and 10 i believe.

It catches the tests that check store-to-load forwarding. These are 6, 7 and 9.

Test 10 has both store-to-load and load-to-load forwarding. This pass catches st->ld but not ld->ld.

I guess in order to catch ld->ld as well we'd need LAA to also track RAR loop-carried dependences. That does not sound difficult.

lib/Transforms/Scalar/LoopLoadElimination.cpp

65 ↗

(On Diff #36003)

Actually after looking at this again, these checks are not necessary.

Since we only include store->load dependences of known types in the candidates, we can't have mismatching address space or type because that would yield an unknown dependence.

Sorry about the confusion, I'll change these into asserts.

Also as a follow-on, I am planning to make dependence distance an attribute of the dependence class and then this function would go away.

100 ↗

(On Diff #36003)

I think you're right. We should be able to enhance it further to also handle cases where to load is only partially redundant.

Needless to say, I am going for fully-redundant loads in this initial version.

137 ↗

(On Diff #36003)

See my explanation in isDependenceDistanceOfOne.

[anemet]

Address dberlin's comments

[rengolin]

In D13259#260230, @anemet wrote:

Test 10 has both store-to-load and load-to-load forwarding. This pass catches st->ld but not ld->ld.
I guess in order to catch ld->ld as well we'd need LAA to also track RAR loop-carried dependences. That does not sound difficult.

Hi Adam,

I believe this is the case I'm looking at, from Livermore Loops, for the RAR case:

a[k] = b[k] + b[k+1];

If the loop cannot be vectorized because of other problems, the LLE pass should be able to catch it, too, like the GVN does today, so best of both worlds.

Regarding this patch, I have some small comments, but overall, looks great!

I'll merge these changes locally and see if I can come up with a few RAR examples to transform.

cheers,
--renato

lib/Transforms/Scalar/LoopLoadElimination.cpp

11 ↗

(On Diff #36568)

Maybe not necessary to specify store-to-load, here, since this could also do load-to-load in the future?

153 ↗

(On Diff #36568)

I'm assuming to add LAL deps we'd need to add the case here, and map the type together, so that you can use the right struct (StoreToLoadForwardingCandidate or LoadToLoadForwardingCandidate).

Or maybe just extend to ForwardingCandidate and add a type to it would be simpler.

194 ↗

(On Diff #36568)

Is this related to D13255?

319 ↗

(On Diff #36568)

Silly nitpick: I originally thought this function would actually find the forward dependencies, not just find the candidates. Maybe changing the name to findPointers... would be more clear.

368 ↗

(On Diff #36568)

would this be meaningful?

assert(PtrSCEV)

[anemet]

In D13259#275336, @rengolin wrote:

Test 10 has both store-to-load and load-to-load forwarding. This pass catches st->ld but not ld->ld.
I guess in order to catch ld->ld as well we'd need LAA to also track RAR loop-carried dependences. That does not sound difficult.

Hi Adam,

I believe this is the case I'm looking at, from Livermore Loops, for the RAR case:

a[k] = b[k] + b[k+1];

If the loop cannot be vectorized because of other problems, the LLE pass should be able to catch it, too, like the GVN does today, so best of both worlds.

Regarding this patch, I have some small comments, but overall, looks great!

I'll merge these changes locally and see if I can come up with a few RAR examples to transform.

Yes, you're right. It looks like we will need RAR for your case as well.

Adam

lib/Transforms/Scalar/LoopLoadElimination.cpp

11 ↗

(On Diff #36568)

Makes sense.

153 ↗

(On Diff #36568)

Yeah probably the latter since they should be pretty similar. But I could be wrong.

194 ↗

(On Diff #36568)

Yes, the comment is meant to explain why the issue I discovered in D13255 does not affect LLE.

319 ↗

(On Diff #36568)

This comment may have shifted or I am just confused. Can you please elaborate what you mean?

368 ↗

(On Diff #36568)

cast has an implicit assert, no?

[rengolin]

lib/Transforms/Scalar/LoopLoadElimination.cpp

319 ↗

(On Diff #36568)

Sorry. It's about:

computePointersWrittenOnForwardingPath(Candidates);

You don't use it elsewhere, and it doesn't seem important on its own. I'm guessing you pulled it out because of collectMemchecks' size. This nitpick was just a personal preference to call it "find" or "collect" instead of "compute", since that's all it's doing. But it's not important.

368 ↗

(On Diff #36568)

ah, yes, of course!

[• dberlin]

I think, assuming all comments are addressed, that this looks fine to commit and iterate from there.

[anemet]

lib/Transforms/Scalar/LoopLoadElimination.cpp

319 ↗

(On Diff #36568)

Ah, got it, either of the two is better than "compute", thanks.

[rengolin]

While working on top of this patch, I noticed that not all DEBUG() messages are prefixed with "LLE:", which may confuse people using -debug-only=.

[• dberlin]

I'm confused.
AFAIK, we normally do *not* prefix the messages explicitly (i see this pass
does it in some places but not others, that should be changed to be not
prefixed, IMHO).

This pass has DEBUG_TYPE set to loop-load-elim at the top, so if you use
debug-only, you won't get them unless you specify debug-only=loop-load-elim.

I would suggest rather than prefix *anything*, anywhere, that if we want
prefixes, we just have the DEBUG macro output the debug type in front of
the message ;-)

Then it's automatic, not confusing, and clear where it came from.

[rengolin]

In D13259#278362, @dberlin wrote:

I would suggest rather than prefix *anything*, anywhere, that if we want
prefixes, we just have the DEBUG macro output the debug type in front of
the message ;-)

Sounds goo to me.

[hfinkel]

LGTM too, except for:

I don't think we check anywhere to disqualify volatile/atomic loads/stores, and we need to do that (especially for volatiles, since you're changing the number of accesses, and I'd need to think more-carefully about atomics).

[anemet]

In D13259#279193, @hfinkel wrote:

LGTM too, except for:

I don't think we check anywhere to disqualify volatile/atomic loads/stores, and we need to do that (especially for volatiles, since you're changing the number of accesses, and I'd need to think more-carefully about atomics).

LAA stops the analysis unless the memops are all isSimple(). Let me add a comment about this.

llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h

public:
   void clearDependences() { Dependences.clear(); }
 
   /// \brief The vector of memory access instructions.  The indices are used as
   /// instruction identifiers in the Dependence class.
   const SmallVectorImpl<Instruction *> &getMemoryInstructions() const {
     return InstMap;
   }
 
+  /// \brief Generate a mapping between the memory instructions and their
+  /// indices according to program order.
+  DenseMap<Instruction *, unsigned> generateInstructionOrderMap() const {
+    DenseMap<Instruction *, unsigned> OrderMap;
+
+    for (unsigned I = 0; I < InstMap.size(); ++I)
+      OrderMap[InstMap[I]] = I;
+
+    return OrderMap;
+  }
+
   /// \brief Find the set of instructions that read or write via \p Ptr.
   SmallVector<Instruction *, 4> getInstructionsForAccess(Value *Ptr,
                                                          bool isWrite) const;
 
 private:
   ScalarEvolution *SE;
   const Loop *InnermostLoop;
 
public:
   static bool
   arePointersInSamePartition(const SmallVectorImpl<int> &PtrToPartition,
                              unsigned PtrIdx1, unsigned PtrIdx2);
 
   /// \brief Decide whether we need to issue a run-time check for pointer at
   /// index \p I and \p J to prove their independence.
   bool needsChecking(unsigned I, unsigned J) const;
 
+  /// \brief Return PointerInfo for pointer at index \p PtrIdx.
+  const PointerInfo &getPointerInfo(unsigned PtrIdx) const {
+    return Pointers[PtrIdx];
+  }
+
 private:
   /// \brief Groups pointers such that a single memcheck is required
   /// between two different groups. This will clear the CheckingGroups vector
   /// and re-compute it. We will only group dependecies if \p UseDependencies
   /// is true, otherwise we will create a separate group for each pointer.
   void groupChecks(MemoryDepChecker::DepCandidates &DepCands,
                    bool UseDependencies);
 

llvm/trunk/include/llvm/InitializePasses.h

 void initializePlaceBackedgeSafepointsImplPass(PassRegistry&);
 void initializePlaceSafepointsPass(PassRegistry&);
 void initializeDwarfEHPreparePass(PassRegistry&);
 void initializeFloat2IntPass(PassRegistry&);
 void initializeLoopDistributePass(PassRegistry&);
 void initializeSjLjEHPreparePass(PassRegistry&);
 void initializeDemandedBitsPass(PassRegistry&);
 void initializeFuncletLayoutPass(PassRegistry &);
+void initializeLoopLoadEliminationPass(PassRegistry&);
 }
 
 #endif

llvm/trunk/include/llvm/Transforms/Scalar.h

 FunctionPass *createNaryReassociatePass();
 
 //===----------------------------------------------------------------------===//
 //
 // LoopDistribute - Distribute loops.
 //
 FunctionPass *createLoopDistributePass();
 
+//===----------------------------------------------------------------------===//
+//
+// LoopLoadElimination - Perform loop-aware load elimination.
+//
+FunctionPass *createLoopLoadEliminationPass();
+
 } // End llvm namespace
 
 #endif

llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp

static cl::opt<bool> EnableLoopDistribute(
     "enable-loop-distribute", cl::init(false), cl::Hidden,
     cl::desc("Enable the new, experimental LoopDistribution Pass"));
 
 static cl::opt<bool> EnableNonLTOGlobalsModRef(
     "enable-non-lto-gmr", cl::init(true), cl::Hidden,
     cl::desc(
         "Enable the GlobalsModRef AliasAnalysis outside of the LTO pipeline."));
 
+static cl::opt<bool> EnableLoopLoadElim(
+    "enable-loop-load-elim", cl::init(false), cl::Hidden,
+    cl::desc("Enable the new, experimental LoopLoadElimination Pass"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
     LibraryInfo = nullptr;
     Inliner = nullptr;
     DisableUnitAtATime = false;
     DisableUnrollLoops = false;
     BBVectorize = RunBBVectorization;
void PassManagerBuilder::populateModulePassManager(
   MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1));
 
   // Distribute loops to allow partial vectorization.  I.e. isolate dependences
   // into separate loop that would otherwise inhibit vectorization.
   if (EnableLoopDistribute)
     MPM.add(createLoopDistributePass());
 
   MPM.add(createLoopVectorizePass(DisableUnrollLoops, LoopVectorize));
+
+  // Eliminate loads by forwarding stores from the previous iteration to loads
+  // of the current iteration.
+  if (EnableLoopLoadElim)
+    MPM.add(createLoopLoadEliminationPass());
+
   // FIXME: Because of #pragma vectorize enable, the passes below are always
   // inserted in the pipeline, even when the vectorizer doesn't run (ex. when
   // on -O1 and no #pragma is found). Would be good to have these two passes
   // as function calls, so that we can only pass them when the vectorizer
   // changed the code.
   MPM.add(createInstructionCombiningPass());
   if (OptLevel > 1 && ExtraVectorizerPasses) {
     // At higher optimization levels, try to clean up any runtime overlap and

llvm/trunk/lib/Transforms/Scalar/CMakeLists.txt

add_llvm_library(LLVMScalarOpts
   JumpThreading.cpp
   LICM.cpp
   LoadCombine.cpp
   LoopDeletion.cpp
   LoopDistribute.cpp
   LoopIdiomRecognize.cpp
   LoopInstSimplify.cpp
   LoopInterchange.cpp
+  LoopLoadElimination.cpp
   LoopRerollPass.cpp
   LoopRotation.cpp
   LoopStrengthReduce.cpp
   LoopUnrollPass.cpp
   LoopUnswitch.cpp
   LowerAtomic.cpp
   LowerExpectIntrinsic.cpp
   MemCpyOptimizer.cpp

llvm/trunk/lib/Transforms/Scalar/LoopLoadElimination.cpp

+//===- LoopLoadElimination.cpp - Loop Load Elimination Pass ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implement a loop-aware load elimination pass.
+//
+// It uses LoopAccessAnalysis to identify loop-carried dependences with a
+// distance of one between stores and loads.  These form the candidates for the
+// transformation.  The source value of each store then propagated to the user
+// of the corresponding load.  This makes the load dead.
+//
+// The pass can also version the loop and add memchecks in order to prove that
+// may-aliasing stores can't change the value in memory before it's read by the
+// load.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/LoopAccessAnalysis.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/LoopVersioning.h"
+#include <forward_list>
+
+#define LLE_OPTION "loop-load-elim"
+#define DEBUG_TYPE LLE_OPTION
+
+using namespace llvm;
+
+static cl::opt<unsigned> CheckPerElim(
+    "runtime-check-per-loop-load-elim", cl::Hidden,
+    cl::desc("Max number of memchecks allowed per eliminated load on average"),
+    cl::init(1));
+
+STATISTIC(NumLoopLoadEliminted, "Number of loads eliminated by LLE");
+
+namespace {
+
+/// \brief Represent a store-to-forwarding candidate.
+struct StoreToLoadForwardingCandidate {
+  LoadInst *Load;
+  StoreInst *Store;
+
+  StoreToLoadForwardingCandidate(LoadInst *Load, StoreInst *Store)
+      : Load(Load), Store(Store) {}
+
+  /// \brief Return true if the dependence from the store to the load has a
+  /// distance of one.  E.g. A[i+1] = A[i]
+  bool isDependenceDistanceOfOne(ScalarEvolution *SE) const {
+    Value *LoadPtr = Load->getPointerOperand();
+    Value *StorePtr = Store->getPointerOperand();
+    Type *LoadPtrType = LoadPtr->getType();
+    Type *StorePtrType = StorePtr->getType();
+    Type *LoadType = LoadPtrType->getPointerElementType();
+
+    assert(LoadPtrType->getPointerAddressSpace() ==
+               StorePtrType->getPointerAddressSpace() &&
+           LoadType == StorePtrType->getPointerElementType() &&
+           "Should be a known dependence");
+
+    auto &DL = Load->getParent()->getModule()->getDataLayout();
+    unsigned TypeByteSize = DL.getTypeAllocSize(const_cast<Type *>(LoadType));
+
+    auto *LoadPtrSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(LoadPtr));
+    auto *StorePtrSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
+
+    // We don't need to check non-wrapping here because forward/backward
+    // dependence wouldn't be valid if these weren't monotonic accesses.
+    auto *Dist =
+        cast<SCEVConstant>(SE->getMinusSCEV(StorePtrSCEV, LoadPtrSCEV));
+    const APInt &Val = Dist->getValue()->getValue();
+    return Val.abs() == TypeByteSize;
+  }
+
+  Value *getLoadPtr() const { return Load->getPointerOperand(); }
+
+#ifndef NDEBUG
+  friend raw_ostream &operator<<(raw_ostream &OS,
+                                 const StoreToLoadForwardingCandidate &Cand) {
+    OS << *Cand.Store << " -->\n";
+    OS.indent(2) << *Cand.Load << "\n";
+    return OS;
+  }
+#endif
+};
+
+/// \brief Check if the store dominates all latches, so as long as there is no
+/// intervening store this value will be loaded in the next iteration.
+bool doesStoreDominatesAllLatches(BasicBlock *StoreBlock, Loop *L,
+                                  DominatorTree *DT) {
+  SmallVector<BasicBlock *, 8> Latches;
+  L->getLoopLatches(Latches);
+  return std::all_of(Latches.begin(), Latches.end(),
+                     [&](const BasicBlock *Latch) {
+                       return DT->dominates(StoreBlock, Latch);
+                     });
+}
+
+/// \brief The per-loop class that does most of the work.
+class LoadEliminationForLoop {
+public:
+  LoadEliminationForLoop(Loop *L, LoopInfo *LI, const LoopAccessInfo &LAI,
+                         DominatorTree *DT, ScalarEvolution *SE)
+      : L(L), LI(LI), LAI(LAI), DT(DT), SE(SE) {}
+
+  /// \brief Look through the loop-carried and loop-independent dependences in
+  /// this loop and find store->load dependences.
+  ///
+  /// Note that no candidate is returned if LAA has failed to analyze the loop
+  /// (e.g. if it's not bottom-tested, contains volatile memops, etc.)
+  std::forward_list<StoreToLoadForwardingCandidate>
+  findStoreToLoadDependences(const LoopAccessInfo &LAI) {
+    std::forward_list<StoreToLoadForwardingCandidate> Candidates;
+
+    const auto *Deps = LAI.getDepChecker().getDependences();
+    if (!Deps)
+      return Candidates;
+
+    // Find store->load dependences (consequently true dep).  Both lexically
+    // forward and backward dependences qualify.  Disqualify loads that have
+    // other unknown dependences.
+
+    SmallSet<Instruction *, 4> LoadsWithUnknownDepedence;
+
+    for (const auto &Dep : *Deps) {
+      Instruction *Source = Dep.getSource(LAI);
+      Instruction *Destination = Dep.getDestination(LAI);
+
+      if (Dep.Type == MemoryDepChecker::Dependence::Unknown) {
+        if (isa<LoadInst>(Source))
+          LoadsWithUnknownDepedence.insert(Source);
+        if (isa<LoadInst>(Destination))
+          LoadsWithUnknownDepedence.insert(Destination);
+        continue;
+      }
+
+      if (Dep.isBackward())
+        // Note that the designations source and destination follow the program
+        // order, i.e. source is always first.  (The direction is given by the
+        // DepType.)
+        std::swap(Source, Destination);
+      else
+        assert(Dep.isForward() && "Needs to be a forward dependence");
+
+      auto *Store = dyn_cast<StoreInst>(Source);
+      if (!Store)
+        continue;
+      auto *Load = dyn_cast<LoadInst>(Destination);
+      if (!Load)
+        continue;
+      Candidates.emplace_front(Load, Store);
+    }
+
+    if (!LoadsWithUnknownDepedence.empty())
+      Candidates.remove_if([&](const StoreToLoadForwardingCandidate &C) {
+        return LoadsWithUnknownDepedence.count(C.Load);
+      });
+
+    return Candidates;
+  }
+
+  /// \brief Return the index of the instruction according to program order.
+  unsigned getInstrIndex(Instruction *Inst) {
+    auto I = InstOrder.find(Inst);
+    assert(I != InstOrder.end() && "No index for instruction");
+    return I->second;
+  }
+
+  /// \brief If a load has multiple candidates associated (i.e. different
+  /// stores), it means that it could be forwarding from multiple stores
+  /// depending on control flow.  Remove these candidates.
+  ///
+  /// Here, we rely on LAA to include the relevant loop-independent dependences.
+  /// LAA is known to omit these in the very simple case when the read and the
+  /// write within an alias set always takes place using the *same* pointer.
+  ///
+  /// However, we know that this is not the case here, i.e. we can rely on LAA
+  /// to provide us with loop-independent dependences for the cases we're
+  /// interested.  Consider the case for example where a loop-independent
+  /// dependece S1->S2 invalidates the forwarding S3->S2.
+  ///
+  ///         A[i]   = ...   (S1)
+  ///         ...    = A[i]  (S2)
+  ///         A[i+1] = ...   (S3)
+  ///
+  /// LAA will perform dependence analysis here because there are two
+  /// *different* pointers involved in the same alias set (&A[i] and &A[i+1]).
+  void removeDependencesFromMultipleStores(
+      std::forward_list<StoreToLoadForwardingCandidate> &Candidates) {
+    // If Store is nullptr it means that we have multiple stores forwarding to
+    // this store.
+    typedef DenseMap<LoadInst *, const StoreToLoadForwardingCandidate *>
+        LoadToSingleCandT;
+    LoadToSingleCandT LoadToSingleCand;
+
+    for (const auto &Cand : Candidates) {
+      bool NewElt;
+      LoadToSingleCandT::iterator Iter;
+
+      std::tie(Iter, NewElt) =
+          LoadToSingleCand.insert(std::make_pair(Cand.Load, &Cand));
+      if (!NewElt) {
+        const StoreToLoadForwardingCandidate *&OtherCand = Iter->second;
+        // Already multiple stores forward to this load.
+        if (OtherCand == nullptr)
+          continue;
+
+        // Handle the very basic of case when the two stores are in the same
+        // block so deciding which one forwards is easy.  The later one forwards
+        // as long as they both have a dependence distance of one to the load.
+        if (Cand.Store->getParent() == OtherCand->Store->getParent() &&
+            Cand.isDependenceDistanceOfOne(SE) &&
+            OtherCand->isDependenceDistanceOfOne(SE)) {
+          // They are in the same block, the later one will forward to the load.
+          if (getInstrIndex(OtherCand->Store) < getInstrIndex(Cand.Store))
+            OtherCand = &Cand;
+        } else
+          OtherCand = nullptr;
+      }
+    }
+
+    Candidates.remove_if([&](const StoreToLoadForwardingCandidate &Cand) {
+      if (LoadToSingleCand[Cand.Load] != &Cand) {
+        DEBUG(dbgs() << "Removing from candidates: \n" << Cand
+                     << "  The load may have multiple stores forwarding to "
+                     << "it\n");
+        return true;
+      }
+      return false;
+    });
+  }
+
+  /// \brief Given two pointers operations by their RuntimePointerChecking
+  /// indices, return true if they require an alias check.
+  ///
+  /// We need a check if one is a pointer for a candidate load and the other is
+  /// a pointer for a possibly intervening store.
+  bool needsChecking(unsigned PtrIdx1, unsigned PtrIdx2,
+                     const SmallSet<Value *, 4> &PtrsWrittenOnFwdingPath,
+                     const std::set<Value *> &CandLoadPtrs) {
+    Value *Ptr1 =
+        LAI.getRuntimePointerChecking()->getPointerInfo(PtrIdx1).PointerValue;
+    Value *Ptr2 =
+        LAI.getRuntimePointerChecking()->getPointerInfo(PtrIdx2).PointerValue;
+    return ((PtrsWrittenOnFwdingPath.count(Ptr1) && CandLoadPtrs.count(Ptr2)) ||
+            (PtrsWrittenOnFwdingPath.count(Ptr2) && CandLoadPtrs.count(Ptr1)));
+  }
+
+  /// \brief Return pointers that are possibly written to on the path from a
+  /// forwarding store to a load.
+  ///
+  /// These pointers need to be alias-checked against the forwarding candidates.
+  SmallSet<Value *, 4> findPointersWrittenOnForwardingPath(
+      const SmallVectorImpl<StoreToLoadForwardingCandidate> &Candidates) {
+    // From FirstStore to LastLoad neither of the elimination candidate loads
+    // should overlap with any of the stores.
+    //
+    // E.g.:
+    //
+    // st1 C[i]
+    // ld1 B[i] <-------,
+    // ld0 A[i] <----,  |              * LastLoad
+    // ...           |  |
+    // st2 E[i]      |  |
+    // st3 B[i+1] -- | -'              * FirstStore
+    // st0 A[i+1] ---'
+    // st4 D[i]
+    //
+    // st0 forwards to ld0 if the accesses in st4 and st1 don't overlap with
+    // ld0.
+
+    LoadInst *LastLoad =
+        std::max_element(Candidates.begin(), Candidates.end(),
+                         [&](const StoreToLoadForwardingCandidate &A,
+                             const StoreToLoadForwardingCandidate &B) {
+                           return getInstrIndex(A.Load) < getInstrIndex(B.Load);
+                         })
+            ->Load;
+    StoreInst *FirstStore =
+        std::min_element(Candidates.begin(), Candidates.end(),
+                         [&](const StoreToLoadForwardingCandidate &A,
+                             const StoreToLoadForwardingCandidate &B) {
+                           return getInstrIndex(A.Store) <
+                                  getInstrIndex(B.Store);
+                         })
+            ->Store;
+
+    // We're looking for stores after the first forwarding store until the end
+    // of the loop, then from the beginning of the loop until the last
+    // forwarded-to load.  Collect the pointer for the stores.
+    SmallSet<Value *, 4> PtrsWrittenOnFwdingPath;
+
+    auto InsertStorePtr = [&](Instruction *I) {
+      if (auto *S = dyn_cast<StoreInst>(I))
+        PtrsWrittenOnFwdingPath.insert(S->getPointerOperand());
+    };
+    const auto &MemInstrs = LAI.getDepChecker().getMemoryInstructions();
+    std::for_each(MemInstrs.begin() + getInstrIndex(FirstStore) + 1,
+                  MemInstrs.end(), InsertStorePtr);
+    std::for_each(MemInstrs.begin(), &MemInstrs[getInstrIndex(LastLoad)],
+                  InsertStorePtr);
+
+    return PtrsWrittenOnFwdingPath;
+  }
+
+  /// \brief Determine the pointer alias checks to prove that there are no
+  /// intervening stores.
+  SmallVector<RuntimePointerChecking::PointerCheck, 4> collectMemchecks(
+      const SmallVectorImpl<StoreToLoadForwardingCandidate> &Candidates) {
+
+    SmallSet<Value *, 4> PtrsWrittenOnFwdingPath =
+        findPointersWrittenOnForwardingPath(Candidates);
+
+    // Collect the pointers of the candidate loads.
+    // FIXME: SmallSet does not work with std::inserter.
+    std::set<Value *> CandLoadPtrs;
+    std::transform(Candidates.begin(), Candidates.end(),
+                   std::inserter(CandLoadPtrs, CandLoadPtrs.begin()),
+                   std::mem_fn(&StoreToLoadForwardingCandidate::getLoadPtr));
+
+    const auto &AllChecks = LAI.getRuntimePointerChecking()->getChecks();
+    SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks;
+
+    std::copy_if(AllChecks.begin(), AllChecks.end(), std::back_inserter(Checks),
+                 [&](const RuntimePointerChecking::PointerCheck &Check) {
+                   for (auto PtrIdx1 : Check.first->Members)
+                     for (auto PtrIdx2 : Check.second->Members)
+                       if (needsChecking(PtrIdx1, PtrIdx2,
+                                         PtrsWrittenOnFwdingPath, CandLoadPtrs))
+                         return true;
+                   return false;
+                 });
+
+    DEBUG(dbgs() << "\nPointer Checks (count: " << Checks.size() << "):\n");
+    DEBUG(LAI.getRuntimePointerChecking()->printChecks(dbgs(), Checks));
+
+    return Checks;
+  }
+
+  /// \brief Perform the transformation for a candidate.
+  void
+  propagateStoredValueToLoadUsers(const StoreToLoadForwardingCandidate &Cand,
+                                  SCEVExpander &SEE) {
+    //
+    // loop:
+    //      %x = load %gep_i
+    //         = ... %x
+    //      store %y, %gep_i_plus_1
+    //
+    // =>
+    //
+    // ph:
+    //      %x.initial = load %gep_0
+    // loop:
+    //      %x.storeforward = phi [%x.initial, %ph] [%y, %loop]
+    //      %x = load %gep_i            <---- now dead
+    //         = ... %x.storeforward
+    //      store %y, %gep_i_plus_1
+
+    Value *Ptr = Cand.Load->getPointerOperand();
+    auto *PtrSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(Ptr));
+    auto *PH = L->getLoopPreheader();
+    Value *InitialPtr = SEE.expandCodeFor(PtrSCEV->getStart(), Ptr->getType(),
+                                          PH->getTerminator());
+    Value *Initial =
+        new LoadInst(InitialPtr, "load_initial", PH->getTerminator());
+    PHINode *PHI = PHINode::Create(Initial->getType(), 2, "store_forwarded",
+                                   L->getHeader()->begin());
+    PHI->addIncoming(Initial, PH);
+    PHI->addIncoming(Cand.Store->getOperand(0), L->getLoopLatch());
+
+    Cand.Load->replaceAllUsesWith(PHI);
+  }
+
+  /// \brief Top-level driver for each loop: find store->load forwarding
+  /// candidates, add run-time checks and perform transformation.
+  bool processLoop() {
+    DEBUG(dbgs() << "\nIn \"" << L->getHeader()->getParent()->getName()
+                 << "\" checking " << *L << "\n");
+    // Look for store-to-load forwarding cases across the
+    // backedge. E.g.:
+    //
+    // loop:
+    //      %x = load %gep_i
+    //         = ... %x
+    //      store %y, %gep_i_plus_1
+    //
+    // =>
+    //
+    // ph:
+    //      %x.initial = load %gep_0
+    // loop:
+    //      %x.storeforward = phi [%x.initial, %ph] [%y, %loop]
+    //      %x = load %gep_i            <---- now dead
+    //         = ... %x.storeforward
+    //      store %y, %gep_i_plus_1
+
+    // First start with store->load dependences.
+    auto StoreToLoadDependences = findStoreToLoadDependences(LAI);
+    if (StoreToLoadDependences.empty())
+      return false;
+
+    // Generate an index for each load and store according to the original
+    // program order.  This will be used later.
+    InstOrder = LAI.getDepChecker().generateInstructionOrderMap();
+
+    // To keep things simple for now, remove those where the load is potentially
+    // fed by multiple stores.
+    removeDependencesFromMultipleStores(StoreToLoadDependences);
+    if (StoreToLoadDependences.empty())
+      return false;
+
+    // Filter the candidates further.
+    SmallVector<StoreToLoadForwardingCandidate, 4> Candidates;
+    unsigned NumForwarding = 0;
+    for (const StoreToLoadForwardingCandidate Cand : StoreToLoadDependences) {
+      DEBUG(dbgs() << "Candidate " << Cand);
+      // Make sure that the stored values is available everywhere in the loop in
+      // the next iteration.
+      if (!doesStoreDominatesAllLatches(Cand.Store->getParent(), L, DT))
+        continue;
+
+      // Check whether the SCEV difference is the same as the induction step,
+      // thus we load the value in the next iteration.
+      if (!Cand.isDependenceDistanceOfOne(SE))
+        continue;
+
+      ++NumForwarding;
+      DEBUG(dbgs()
+            << NumForwarding
+            << ". Valid store-to-load forwarding across the loop backedge\n");
+      Candidates.push_back(Cand);
+    }
+    if (Candidates.empty())
+      return false;
+
+    // Check intervening may-alias stores.  These need runtime checks for alias
+    // disambiguation.
+    SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks =
+        collectMemchecks(Candidates);
+
+    // Too many checks are likely to outweigh the benefits of forwarding.
+    if (Checks.size() > Candidates.size() * CheckPerElim) {
+      DEBUG(dbgs() << "Too many run-time checks needed.\n");
+      return false;
+    }
+
+    // Point of no-return, start the transformation.  First, version the loop if
+    // necessary.
+    if (!Checks.empty()) {
+      LoopVersioning LV(std::move(Checks), LAI, L, LI, DT);
+      LV.versionLoop();
+    }
+
+    // Next, propagate the value stored by the store to the users of the load.
+    // Also for the first iteration, generate the initial value of the load.
+    SCEVExpander SEE(*SE, L->getHeader()->getModule()->getDataLayout(),
+                     "storeforward");
+    for (const auto &Cand : Candidates)
+      propagateStoredValueToLoadUsers(Cand, SEE);
+    NumLoopLoadEliminted += NumForwarding;
+
+    return true;
+  }
+
+private:
+  Loop *L;
+
+  /// \brief Maps the load/store instructions to their index according to
+  /// program order.
+  DenseMap<Instruction *, unsigned> InstOrder;
+
+  // Analyses used.
+  LoopInfo *LI;
+  const LoopAccessInfo &LAI;
+  DominatorTree *DT;
+  ScalarEvolution *SE;
+};
+
+/// \brief The pass.  Most of the work is delegated to the per-loop
+/// LoadEliminationForLoop class.
+class LoopLoadElimination : public FunctionPass {
+public:
+  LoopLoadElimination() : FunctionPass(ID) {
+    initializeLoopLoadEliminationPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override {
+    auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+    auto *LAA = &getAnalysis<LoopAccessAnalysis>();
+    auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+    auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+
+    // Build up a worklist of inner-loops to vectorize. This is necessary as the
+    // act of distributing a loop creates new loops and can invalidate iterators
+    // across the loops.
+    SmallVector<Loop *, 8> Worklist;
+
+    for (Loop *TopLevelLoop : *LI)
+      for (Loop *L : depth_first(TopLevelLoop))
+        // We only handle inner-most loops.
+        if (L->empty())
+          Worklist.push_back(L);
+
+    // Now walk the identified inner loops.
+    bool Changed = false;
+    for (Loop *L : Worklist) {
+      const LoopAccessInfo &LAI = LAA->getInfo(L, ValueToValueMap());
+      // The actual work is performed by LoadEliminationForLoop.
+      LoadEliminationForLoop LEL(L, LI, LAI, DT, SE);
+      Changed |= LEL.processLoop();
+    }
+
+    // Process each loop nest in the function.
+    return Changed;
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<LoopInfoWrapperPass>();
+    AU.addPreserved<LoopInfoWrapperPass>();
+    AU.addRequired<LoopAccessAnalysis>();
+    AU.addRequired<ScalarEvolutionWrapperPass>();
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addPreserved<DominatorTreeWrapperPass>();
+  }
+
+  static char ID;
+};
+}
+
+char LoopLoadElimination::ID;
+static const char LLE_name[] = "Loop Load Elimination";
+
+INITIALIZE_PASS_BEGIN(LoopLoadElimination, LLE_OPTION, LLE_name, false, false)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopAccessAnalysis)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(LoopLoadElimination, LLE_OPTION, LLE_name, false, false)
+
+namespace llvm {
+FunctionPass *createLoopLoadEliminationPass() {
+  return new LoopLoadElimination();
+}
+}

llvm/trunk/lib/Transforms/Scalar/Scalar.cpp

void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeSeparateConstOffsetFromGEPPass(Registry);
   initializeSpeculativeExecutionPass(Registry);
   initializeStraightLineStrengthReducePass(Registry);
   initializeLoadCombinePass(Registry);
   initializePlaceBackedgeSafepointsImplPass(Registry);
   initializePlaceSafepointsPass(Registry);
   initializeFloat2IntPass(Registry);
   initializeLoopDistributePass(Registry);
+  initializeLoopLoadEliminationPass(Registry);
 }
 
 void LLVMInitializeScalarOpts(LLVMPassRegistryRef R) {
   initializeScalarOpts(*unwrap(R));
 }
 
 void LLVMAddAggressiveDCEPass(LLVMPassManagerRef PM) {
   unwrap(PM)->add(createAggressiveDCEPass());

llvm/trunk/test/Transforms/LoopLoadElim/backward.ll

+; RUN: opt -loop-load-elim -S < %s | FileCheck %s
+
+; Simple st->ld forwarding derived from a lexical backward dep.
+;
+;   for (unsigned i = 0; i < 100; i++)
+;     A[i+1] = A[i] + B[i];
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @f(i32* noalias nocapture %A, i32* noalias nocapture readonly %B, i64 %N) {
+entry:
+; CHECK: %load_initial = load i32, i32* %A
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK: %store_forwarded = phi i32 [ %load_initial, %entry ], [ %add, %for.body ]
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+  %load = load i32, i32* %arrayidx, align 4
+  %arrayidx2 = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  %load_1 = load i32, i32* %arrayidx2, align 4
+; CHECK: %add = add i32 %load_1, %store_forwarded
+  %add = add i32 %load_1, %load
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %arrayidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  store i32 %add, i32* %arrayidx_next, align 4
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}

llvm/trunk/test/Transforms/LoopLoadElim/def-store-before-load.ll

+; RUN: opt -loop-load-elim -S < %s | FileCheck %s
+
+;  No loop-carried forwarding: The intervening store to A[i] kills the stored
+;  value from the previous iteration.
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     A[i] = 1;
+;     A[i+1] = A[i] + B[i];
+;   }
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @f(i32* noalias nocapture %A, i32* noalias nocapture readonly %B, i64 %N) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK-NOT: %store_forwarded
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+  store i32 1, i32* %arrayidx, align 4
+  %a = load i32, i32* %arrayidx, align 4
+  %arrayidxB = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  %b = load i32, i32* %arrayidxB, align 4
+; CHECK: %add = add i32 %b, %a
+  %add = add i32 %b, %a
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %arrayidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  store i32 %add, i32* %arrayidx_next, align 4
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}

llvm/trunk/test/Transforms/LoopLoadElim/forward.ll

+; RUN: opt -loop-load-elim -S < %s | FileCheck %s
+
+; Simple st->ld forwarding derived from a lexical forwrad dep.
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     A[i+1] = B[i] + 2;
+;     C[i] = A[i] * 2;
+;   }
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @f(i32* %A, i32* %B, i32* %C, i64 %N) {
+
+; CHECK:   for.body.lver.memcheck:
+; CHECK:     %found.conflict{{.*}} =
+; CHECK-NOT: %found.conflict{{.*}} =
+
+entry:
+; for.body.ph:
+; CHECK: %load_initial = load i32, i32* %A
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK: %store_forwarded = phi i32 [ %load_initial, %for.body.ph ], [ %a_p1, %for.body ]
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+
+  %Aidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  %Bidx = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  %Cidx = getelementptr inbounds i32, i32* %C, i64 %indvars.iv
+  %Aidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+
+  %b = load i32, i32* %Bidx, align 4
+  %a_p1 = add i32 %b, 2
+  store i32 %a_p1, i32* %Aidx_next, align 4
+
+  %a = load i32, i32* %Aidx, align 4
+; CHECK: %c = mul i32 %store_forwarded, 2
+  %c = mul i32 %a, 2
+  store i32 %c, i32* %Cidx, align 4
+
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}

llvm/trunk/test/Transforms/LoopLoadElim/memcheck.ll

+; RUN: opt -loop-load-elim -S < %s | FileCheck %s
+; RUN: opt -loop-load-elim -S -runtime-check-per-loop-load-elim=2 < %s | FileCheck %s --check-prefix=AGGRESSIVE
+
+; This needs two pairs of memchecks (A * { C, D }) for a single load
+; elimination which is considered to expansive by default.
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     A[i+1] = B[i] + 2;
+;     C[i] = A[i] * 2;
+;     D[i] = 2;
+;   }
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @f(i32*  %A, i32*  %B, i32*  %C, i64 %N, i32* %D) {
+entry:
+  br label %for.body
+
+; AGGRESSIVE: for.body.lver.memcheck:
+; AGGRESSIVE: %found.conflict{{.*}} =
+; AGGRESSIVE: %found.conflict{{.*}} =
+; AGGRESSIVE-NOT: %found.conflict{{.*}} =
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK-NOT: %store_forwarded =
+; AGGRESSIVE: %store_forwarded =
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+
+  %Aidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  %Bidx = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  %Cidx = getelementptr inbounds i32, i32* %C, i64 %indvars.iv
+  %Aidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+  %Didx = getelementptr inbounds i32, i32* %D, i64 %indvars.iv
+
+  %b = load i32, i32* %Bidx, align 4
+  %a_p1 = add i32 %b, 2
+  store i32 %a_p1, i32* %Aidx_next, align 4
+
+  %a = load i32, i32* %Aidx, align 4
+; CHECK: %c = mul i32 %a, 2
+; AGGRESSIVE: %c = mul i32 %store_forwarded, 2
+  %c = mul i32 %a, 2
+  store i32 %c, i32* %Cidx, align 4
+  store i32 2, i32* %Didx, align 4
+
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}

llvm/trunk/test/Transforms/LoopLoadElim/multiple-stores-same-block.ll

+; RUN: opt -basicaa -loop-load-elim -S < %s | FileCheck %s
+
+; In this case the later store forward to the load:
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     B[i] = A[i] + 1;
+;     A[i+1] = C[i] + 2;
+;     A[i+1] = D[i] + 3;
+;   }
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @f(i32* noalias nocapture %A, i32* noalias nocapture readonly %B,
+               i32* noalias nocapture %C, i32* noalias nocapture readonly %D,
+               i64 %N) {
+entry:
+; CHECK: %load_initial = load i32, i32* %A
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK: %store_forwarded = phi i32 [ %load_initial, %entry ], [ %addD, %for.body ]
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidxA = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+  %loadA = load i32, i32* %arrayidxA, align 4
+; CHECK: %addA = add i32 %store_forwarded, 1
+  %addA = add i32 %loadA, 1
+
+  %arrayidxB = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  store i32 %addA, i32* %arrayidxB, align 4
+
+  %arrayidxC = getelementptr inbounds i32, i32* %C, i64 %indvars.iv
+  %loadC = load i32, i32* %arrayidxC, align 4
+  %addC = add i32 %loadC, 2
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %arrayidxA_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  store i32 %addC, i32* %arrayidxA_next, align 4
+
+  %arrayidxD = getelementptr inbounds i32, i32* %D, i64 %indvars.iv
+  %loadD = load i32, i32* %arrayidxD, align 4
+  %addD = add i32 %loadD, 3
+  store i32 %addD, i32* %arrayidxA_next, align 4
+
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}

llvm/trunk/test/Transforms/LoopLoadElim/unknown-dep.ll

+; RUN: opt -basicaa -loop-load-elim -S < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; Give up in the presence of unknown deps. Here, the different strides result
+; in unknown dependence:
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     A[i+1] = B[i] + 2;
+;     A[2*i] = C[i] + 2;
+;     D[i] = A[i] + 2;
+;   }
+
+define void @f(i32* noalias %A, i32* noalias %B, i32* noalias %C,
+               i32* noalias %D, i64 %N) {
+
+entry:
+; for.body.ph:
+; CHECK-NOT: %load_initial =
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+; CHECK-NOT: %store_forwarded =
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+
+  %Aidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
+  %Bidx = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
+  %Cidx = getelementptr inbounds i32, i32* %C, i64 %indvars.iv
+  %Didx = getelementptr inbounds i32, i32* %D, i64 %indvars.iv
+  %Aidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
+  %indvars.m2 = mul nuw nsw i64 %indvars.iv, 2
+  %A2idx = getelementptr inbounds i32, i32* %A, i64 %indvars.m2
+
+  %b = load i32, i32* %Bidx, align 4
+  %a_p1 = add i32 %b, 2
+  store i32 %a_p1, i32* %Aidx_next, align 4
+
+  %c = load i32, i32* %Cidx, align 4
+  %a_m2 = add i32 %c, 2
+  store i32 %a_m2, i32* %A2idx, align 4
+
+  %a = load i32, i32* %Aidx, align 4
+; CHECK-NOT: %d = add i32 %store_forwarded, 2
+; CHECK: %d = add i32 %a, 2
+  %d = add i32 %a, 2
+  store i32 %d, i32* %Didx, align 4
+
+  %exitcond = icmp eq i64 %indvars.iv.next, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}