[CGP] Split large data structres to sink more GEPs

[haicheng]

Summary

Accessing the members of a large data structures needs a lot of GEPs which usually have large offsets due to the size of the underlying data structure. If the offsets are too large to fit into the r+i addressing mode, these GEPs cannot be sunk to their users' blocks and many extra registers are needed then to carry the values of these GEPs.

This patch tries to split a large data struct starting from %base like the following.

Before:

BB0:
  %base     =

BB1:
  %gep0     = gep %base, off0
  %gep1     = gep %base, off1
  %gep2     = gep %base, off2

BB2:
  %load1    = load %gep0
  %load2    = load %gep1
  %load3    = load %gep2

After:

BB0:
  %base     =
  %new_base = gep %base, off0

BB1:
  %new_gep0 = %new_base
  %new_gep1 = gep %new_base, off1 - off0
  %new_gep2 = gep %new_base, off2 - off0

BB2:
  %load1    = load i32, i32* %new_gep0
  %load2    = load i32, i32* %new_gep1
  %load3    = load i32, i32* %new_gep2

In the above example, the struct is split into two parts. The first part still starts from %base and the second part starts from %new_base. After the splitting, %new_gep1 and %new_gep2 have smaller offsets and then can be sunk to BB2 and folded into their users.

The algorithm to split data structure is simple and very similar to the work of merging SExts. First, it collects GEPs that have large offsets when iterating the blocks. Second, it splits the underlying data structures and updates the collected GEPs to use smaller offsets.

The code size and performance results of spec20xx is listed as below

	Code Size (%)	Performance (%)
	(- is smaller)	(+ is faster)
spec2006/bzip2	-3.47	+2.89
spec2017/imagick	-0.01	+1.02
spec2000/mesa	-0.76	+0.71
spec2017/x264	-0.91	+0.39
spec2017/leela	-0.01	+0.31
spec2000/ammp	-0.04	+0.17
spec2017/parest	0	0
spec2017/xalancbmk	0	-0.13
spec2017/blender	0	-0.18
spec2006/xalancbmk	0	-0.51

[efriedma]

lib/CodeGen/CodeGenPrepare.cpp

3751

I haven't really reviewed the whole patch closely, so I might be missing something, but why does it matter that the element type is a struct, as opposed to something else large like an array?

[haicheng]

lib/CodeGen/CodeGenPrepare.cpp

3751

Nothing special, I am just being conservative in the beginning. I find that at this stage, LLVM can bitcast arbitrary pointers to i8* and do random stuffs. I am thinking supporting other large data structures as the next step.

[tobiasvk]

lib/CodeGen/CodeGenPrepare.cpp

429

Drive-by comment: it's not a good idea to modify a cl::opt here as this will create (benign) races between multiple backend threads in ThinLTO.

[haicheng]

I made two changes to address the comments.

• Add the support of splitting large arrays.

• Introduce a target hook to enable/disable the change.

I am also welcome to any alternative implementation suggestions.

[mcrosier]

A few quick comments. Will follow up with a more complete review later this week.

lib/CodeGen/CodeGenPrepare.cpp

3743

Perhaps something like:

// Record GEPs with a non-zero offsets as candidates for splitting in the event that the offset cannot fit into the r+i addressing mode.

3746

Add isa<GetElementPtrInst>(AddrInst) check?

3755

You could reduce the indent here by putting a 'isa<GetElementPtrInst>(AddrInst) in the outer most if statement and then make this a cast<>. See comment above.

3756

Also, I need some clarification here. The first check

(BaseI && GEP->getParent() != BaseI->getParent())

seems to be inline with the comments below, but I don't completely follow the second check (i.e., checking that the PHI is not in the entry block, if BaseI is null).

[efriedma]

lib/Target/AArch64/AArch64ISelLowering.cpp

8012

The base of a GEP is always a pointer.

8014

I'm still not sure what the purpose of this check is; why does it matter how many dimensions an array has?

[haicheng]

• Completely remove the check of base types. So, any large data structure is supported.

• Clarify some comments.

Thank you for the review.

[haicheng]

Made some changes to improve the readability.

[haicheng]

Kindly ping. I am appreciated to take any advice.

[haicheng]

Kindly Ping #2

[haicheng]

Kindly Ping #3

[junbuml]

Just minor nits inlined.

lib/CodeGen/CodeGenPrepare.cpp

3743

a non-zero offsets -> a non-zero offset

3749

It seems that you can move this check to "else" by changing "else" to "else if".

4293

You may want to have a new variable for GetElementPtrInst instead of doing LargeOffsetGEP.first several times.

4295

!NewGEPBases.count(LargeOffsetGEP.first)

4297

I'm not clear about this comment. Can you clarify it little bit?

typo: spllit

lib/Target/AArch64/AArch64ISelLowering.cpp

8011

Should we have BaseType as parameter ?

[haicheng]

Address Jun's comments.

Thank you very much for taking a look.

[skatkov]

lib/CodeGen/CodeGenPrepare.cpp

3742

Why do you need the check isa<GetElementPtrInst>(AddrInst)?
You are in "case Instruction::GetElementPtr" basing on "switch (Opcode) {" where Opcode is actually AddrInst->getOpcode().
So to me it is redundant...

3761

getParent()->getParent() => getFunction()

4802

Please add a comment for nontrivial comparison function.

4856

I wonder if you sort the an array anyway why not to use set like data-structure to keep them and avoid erase of unique elements?
Is there any hidden sense for that?

4868

How can we get GEP == nullptr here?

4912

getParent()->getParent() => getFunction()

[haicheng]

Update to address Serguei's comments. Thank you for taking a look.

lib/CodeGen/CodeGenPrepare.cpp

3742

AddrInst is a user. It can be a GetElementPtrConstantExpr which is not supported by CGP yet.

4856

I need a data structure that can be iterated in a sorted order (ascending by offsets) and all elements (pointers to GEPs) are unique. I think the data in the set are unique but I cannot access them in the order that I want If I use a set.

[haicheng]

Kindly Ping

[efriedma]

lib/CodeGen/CodeGenPrepare.cpp

2545

AssertingVH<GetElementPtrInst>

4866

This is going to visit GEPs with the same offset in non-deterministic order. I guess it might not be that important, but it'll probably mess with the use-lists, which could affect some later pass. I would rather stay on the safe side and sort GEPs with the same offset based on the order they were added to the map, or some other deterministic ordering.

4873

The result type of the GEP might not be the type of the memory access... but I don't know how you'd get the right type off the top of my head, and it might not be important in practice. Maybe worth noting with a comment, though.

4907

I think you have to be a bit more careful here to avoid inserting a GEP into a block with a catchswitch. (This is Windows-only exception-handling, so maybe difficult to trigger.)

[haicheng]

Address Eli's comments. Thank you very much, Eli.

[haicheng]

lib/CodeGen/CodeGenPrepare.cpp

4907

Now I check if the blocks contain catchswitch when collecting GEPs.

[efriedma]

This is looking good. Just a few more small comments.

lib/CodeGen/CodeGenPrepare.cpp

278

Probably these maps should also use AssertingVH.

3753

I'm pretty sure it's impossible for BaseI to be a BinaryOperator, given it's a pointer.

[haicheng]

Address Eli's comments. Thank you.

[efriedma]

lib/CodeGen/CodeGenPrepare.cpp

278

By "these maps", I meant NewGEPBases and LargeOffsetGEPMap too.

[haicheng]

Add more AssertingVH.

[efriedma]

lib/CodeGen/CodeGenPrepare.cpp

278

LargeOffsetGEPMap still has a GetElementPtrInst * that isn't an AssertingVH.

[haicheng]

Add one more AssertingVH

[haicheng]

Kindly Ping

[efriedma]

LGTM

[jonpa]

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

3816 ↗

(On Diff #146171)

Is there any reason to not use '!=' as oppsed to '>' here (which would match the comment below)?

On SystemZ negative offsets are not supported on vector instructions.

[efriedma]

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

3816 ↗

(On Diff #146171)

It could work. Would probably need a few other changes to make sure we don't do anything silly like grouping together positive and negative offsets.

[jonpa]

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

3816 ↗

(On Diff #146171)

I may be missing something, but as far as I can tell there should not be any problem just because the offset is negative. The type for offsets is already int64_t, and the algorithm of sorting them by offset and inserting a new GEP whenever needed seems to work regardless of a negative offset, since the most negative offset is generated first and then the remainder will be positive, or?

[efriedma]

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

3816 ↗

(On Diff #146171)

I don't remember exactly how the algorithm for grouping offsets together functions; maybe it just works.

include/llvm/CodeGen/TargetLowering.h

public:
   }
 
   // Return true if it is profitable to use a scalar input to a BUILD_VECTOR
   // even if the vector itself has multiple uses.
   virtual bool aggressivelyPreferBuildVectorSources(EVT VecVT) const {
     return false;
   }
 
+  // Return true if CodeGenPrepare should consider splitting large offset of a
+  // GEP to make the GEP fit into the addressing mode and can be sunk into the
+  // same blocks of its users.
+  virtual bool shouldConsiderGEPOffsetSplit(Type* BaseType) const {
+    return false;
+  }
+
   //===--------------------------------------------------------------------===//
   // Runtime Library hooks
   //
 
   /// Rename the default libcall routine name for the specified libcall.
   void setLibcallName(RTLIB::Libcall Call, const char *Name) {
     LibcallRoutineNames[Call] = Name;
   }

lib/CodeGen/CodeGenPrepare.cpp

 static cl::opt<bool> AddrSinkCombineBaseOffs(
     "addr-sink-combine-base-offs", cl::Hidden, cl::init(true),
     cl::desc("Allow combining of BaseOffs field in Address sinking."));
 
 static cl::opt<bool> AddrSinkCombineScaledReg(
     "addr-sink-combine-scaled-reg", cl::Hidden, cl::init(true),
     cl::desc("Allow combining of ScaledReg field in Address sinking."));
 
+static cl::opt<bool>
+    EnableGEPOffsetSplit("cgp-split-large-offset-gep", cl::Hidden,
+                         cl::init(true),
+                         cl::desc("Enable splitting large offset of GEP."));
+
 namespace {
 
 using SetOfInstrs = SmallPtrSet<Instruction *, 16>;
 using TypeIsSExt = PointerIntPair<Type *, 1, bool>;
 using InstrToOrigTy = DenseMap<Instruction *, TypeIsSExt>;
 using SExts = SmallVector<Instruction *, 16>;
 using ValueToSExts = DenseMap<Value *, SExts>;
 
class CodeGenPrepare : public FunctionPass {
     InstrToOrigTy PromotedInsts;
 
     /// Keep track of instructions removed during promotion.
     SetOfInstrs RemovedInsts;
 
     /// Keep track of sext chains based on their initial value.
     DenseMap<Value *, Instruction *> SeenChainsForSExt;
 
+    // Keep track of GEPs accessing the same data structures such as structs or
+    // arrays that are candidates to be split later because of their large size.
+    DenseMap<Value *, SmallVector<std::pair<GetElementPtrInst *, int64_t>, 32>>
+        LargeOffsetGEPMap;
+
     /// Keep track of SExt promoted.
     ValueToSExts ValToSExtendedUses;
 
     /// True if CFG is modified in any way.
     bool ModifiedDT;
 
     /// True if optimizing for size.
     bool OptSize;
 
     /// DataLayout for the Function being processed.
     const DataLayout *DL = nullptr;
 
   public:
     static char ID; // Pass identification, replacement for typeid
private:
     bool placeDbgValues(Function &F);
     bool canFormExtLd(const SmallVectorImpl<Instruction *> &MovedExts,
                       LoadInst *&LI, Instruction *&Inst, bool HasPromoted);
     bool tryToPromoteExts(TypePromotionTransaction &TPT,
                           const SmallVectorImpl<Instruction *> &Exts,
                           SmallVectorImpl<Instruction *> &ProfitablyMovedExts,
                           unsigned CreatedInstsCost = 0);
     bool mergeSExts(Function &F);
+    bool splitLargeGEPOffsets();
     bool performAddressTypePromotion(
         Instruction *&Inst,
         bool AllowPromotionWithoutCommonHeader,
         bool HasPromoted, TypePromotionTransaction &TPT,
         SmallVectorImpl<Instruction *> &SpeculativelyMovedExts);
     bool splitBranchCondition(Function &F);
     bool simplifyOffsetableRelocate(Instruction &I);
   };
bool CodeGenPrepare::runOnFunction(Function &F) {
   EverMadeChange |= SplitIndirectBrCriticalEdges(F);
 
   bool MadeChange = true;
   while (MadeChange) {
     MadeChange = false;
     SeenChainsForSExt.clear();
     ValToSExtendedUses.clear();
     RemovedInsts.clear();
+    LargeOffsetGEPMap.clear();
     for (Function::iterator I = F.begin(); I != F.end(); ) {
       BasicBlock *BB = &*I++;
       bool ModifiedDTOnIteration = false;
       MadeChange |= optimizeBlock(*BB, ModifiedDTOnIteration);
 
       // Restart BB iteration if the dominator tree of the Function was changed
       if (ModifiedDTOnIteration)
         break;
     }
     if (EnableTypePromotionMerge && !ValToSExtendedUses.empty())
       MadeChange |= mergeSExts(F);
+    if (!LargeOffsetGEPMap.empty())
+      MadeChange |= splitLargeGEPOffsets();
 
     // Really free removed instructions during promotion.
     for (Instruction *I : RemovedInsts)
       I->deleteValue();
 
     EverMadeChange |= MadeChange;
   }
 
class AddressingModeMatcher {
   const SetOfInstrs &InsertedInsts;
 
   /// A map from the instructions to their type before promotion.
   InstrToOrigTy &PromotedInsts;
 
   /// The ongoing transaction where every action should be registered.
   TypePromotionTransaction &TPT;
 
+  // A GEP which has too large offset to be folded into the addressing mode.
+  std::pair<GetElementPtrInst *, int64_t> &LargeOffsetGEP;
+
   /// This is set to true when we should not do profitability checks.
   /// When true, IsProfitableToFoldIntoAddressingMode always returns true.
   bool IgnoreProfitability;
 
   AddressingModeMatcher(SmallVectorImpl<Instruction *> &AMI,
                         const TargetLowering &TLI,
-                        const TargetRegisterInfo &TRI,
-                        Type *AT, unsigned AS,
+                        const TargetRegisterInfo &TRI, Type *AT, unsigned AS,
                         Instruction *MI, ExtAddrMode &AM,
                         const SetOfInstrs &InsertedInsts,
                         InstrToOrigTy &PromotedInsts,
-                        TypePromotionTransaction &TPT)
+                        TypePromotionTransaction &TPT,
+                        std::pair<GetElementPtrInst *, int64_t> &LargeOffsetGEP)
       : AddrModeInsts(AMI), TLI(TLI), TRI(TRI),
         DL(MI->getModule()->getDataLayout()), AccessTy(AT), AddrSpace(AS),
         MemoryInst(MI), AddrMode(AM), InsertedInsts(InsertedInsts),
-        PromotedInsts(PromotedInsts), TPT(TPT) {
+        PromotedInsts(PromotedInsts), TPT(TPT), LargeOffsetGEP(LargeOffsetGEP) {
     IgnoreProfitability = false;
   }
 
 public:
   /// Find the maximal addressing mode that a load/store of V can fold,
   /// give an access type of AccessTy.  This returns a list of involved
   /// instructions in AddrModeInsts.
   /// \p InsertedInsts The instructions inserted by other CodeGenPrepare
   /// optimizations.
   /// \p PromotedInsts maps the instructions to their type before promotion.
   /// \p The ongoing transaction where every action should be registered.
-  static ExtAddrMode Match(Value *V, Type *AccessTy, unsigned AS,
-                           Instruction *MemoryInst,
-                           SmallVectorImpl<Instruction*> &AddrModeInsts,
-                           const TargetLowering &TLI,
-                           const TargetRegisterInfo &TRI,
-                           const SetOfInstrs &InsertedInsts,
-                           InstrToOrigTy &PromotedInsts,
-                           TypePromotionTransaction &TPT) {
+  static ExtAddrMode
+  Match(Value *V, Type *AccessTy, unsigned AS, Instruction *MemoryInst,
+        SmallVectorImpl<Instruction *> &AddrModeInsts,
+        const TargetLowering &TLI, const TargetRegisterInfo &TRI,
+        const SetOfInstrs &InsertedInsts, InstrToOrigTy &PromotedInsts,
+        TypePromotionTransaction &TPT,
+        std::pair<GetElementPtrInst *, int64_t> &LargeOffsetGEP) {
     ExtAddrMode Result;
 
-    bool Success = AddressingModeMatcher(AddrModeInsts, TLI, TRI,
-                                         AccessTy, AS,
+    bool Success = AddressingModeMatcher(AddrModeInsts, TLI, TRI, AccessTy, AS,
                                          MemoryInst, Result, InsertedInsts,
-                                         PromotedInsts, TPT).matchAddr(V, 0);
+                                         PromotedInsts, TPT, LargeOffsetGEP)
+                       .matchAddr(V, 0);
     (void)Success; assert(Success && "Couldn't select *anything*?");
     return Result;
   }
 
 private:
   bool matchScaledValue(Value *ScaleReg, int64_t Scale, unsigned Depth);
   bool matchAddr(Value *V, unsigned Depth);
   bool matchOperationAddr(User *Operation, unsigned Opcode, unsigned Depth,
case Instruction::GetElementPtr: {
     // just add it to the disp field and check validity.
     if (VariableOperand == -1) {
       AddrMode.BaseOffs += ConstantOffset;
       if (ConstantOffset == 0 ||
           TLI.isLegalAddressingMode(DL, AddrMode, AccessTy, AddrSpace)) {
         // Check to see if we can fold the base pointer in too.
         if (matchAddr(AddrInst->getOperand(0), Depth+1))
           return true;
+      } else {
+        // Record a GEP which has too large offset that cannot fit into the r+i
+        // addressing mode.  The GEP is the candidate to be split later.
+        Value *Base = AddrInst->getOperand(0);
+        if (EnableGEPOffsetSplit &&
+            TLI.shouldConsiderGEPOffsetSplit(Base->getType()) && Depth == 0 &&
+            ConstantOffset > 0) {
+          // Simple and common case that only one GEP is used in calculating the
+          // address for the memory access.
+          Instruction *BaseI = dyn_cast<Instruction>(Base);
+          if (isa<Argument>(Base) || isa<GlobalValue>(Base) ||
+              (BaseI && !isa<CastInst>(BaseI) && !isa<BinaryOperator>(BaseI) &&
+               !isa<GetElementPtrInst>(BaseI)))
+            if (auto *GEP = dyn_cast<GetElementPtrInst>(AddrInst))
+              if ((BaseI && GEP->getParent() != BaseI->getParent()) ||
+                  (!BaseI &&
+                   GEP->getParent() !=
+                       &GEP->getParent()->getParent()->getEntryBlock()))
+                // Make sure the base is not in the same basic block as the
+                // GEP.  Otherwise, the split might be undone.
+                LargeOffsetGEP = std::make_pair(GEP, ConstantOffset);
+        }
       }
       AddrMode.BaseOffs -= ConstantOffset;
       return false;
     }
 
     // Save the valid addressing mode in case we can't match.
     ExtAddrMode BackupAddrMode = AddrMode;
     unsigned OldSize = AddrModeInsts.size();
if (!AddrTy)
       return false;
     Type *AddressAccessTy = AddrTy->getElementType();
     unsigned AS = AddrTy->getAddressSpace();
 
     // Do a match against the root of this address, ignoring profitability. This
     // will tell us if the addressing mode for the memory operation will
     // *actually* cover the shared instruction.
     ExtAddrMode Result;
+    std::pair<GetElementPtrInst *, int64_t> LargeOffsetGEP(nullptr, 0);
     TypePromotionTransaction::ConstRestorationPt LastKnownGood =
         TPT.getRestorationPoint();
-    AddressingModeMatcher Matcher(MatchedAddrModeInsts, TLI, TRI,
-                                  AddressAccessTy, AS,
-                                  MemoryInst, Result, InsertedInsts,
-                                  PromotedInsts, TPT);
+    AddressingModeMatcher Matcher(
+        MatchedAddrModeInsts, TLI, TRI, AddressAccessTy, AS, MemoryInst, Result,
+        InsertedInsts, PromotedInsts, TPT, LargeOffsetGEP);
     Matcher.IgnoreProfitability = true;
     bool Success = Matcher.matchAddr(Address, 0);
     (void)Success; assert(Success && "Couldn't select *anything*?");
 
     // The match was to check the profitability, the changes made are not
     // part of the original matcher. Therefore, they should be dropped
     // otherwise the original matcher will not present the right state.
     TPT.rollback(LastKnownGood);
if (SelectInst *SI = dyn_cast<SelectInst>(V)) {
       PhiOrSelectSeen = true;
       continue;
     }
 
     // For non-PHIs, determine the addressing mode being computed.  Note that
     // the result may differ depending on what other uses our candidate
     // addressing instructions might have.
     AddrModeInsts.clear();
+    std::pair<GetElementPtrInst *, int64_t> LargeOffsetGEP(nullptr, 0);
     ExtAddrMode NewAddrMode = AddressingModeMatcher::Match(
         V, AccessTy, AddrSpace, MemoryInst, AddrModeInsts, *TLI, *TRI,
-        InsertedInsts, PromotedInsts, TPT);
-    NewAddrMode.OriginalValue = V;
+        InsertedInsts, PromotedInsts, TPT, LargeOffsetGEP);
+
+    if (LargeOffsetGEP.first &&
+        LargeOffsetGEP.first->getParent() != MemoryInst->getParent())
+      // Only collect GEPs that cannot be sunk unless the underlying data
+      // structures are split.
+      LargeOffsetGEPMap[LargeOffsetGEP.first->getPointerOperand()].push_back(
+          LargeOffsetGEP);
 
+    NewAddrMode.OriginalValue = V;
     if (!AddrModes.addNewAddrMode(NewAddrMode))
       break;
   }
 
   // Try to combine the AddrModes we've collected. If we couldn't collect any,
   // or we have multiple but either couldn't combine them or combining them
   // wouldn't do anything useful, bail out now.
   if (!AddrModes.combineAddrModes()) {
     TPT.rollback(LastKnownGood);
     return false;
   }
   TPT.commit();
for (Instruction *Inst : Insts) {
       }
       if (!inserted)
         CurPts.push_back(Inst);
     }
   }
   return Changed;
 }
 
+static int
+compareGEPOffset(const std::pair<GetElementPtrInst *, int64_t> *LHS,
+                 const std::pair<GetElementPtrInst *, int64_t> *RHS) {
+  if (LHS->first == RHS->first)
+    return 0;
+  if (LHS->second != RHS->second)
+    return LHS->second > RHS->second ? 1 : -1;
+  return LHS->first > RHS->first ? 1 : -1;
+}
+
+// Spliting large data structures so that the GEPs accessing them can have
+// smaller offsets so that they can be sunk to the same blocks as their users.
+// For example, a large struct starting from %base is splitted into two parts
+// where the second part starts from %new_base.
+//
+// Before:
+// BB0:
+//   %base     =
+//
+// BB1:
+//   %gep0     = gep %base, off0
+//   %gep1     = gep %base, off1
+//   %gep2     = gep %base, off2
+//
+// BB2:
+//   %load1    = load %gep0
+//   %load2    = load %gep1
+//   %load3    = load %gep2
+//
+// After:
+// BB0:
+//   %base     =
+//   %new_base = gep %base, off0
+//
+// BB1:
+//   %new_gep0 = %new_base
+//   %new_gep1 = gep %new_base, off1 - off0
+//   %new_gep2 = gep %new_base, off2 - off0
+//
+// BB2:
+//   %load1    = load i32, i32* %new_gep0
+//   %load2    = load i32, i32* %new_gep1
+//   %load3    = load i32, i32* %new_gep2
+//
+// %new_gep1 and %new_gep2 can be sunk to BB2 now after the splitting because
+// their offsets are smaller enough to fit into the addressing mode.
+bool CodeGenPrepare::splitLargeGEPOffsets() {
+  bool Changed = false;
+  for (auto &Entry : LargeOffsetGEPMap) {
+    SmallVectorImpl<std::pair<GetElementPtrInst *, int64_t>> &LargeOffsetGEPs =
+        Entry.second;
+    // Sorting all the GEPs of the same data structures based on the offsets.
+    array_pod_sort(LargeOffsetGEPs.begin(), LargeOffsetGEPs.end(),
+                   compareGEPOffset);
+    LargeOffsetGEPs.erase(
+        std::unique(LargeOffsetGEPs.begin(), LargeOffsetGEPs.end()),
+        LargeOffsetGEPs.end());
+    // Skip if all the GEPs have the same offsets.
+    if (LargeOffsetGEPs.front().second == LargeOffsetGEPs.back().second)
+      continue;
+    GetElementPtrInst *BaseGEP = LargeOffsetGEPs.begin()->first;
+    int64_t BaseOffset = LargeOffsetGEPs.begin()->second;
+    Value *NewBaseGEP = nullptr;
+    for (auto &LargeOffsetGEP : LargeOffsetGEPs) {
+      GetElementPtrInst *GEP = LargeOffsetGEP.first;
+      int64_t Offset = LargeOffsetGEP.second;
+      if (!GEP)
+        continue;
+      if (Offset != BaseOffset) {
+        TargetLowering::AddrMode AddrMode;
+        AddrMode.BaseOffs = Offset - BaseOffset;
+        if (!TLI->isLegalAddressingMode(*DL, AddrMode,
+                                        GEP->getResultElementType(),
+                                        GEP->getAddressSpace())) {
+          // We need to create a new base if the offset to the current base is
+          // too large to fit into the addressing mode. So, a very large struct
+          // may be splitted into several parts.
+          BaseGEP = GEP;
+          BaseOffset = Offset;
+          NewBaseGEP = nullptr;
+        }
+      }
+
+      // Generate a new GEP to replace the current one.
+      IRBuilder<> Builder(GEP);
+      Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
+      Type *I8PtrTy =
+          Builder.getInt8PtrTy(GEP->getType()->getPointerAddressSpace());
+      Type *I8Ty = Builder.getInt8Ty();
+
+      if (!NewBaseGEP) {
+        // Create a new base if we don't have one yet.  Find the insertion
+        // pointer for the new base first.
+        BasicBlock::iterator NewBaseInsertPt;
+        BasicBlock *NewBaseInsertBB;
+        if (auto *BaseI = dyn_cast<Instruction>(Entry.first)) {
+          // If the base of the struct is an instruction, the new base will be
+          // inserted close to it.
+          NewBaseInsertBB = BaseI->getParent();
+          if (isa<PHINode>(BaseI))
+            NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+          else if (InvokeInst *Invoke = dyn_cast<InvokeInst>(BaseI)) {
+            NewBaseInsertBB =
+                SplitEdge(NewBaseInsertBB, Invoke->getNormalDest());
+            NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+          } else
+            NewBaseInsertPt = std::next(BaseI->getIterator());
+        } else {
+          // If the current base is an argument or global value, the new base
+          // will be inserted to the entry block.
+          NewBaseInsertBB = &BaseGEP->getParent()->getParent()->getEntryBlock();
+          NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+        }
+        IRBuilder<> NewBaseBuilder(NewBaseInsertBB, NewBaseInsertPt);
+        // Create a new base.
+        Value *BaseIndex = ConstantInt::get(IntPtrTy, BaseOffset);
+        NewBaseGEP = Entry.first;
+        if (NewBaseGEP->getType() != I8PtrTy)
+          NewBaseGEP = NewBaseBuilder.CreatePointerCast(NewBaseGEP, I8PtrTy);
+        NewBaseGEP =
+            NewBaseBuilder.CreateGEP(I8Ty, NewBaseGEP, BaseIndex, "splitgep");
+      }
+
+      Value *NewGEP = NewBaseGEP;
+      if (Offset == BaseOffset) {
+        if (GEP->getType() != I8PtrTy)
+          NewGEP = Builder.CreatePointerCast(NewGEP, GEP->getType());
+      } else {
+        // Calculate the new offset for the new GEP.
+        Value *Index = ConstantInt::get(IntPtrTy, Offset - BaseOffset);
+        NewGEP = Builder.CreateGEP(I8Ty, NewBaseGEP, Index);
+
+        if (GEP->getType() != I8PtrTy)
+          NewGEP = Builder.CreatePointerCast(NewGEP, GEP->getType());
+      }
+      GEP->replaceAllUsesWith(NewGEP);
+      GEP->eraseFromParent();
+      Changed = true;
+    }
+  }
+  return Changed;
+}
+
 /// Return true, if an ext(load) can be formed from an extension in
 /// \p MovedExts.
 bool CodeGenPrepare::canFormExtLd(
     const SmallVectorImpl<Instruction *> &MovedExts, LoadInst *&LI,
     Instruction *&Inst, bool HasPromoted) {
   for (auto *MovedExtInst : MovedExts) {
     if (isa<LoadInst>(MovedExtInst->getOperand(0))) {
       LI = cast<LoadInst>(MovedExtInst->getOperand(0));

lib/Target/AArch64/AArch64ISelLowering.h

bool lowerInterleavedLoad(LoadInst *LI,
                             ArrayRef<unsigned> Indices,
                             unsigned Factor) const override;
   bool lowerInterleavedStore(StoreInst *SI, ShuffleVectorInst *SVI,
                              unsigned Factor) const override;
 
   bool isLegalAddImmediate(int64_t) const override;
   bool isLegalICmpImmediate(int64_t) const override;
 
+  bool shouldConsiderGEPOffsetSplit(Type *BaseType) const override;
+
   EVT getOptimalMemOpType(uint64_t Size, unsigned DstAlign, unsigned SrcAlign,
                           bool IsMemset, bool ZeroMemset, bool MemcpyStrSrc,
                           MachineFunction &MF) const override;
 
   /// Return true if the addressing mode represented by AM is legal for this
   /// target, for a load/store of the specified type.
   bool isLegalAddressingMode(const DataLayout &DL, const AddrMode &AM, Type *Ty,
                              unsigned AS,

lib/Target/AArch64/AArch64ISelLowering.cpp

if (!AM.Scale) {
     return false;
   }
 
   // Check reg1 + SIZE_IN_BYTES * reg2 and reg1 + reg2
 
   return AM.Scale == 1 || (AM.Scale > 0 && (uint64_t)AM.Scale == NumBytes);
 }
 
+bool AArch64TargetLowering::shouldConsiderGEPOffsetSplit(Type *BaseType) const {
+  if (auto *Ty = dyn_cast<PointerType>(BaseType))
+    // Consider splitting large offset of struct or array.
+    if (Ty->getElementType()->isAggregateType())
+      return true;
+
+  return false;
+}
+
 int AArch64TargetLowering::getScalingFactorCost(const DataLayout &DL,
                                                 const AddrMode &AM, Type *Ty,
                                                 unsigned AS) const {
   // Scaling factors are not free at all.
   // Operands                     | Rt Latency
   // -------------------------------------------
   // Rt, [Xn, Xm]                 | 4
   // -------------------------------------------

test/Transforms/CodeGenPrepare/AArch64/large-offset-gep.ll

+; RUN: llc -mtriple=aarch64-linux-gnu -verify-machineinstrs -o - %s | FileCheck %s
+
+%struct_type = type { [10000 x i32], i32, i32 }
+
+define void @test1(%struct_type** %s, i32 %n) {
+; CHECK-LABEL: test1
+entry:
+  %struct = load %struct_type*, %struct_type** %s
+  br label %while_cond
+
+while_cond:
+  %phi = phi i32 [ 0, %entry ], [ %i, %while_body ]
+; CHECK:     mov     w{{[0-9]+}}, #40000
+; CHECK-NOT: mov     w{{[0-9]+}}, #40004
+  %gep0 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 1
+  %gep1 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 2
+  %cmp = icmp slt i32 %phi, %n
+  br i1 %cmp, label %while_body, label %while_end
+
+while_body:
+; CHECK:     str      w{{[0-9]+}}, [x{{[0-9]+}}, #4]
+  %i = add i32 %phi, 1
+  store i32 %i, i32* %gep0
+  store i32 %phi, i32* %gep1
+  br label %while_cond
+
+while_end:
+  ret void
+}
+
+define void @test2(%struct_type* %struct, i32 %n) {
+; CHECK-LABEL: test2
+entry:
+  %cmp = icmp eq %struct_type* %struct, null
+  br i1 %cmp, label %while_end, label %while_cond
+
+while_cond:
+  %phi = phi i32 [ 0, %entry ], [ %i, %while_body ]
+; CHECK:     mov     w{{[0-9]+}}, #40000
+; CHECK-NOT: mov     w{{[0-9]+}}, #40004
+  %gep0 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 1
+  %gep1 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 2
+  %cmp1 = icmp slt i32 %phi, %n
+  br i1 %cmp1, label %while_body, label %while_end
+
+while_body:
+; CHECK:     str      w{{[0-9]+}}, [x{{[0-9]+}}, #4]
+  %i = add i32 %phi, 1
+  store i32 %i, i32* %gep0
+  store i32 %phi, i32* %gep1
+  br label %while_cond
+
+while_end:
+  ret void
+}
+
+define void @test3(%struct_type* %s1, %struct_type* %s2, i1 %cond, i32 %n) {
+; CHECK-LABEL: test3
+entry:
+  br i1 %cond, label %if_true, label %if_end
+
+if_true:
+  br label %if_end
+
+if_end:
+  %struct = phi %struct_type* [ %s1, %entry ], [ %s2, %if_true ]
+  %cmp = icmp eq %struct_type* %struct, null
+  br i1 %cmp, label %while_end, label %while_cond
+
+while_cond:
+  %phi = phi i32 [ 0, %if_end ], [ %i, %while_body ]
+; CHECK:     mov     w{{[0-9]+}}, #40000
+; CHECK-NOT: mov     w{{[0-9]+}}, #40004
+  %gep0 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 1
+  %gep1 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 2
+  %cmp1 = icmp slt i32 %phi, %n
+  br i1 %cmp1, label %while_body, label %while_end
+
+while_body:
+; CHECK:     str      w{{[0-9]+}}, [x{{[0-9]+}}, #4]
+  %i = add i32 %phi, 1
+  store i32 %i, i32* %gep0
+  store i32 %phi, i32* %gep1
+  br label %while_cond
+
+while_end:
+  ret void
+}
+
+declare %struct_type* @foo()
+
+define void @test4(i32 %n) personality i32 (...)* @__FrameHandler {
+; CHECK-LABEL: test4
+entry:
+  %struct = invoke %struct_type* @foo() to label %while_cond unwind label %cleanup
+
+while_cond:
+  %phi = phi i32 [ 0, %entry ], [ %i, %while_body ]
+; CHECK:     mov     w{{[0-9]+}}, #40000
+; CHECK-NOT: mov     w{{[0-9]+}}, #40004
+  %gep0 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 1
+  %gep1 = getelementptr %struct_type, %struct_type* %struct, i64 0, i32 2
+  %cmp = icmp slt i32 %phi, %n
+  br i1 %cmp, label %while_body, label %while_end
+
+while_body:
+; CHECK:     str      w{{[0-9]+}}, [x{{[0-9]+}}, #4]
+  %i = add i32 %phi, 1
+  store i32 %i, i32* %gep0
+  store i32 %phi, i32* %gep1
+  br label %while_cond
+
+while_end:
+  ret void
+
+cleanup:
+  landingpad { i8*, i32 } cleanup
+  unreachable
+}
+
+declare i32 @__FrameHandler(...)
+
+define void @test5([65536 x i32]** %s, i32 %n) {
+; CHECK-LABEL: test5
+entry:
+  %struct = load [65536 x i32]*, [65536 x i32]** %s
+  br label %while_cond
+
+while_cond:
+  %phi = phi i32 [ 0, %entry ], [ %i, %while_body ]
+; CHECK:     mov     w{{[0-9]+}}, #14464
+; CHECK-NOT: mov     w{{[0-9]+}}, #14468
+  %gep0 = getelementptr [65536 x i32], [65536 x i32]* %struct, i64 0, i32 20000
+  %gep1 = getelementptr [65536 x i32], [65536 x i32]* %struct, i64 0, i32 20001
+  %cmp = icmp slt i32 %phi, %n
+  br i1 %cmp, label %while_body, label %while_end
+
+while_body:
+; CHECK:     str      w{{[0-9]+}}, [x{{[0-9]+}}, #4]
+  %i = add i32 %phi, 1
+  store i32 %i, i32* %gep0
+  store i32 %phi, i32* %gep1
+  br label %while_cond
+
+while_end:
+  ret void
+}