[LoopIdiom] Transform loop containing memcpy into memmove

[yurai007]

Summary

The purpose of patch is to learn Loop idiom recognition pass how to recognize simple memmove patterns
in similar way like GCC does: https://godbolt.org/z/dKjGvTGff
It's follow-up of following change: https://reviews.llvm.org/D104464

[zhuhan0]

Sorry for the delay. I've been occupied with other projects.

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

1204–1208

Not sure I fully understand the FIXME. Why not drop the equals case?

1340

Nit: please fix lint.

1348

I feel the logic here is a bit hard to read. Whether it's memcpy or not, it both requires IsSameObject to be true. Maybe change this to

bool UseMemMove = IsMemCpy || LoopAccessStore;

And then move the IsSameObject check into MemmoveVerifier?

[yurai007]

Thank you for looking into this change. Since I'm busy with other projects too I will address comments and update patch tomorrow morning.

[yurai007]

Rebased and addressed comments.

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

1204–1208

Right, apparently I didn't think enough about that case so left fixme without good reason.

1348

I agree logic is not very straightforward here. Your idea would work except isMemcpy and !IsSameObject case (like in memcpy-intrinsic.ll).
For that case we just want to hoist body memcpy to big header memcpy without reaching MemmoveVerifier.
We could still move some checks there to make main logic more readable but then that wouldn't be "MemmoveVerifier" anymore I guess.

[yurai007]

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

1191

Added some cosmetic improvements - more consts, refs instead pointers etc.

[zhuhan0]

LGTM.

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(StoreInst *SI,
   // random load we can't handle.
   Value *LoadPtr = LI->getPointerOperand();
   const SCEVAddRecExpr *LoadEv = cast<SCEVAddRecExpr>(SE->getSCEV(LoadPtr));
   return processLoopStoreOfLoopLoad(StorePtr, LoadPtr, StoreSize,
                                     SI->getAlign(), LI->getAlign(), SI, LI,
                                     StoreEv, LoadEv, BECount);
 }
 
+class MemmoveVerifier {
+public:
+  explicit MemmoveVerifier(Value *LoadBasePtr, Value *StoreBasePtr,
+                           const DataLayout *DL)
+      : DL(DL), LoadOff(0), StoreOff(0),
+        BP1(llvm::GetPointerBaseWithConstantOffset(
+            LoadBasePtr->stripPointerCasts(), LoadOff, *DL)),
+        BP2(llvm::GetPointerBaseWithConstantOffset(
+            StoreBasePtr->stripPointerCasts(), StoreOff, *DL)),
+        IsSameObject(BP1 == BP2) {}
+
+  bool loadAndStoreMayFormMemmove(unsigned StoreSize, bool NegStride,
+                                  Instruction *TheLoad, bool IsMemCpy) const {
+    if (IsMemCpy) {
+      // Ensure that LoadBasePtr is after StoreBasePtr or before StoreBasePtr
+      // for negative stride. FIXME: since llvm.memcpy dest and src may be equal
+      // extra check is needed here.
+      if ((!NegStride && LoadOff <= StoreOff) ||
+          (NegStride && LoadOff >= StoreOff))
+        return false;
+    } else {
+      // Ensure that LoadBasePtr is after StoreBasePtr or before StoreBasePtr
+      // for negative stride. LoadBasePtr shouldn't overlap with StoreBasePtr.
+      int64_t LoadSize =
+          DL->getTypeSizeInBits(TheLoad->getType()).getFixedSize() / 8;
+      if (BP1 != BP2 || LoadSize != int64_t(StoreSize))
+        return false;
+      if ((!NegStride && LoadOff < StoreOff + int64_t(StoreSize)) ||
+          (NegStride && LoadOff + LoadSize > StoreOff))
+        return false;
+    }
+    return true;
+  }
+
+private:
+  const DataLayout *DL;
+  int64_t LoadOff;
+  int64_t StoreOff;
+  const Value *BP1;
+  const Value *BP2;
+
+public:
+  const bool IsSameObject;
+};
+
 bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
     Value *DestPtr, Value *SourcePtr, unsigned StoreSize, MaybeAlign StoreAlign,
     MaybeAlign LoadAlign, Instruction *TheStore, Instruction *TheLoad,
     const SCEVAddRecExpr *StoreEv, const SCEVAddRecExpr *LoadEv,
     const SCEV *BECount) {
 
   // FIXME: until llvm.memcpy.inline supports dynamic sizes, we need to
   // conservatively bail here, since otherwise we may have to transform
bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
   Changed = true;
 
   SmallPtrSet<Instruction *, 2> Stores;
   Stores.insert(TheStore);
 
   bool IsMemCpy = isa<MemCpyInst>(TheStore);
   const StringRef InstRemark = IsMemCpy ? "memcpy" : "load and store";
 
-  bool UseMemMove =
+  bool LoopAccessStore =
       mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                             StoreSize, *AA, Stores);
-  if (UseMemMove) {
+  if (LoopAccessStore) {
     Stores.insert(TheLoad);
     if (mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop,
                               BECount, StoreSize, *AA, Stores)) {
       ORE.emit([&]() {
         return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessStore",
                                         TheStore)
                << ore::NV("Inst", InstRemark) << " in "
                << ore::NV("Function", TheStore->getFunction())
bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
   // mutated by the loop.
   Value *LoadBasePtr = Expander.expandCodeFor(
       LdStart, Builder.getInt8PtrTy(LdAS), Preheader->getTerminator());
 
   // If the store is a memcpy instruction, we must check if it will write to
   // the load memory locations. So remove it from the ignored stores.
   if (IsMemCpy)
     Stores.erase(TheStore);
+  MemmoveVerifier Verifier(LoadBasePtr, StoreBasePtr, DL);
   if (mayLoopAccessLocation(LoadBasePtr, ModRefInfo::Mod, CurLoop, BECount,
                             StoreSize, *AA, Stores)) {
+    if (!IsMemCpy) {
       ORE.emit([&]() {
-      return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessLoad", TheLoad)
+        return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessLoad",
+                                        TheLoad)
                << ore::NV("Inst", InstRemark) << " in "
                << ore::NV("Function", TheStore->getFunction())
                << " function will not be hoisted: "
                << ore::NV("Reason", "The loop may access load location");
       });
       return Changed;
-  }
-  if (UseMemMove) {
-    // Ensure that LoadBasePtr is after StoreBasePtr or before StoreBasePtr for
-    // negative stride. LoadBasePtr shouldn't overlap with StoreBasePtr.
-    int64_t LoadOff = 0, StoreOff = 0;
-    const Value *BP1 = llvm::GetPointerBaseWithConstantOffset(
-        LoadBasePtr->stripPointerCasts(), LoadOff, *DL);
-    const Value *BP2 = llvm::GetPointerBaseWithConstantOffset(
-        StoreBasePtr->stripPointerCasts(), StoreOff, *DL);
-    int64_t LoadSize =
-        DL->getTypeSizeInBits(TheLoad->getType()).getFixedSize() / 8;
-    if (BP1 != BP2 || LoadSize != int64_t(StoreSize))
-      return Changed;
-    if ((!NegStride && LoadOff < StoreOff + int64_t(StoreSize)) ||
-        (NegStride && LoadOff + LoadSize > StoreOff))
+    } else {
+      // At this point loop may access load only for memcpy in same underlying
+      // object. If that's not the case bail out.
+      if (!Verifier.IsSameObject)
         return Changed;
     }
+  }
+
+  bool UseMemMove = IsMemCpy ? Verifier.IsSameObject : LoopAccessStore;
+  if (UseMemMove)
+    if (!Verifier.loadAndStoreMayFormMemmove(StoreSize, NegStride, TheLoad,
+                                             IsMemCpy))
+      return Changed;
 
   if (avoidLIRForMultiBlockLoop())
     return Changed;
 
   // Okay, everything is safe, we can transform this!
 
   const SCEV *NumBytesS =
       getNumBytes(BECount, IntIdxTy, StoreSize, CurLoop, DL, SE);

llvm/test/Transforms/LoopIdiom/basic.ll

for.body:                                         ; preds = %bb.nph, %for.body
   %indvar.next = add i64 %indvar, 1
   %exitcond = icmp eq i64 %indvar.next, %Size
   br i1 %exitcond, label %for.end, label %for.body
 
 for.end:                                          ; preds = %for.body, %entry
   ret void
 }
 
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg)
+
+;; Memmove formation. We expect exactly same memmove result like in PR46179_positive_stride output.
+define void @loop_with_memcpy_PR46179_positive_stride(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @loop_with_memcpy_PR46179_positive_stride(
+; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0i8.p0i8.i64(i8* align 1 [[SRC]], i8* align 1 [[SCEVGEP]], i64 [[SIZE:%.*]], i1 false)
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ 0, [[BB_NPH:%.*]] ], [ [[INDVAR_NEXT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], 1
+; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr i8, i8* [[SRC]], i64 [[STEP]]
+; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr i8, i8* [[SRC]], i64 [[INDVAR]]
+; CHECK-NEXT:    [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVAR_NEXT]], [[SIZE:%.*]]
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+bb.nph:
+  br label %for.body
+
+for.body:                                         ; preds = %bb.nph, %for.body
+  %indvar = phi i64 [ 0, %bb.nph ], [ %indvar.next, %for.body ]
+  %Step = add nuw nsw i64 %indvar, 1
+  %SrcI = getelementptr i8, i8* %Src, i64 %Step
+  %DestI = getelementptr i8, i8* %Src, i64 %indvar
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 1, i1 false)
+  %indvar.next = add i64 %indvar, 1
+  %exitcond = icmp eq i64 %indvar.next, %Size
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+}
+
 ;; Memmove formation.
 define void @PR46179_negative_stride(i8* %Src, i64 %Size) {
 ; CHECK-LABEL: @PR46179_negative_stride(
 ; CHECK-NEXT:  bb.nph:
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i64 [[SIZE:%.*]], 0
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_END:%.*]]
 ; CHECK:       for.body.preheader:
 ; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 1
for.body:                                           ; preds = %bb.nph, %.for.body
   store i8 %V, i8* %DestI, align 1
   %exitcond = icmp sgt i64 %indvar, 1
   br i1 %exitcond, label %for.body, label %for.end
 
 for.end:                                      ; preds = %.for.body, %bb.nph
   ret void
 }
 
-;; Do not form memmove from previous store when stride is positive.
+;; Memmove formation. We expect exactly same memmove result like in PR46179_negative_stride output.
+define void @loop_with_memcpy_PR46179_negative_stride(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @loop_with_memcpy_PR46179_negative_stride(
+; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i64 [[SIZE:%.*]], 0
+; CHECK-NEXT:    br i1 [[CMP1]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body.preheader:
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 1
+; CHECK-NEXT:    call void @llvm.memmove.p0i8.p0i8.i64(i8* align 1 [[SCEVGEP]], i8* align 1 [[SRC]], i64 [[SIZE]], i1 false)
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ [[STEP]], [[FOR_BODY]] ], [ [[SIZE]], [[FOR_BODY_PREHEADER]] ]
+; CHECK-NEXT:    [[STEP:%.*]] = add nsw i64 [[INDVAR]], -1
+; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr inbounds i8, i8* [[SRC:%.*]], i64 [[STEP]]
+; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr inbounds i8, i8* [[SRC]], i64 [[INDVAR]]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp sgt i64 [[INDVAR]], 1
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[FOR_END]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+bb.nph:
+  %cmp1 = icmp sgt i64 %Size, 0
+  br i1 %cmp1, label %for.body, label %for.end
+
+for.body:                                           ; preds = %bb.nph, %.for.body
+  %indvar = phi i64 [ %Step, %for.body ], [ %Size, %bb.nph ]
+  %Step = add nsw i64 %indvar, -1
+  %SrcI = getelementptr inbounds i8, i8* %Src, i64 %Step
+  %DestI = getelementptr inbounds i8, i8* %Src, i64 %indvar
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 1, i1 false)
+  %exitcond = icmp sgt i64 %indvar, 1
+  br i1 %exitcond, label %for.body, label %for.end
+
+for.end:                                      ; preds = %.for.body, %bb.nph
+  ret void
+}
+
+;; Memmove formation.
+define void @loop_with_memcpy_stride16(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @loop_with_memcpy_stride16(
+; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 16
+; CHECK-NEXT:    [[SMAX:%.*]] = call i64 @llvm.smax.i64(i64 [[SIZE:%.*]], i64 16)
+; CHECK-NEXT:    [[TMP0:%.*]] = add nsw i64 [[SMAX]], -1
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i64 [[TMP0]], 4
+; CHECK-NEXT:    [[TMP2:%.*]] = shl nuw nsw i64 [[TMP1]], 4
+; CHECK-NEXT:    [[TMP3:%.*]] = add nuw i64 [[TMP2]], 16
+; CHECK-NEXT:    call void @llvm.memmove.p0i8.p0i8.i64(i8* align 1 [[SRC]], i8* align 1 [[SCEVGEP]], i64 [[TMP3]], i1 false)
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ [[STEP]], [[FOR_BODY]] ], [ 0, [[BB_NPH:%.*]] ]
+; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], 16
+; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr inbounds i8, i8* [[SRC]], i64 [[STEP]]
+; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr inbounds i8, i8* [[SRC]], i64 [[INDVAR]]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp slt i64 [[STEP]], [[SIZE:%.*]]
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[FOR_END:%.*]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+bb.nph:
+  br label %for.body
+
+for.body:                                                ; preds = %for.body, %bb.nph
+  %indvar = phi i64 [ %Step, %for.body ], [ 0, %bb.nph ]
+  %Step = add nuw nsw i64 %indvar, 16
+  %SrcI = getelementptr inbounds i8, i8* %Src, i64 %Step
+  %DestI = getelementptr inbounds i8, i8* %Src, i64 %indvar
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 16, i1 false)
+  %exitcond = icmp slt i64 %Step, %Size
+  br i1 %exitcond, label %for.body, label %for.end
+
+for.end:                                                ; preds = %for.body
+  ret void
+}
+
+;; Do not form memmove from previous load when stride is positive.
 define void @do_not_form_memmove1(i8* %Src, i64 %Size) {
 ; CHECK-LABEL: @do_not_form_memmove1(
 ; CHECK-NEXT:  bb.nph:
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ 1, [[BB_NPH:%.*]] ], [ [[INDVAR_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], -1
 ; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 [[STEP]]
for.body:                                         ; preds = %bb.nph, %for.body
   %indvar.next = add i64 %indvar, 1
   %exitcond = icmp eq i64 %indvar.next, %Size
   br i1 %exitcond, label %for.end, label %for.body
 
 for.end:                                          ; preds = %for.body, %entry
   ret void
 }
 
-;; Do not form memmove from next store when stride is negative.
+;; Do not form memmove from previous load in memcpy when stride is positive.
 define void @do_not_form_memmove2(i8* %Src, i64 %Size) {
 ; CHECK-LABEL: @do_not_form_memmove2(
 ; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ 1, [[BB_NPH:%.*]] ], [ [[INDVAR_NEXT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], -1
+; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr i8, i8* [[SRC:%.*]], i64 [[STEP]]
+; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr i8, i8* [[SRC]], i64 [[INDVAR]]
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 [[DESTI]], i8* align 1 [[SRCI]], i64 1, i1 false)
+; CHECK-NEXT:    [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVAR_NEXT]], [[SIZE:%.*]]
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+bb.nph:
+  br label %for.body
+
+for.body:                                         ; preds = %bb.nph, %for.body
+  %indvar = phi i64 [ 1, %bb.nph ], [ %indvar.next, %for.body ]
+  %Step = add nuw nsw i64 %indvar, -1
+  %SrcI = getelementptr i8, i8* %Src, i64 %Step
+  %DestI = getelementptr i8, i8* %Src, i64 %indvar
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 1, i1 false)
+  %indvar.next = add i64 %indvar, 1
+  %exitcond = icmp eq i64 %indvar.next, %Size
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+}
+
+;; Do not form memmove from next load when stride is negative.
+define void @do_not_form_memmove3(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @do_not_form_memmove3(
+; CHECK-NEXT:  bb.nph:
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i64 [[SIZE:%.*]], 0
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_END:%.*]]
 ; CHECK:       for.body.preheader:
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT]], [[FOR_BODY]] ], [ [[SIZE]], [[FOR_BODY_PREHEADER]] ]
 ; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], 1
 ; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr inbounds i8, i8* [[SRC:%.*]], i64 [[STEP]]
for.body:                                           ; preds = %bb.nph, %.for.body
   %indvar.next = add nsw i64 %indvar, -1
   %exitcond = icmp sgt i64 %indvar, 1
   br i1 %exitcond, label %for.body, label %for.end
 
 for.end:                                      ; preds = %.for.body, %bb.nph
   ret void
 }
 
+;; Do not form memmove from next load in memcpy when stride is negative.
+define void @do_not_form_memmove4(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @do_not_form_memmove4(
+; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i64 [[SIZE:%.*]], 0
+; CHECK-NEXT:    br i1 [[CMP1]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body.preheader:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT]], [[FOR_BODY]] ], [ [[SIZE]], [[FOR_BODY_PREHEADER]] ]
+; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], 1
+; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr inbounds i8, i8* [[SRC:%.*]], i64 [[STEP]]
+; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr inbounds i8, i8* [[SRC]], i64 [[INDVAR]]
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 [[DESTI]], i8* align 1 [[SRCI]], i64 1, i1 false)
+; CHECK-NEXT:    [[INDVAR_NEXT:%.*]] = add nsw i64 [[INDVAR]], -1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp sgt i64 [[INDVAR]], 1
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[FOR_END]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+bb.nph:
+  %cmp1 = icmp sgt i64 %Size, 0
+  br i1 %cmp1, label %for.body, label %for.end
+
+for.body:                                           ; preds = %bb.nph, %.for.body
+  %indvar = phi i64 [ %indvar.next, %for.body ], [ %Size, %bb.nph ]
+  %Step = add nuw nsw i64 %indvar, 1
+  %SrcI = getelementptr inbounds i8, i8* %Src, i64 %Step
+  %DestI = getelementptr inbounds i8, i8* %Src, i64 %indvar
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 1, i1 false)
+  %indvar.next = add nsw i64 %indvar, -1
+  %exitcond = icmp sgt i64 %indvar, 1
+  br i1 %exitcond, label %for.body, label %for.end
+
+for.end:                                      ; preds = %.for.body, %bb.nph
+  ret void
+}
+
 ;; Do not form memmove when underaligned load is overlapped with store.
-define void @do_not_form_memmove3(i32* %s, i64 %size) {
-; CHECK-LABEL: @do_not_form_memmove3(
+define void @do_not_form_memmove5(i32* %s, i64 %size) {
+; CHECK-LABEL: @do_not_form_memmove5(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[END_IDX:%.*]] = add i64 [[SIZE:%.*]], -1
 ; CHECK-NEXT:    [[END_PTR:%.*]] = getelementptr inbounds i32, i32* [[S:%.*]], i64 [[END_IDX]]
 ; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
 ; CHECK:       while.body:
 ; CHECK-NEXT:    [[PHI_PTR:%.*]] = phi i32* [ [[S]], [[ENTRY:%.*]] ], [ [[NEXT_PTR:%.*]], [[WHILE_BODY]] ]
 ; CHECK-NEXT:    [[NEXT:%.*]] = bitcast i32* [[PHI_PTR]] to i16*
 ; CHECK-NEXT:    [[SRC_PTR:%.*]] = getelementptr i16, i16* [[NEXT]], i64 1
while.body:
   %next.ptr = getelementptr i32, i32* %phi.ptr, i64 1
   %cmp = icmp eq i32* %next.ptr, %end.ptr
   br i1 %cmp, label %exit, label %while.body
 
 exit:
   ret void
 }
 
-declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg)
-
-;; FIXME: Do not form memmove from loop body containing memcpy.
-define void @do_not_form_memmove4(i8* %Src, i64 %Size) {
-; CHECK-LABEL: @do_not_form_memmove4(
+;; Do not form memmove for memcpy with aliasing store.
+define void @do_not_form_memmove6(i8* %Src, i64 %Size) {
+; CHECK-LABEL: @do_not_form_memmove6(
 ; CHECK-NEXT:  bb.nph:
+; CHECK-NEXT:    [[BASEALIAS:%.*]] = call i8* @external(i8* [[SRC:%.*]])
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ 0, [[BB_NPH:%.*]] ], [ [[INDVAR_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[STEP:%.*]] = add nuw nsw i64 [[INDVAR]], 1
 ; CHECK-NEXT:    [[SRCI:%.*]] = getelementptr i8, i8* [[SRC]], i64 [[STEP]]
 ; CHECK-NEXT:    [[DESTI:%.*]] = getelementptr i8, i8* [[SRC]], i64 [[INDVAR]]
 ; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 [[DESTI]], i8* align 1 [[SRCI]], i64 1, i1 false)
+; CHECK-NEXT:    store i8 4, i8* [[BASEALIAS]], align 1
 ; CHECK-NEXT:    [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVAR_NEXT]], [[SIZE:%.*]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
 ; CHECK:       for.end:
 ; CHECK-NEXT:    ret void
 ;
 bb.nph:
+  %BaseAlias = call i8* @external(i8* %Src)
   br label %for.body
 
 for.body:                                         ; preds = %bb.nph, %for.body
   %indvar = phi i64 [ 0, %bb.nph ], [ %indvar.next, %for.body ]
   %Step = add nuw nsw i64 %indvar, 1
   %SrcI = getelementptr i8, i8* %Src, i64 %Step
   %DestI = getelementptr i8, i8* %Src, i64 %indvar
   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 %DestI, i8* align 1 %SrcI, i64 1, i1 false)
+  store i8 4, i8* %BaseAlias
   %indvar.next = add i64 %indvar, 1
   %exitcond = icmp eq i64 %indvar.next, %Size
   br i1 %exitcond, label %for.end, label %for.body
 
 for.end:                                          ; preds = %for.body, %entry
   ret void
 }