diff --git a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
--- a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -200,7 +200,8 @@
   /// @{
 
   bool runOnCountableLoop();
-  bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
+  bool runOnLoopBlock(bool isFromRotatedLoop, BasicBlock *BB,
+                      const SCEV *BECount,
                       SmallVectorImpl<BasicBlock *> &ExitBlocks);
 
   void collectStores(BasicBlock *BB);
@@ -418,7 +419,8 @@
     if (LI->getLoopFor(BB) != CurLoop)
       continue;
 
-    MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks);
+    MadeChange |=
+        runOnLoopBlock(CurLoop->isRotatedForm(), BB, BECount, ExitBlocks);
   }
   return MadeChange;
 }
@@ -620,14 +622,18 @@
 /// with the specified backedge count.  This block is known to be in the current
 /// loop and not in any subloops.
 bool LoopIdiomRecognize::runOnLoopBlock(
-    BasicBlock *BB, const SCEV *BECount,
+    bool isFromRotatedLoop, BasicBlock *BB, const SCEV *BECount,
     SmallVectorImpl<BasicBlock *> &ExitBlocks) {
+
   // We can only promote stores in this block if they are unconditionally
-  // executed in the loop.  For a block to be unconditionally executed, it has
-  // to dominate all the exit blocks of the loop.  Verify this now.
-  for (BasicBlock *ExitBlock : ExitBlocks)
-    if (!DT->dominates(BB, ExitBlock))
-      return false;
+  // executed in the loop. For a block to be unconditionally executed, it has
+  // to dominate all the exit blocks of the loop. Verify this now.
+  // On countable unrotated loops, checking the exit blocks fails.
+  // Hence, this is only run on rotated loops.
+  if (isFromRotatedLoop)
+    for (BasicBlock *ExitBlock : ExitBlocks)
+      if (!DT->dominates(BB, ExitBlock))
+        return false;
 
   bool MadeChange = false;
   // Look for store instructions, which may be optimized to memset/memcpy.
diff --git a/llvm/test/Transforms/LoopIdiom/memset-unrotated-loop.ll b/llvm/test/Transforms/LoopIdiom/memset-unrotated-loop.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/LoopIdiom/memset-unrotated-loop.ll
@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-idiom -S %s | FileCheck %s
+
+define void @test(i8* noalias nonnull align 1 %start, i8* %end) unnamed_addr {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[START2:%.*]] = ptrtoint i8* [[START:%.*]] to i64
+; CHECK-NEXT:    [[END1:%.*]] = ptrtoint i8* [[END:%.*]] to i64
+; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[END1]], 1
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i64 [[TMP0]], [[START2]]
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[START]], i8 1, i64 [[TMP1]], i1 false)
+; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
+; CHECK:       loop.header:
+; CHECK-NEXT:    [[PTR_IV:%.*]] = phi i8* [ [[START]], [[ENTRY:%.*]] ], [ [[PTR_IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[_12_I:%.*]] = icmp eq i8* [[PTR_IV]], [[END]]
+; CHECK-NEXT:    br i1 [[_12_I]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
+; CHECK:       loop.latch:
+; CHECK-NEXT:    [[PTR_IV_NEXT]] = getelementptr inbounds i8, i8* [[PTR_IV]], i64 1
+; CHECK-NEXT:    br label [[LOOP_HEADER]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop.header
+
+loop.header:
+  %ptr.iv = phi i8* [ %start, %entry ], [ %ptr.iv.next, %loop.latch ]
+  %_12.i = icmp eq i8* %ptr.iv, %end
+  br i1 %_12.i, label %exit, label %loop.latch
+
+loop.latch:
+  %ptr.iv.next = getelementptr inbounds i8, i8* %ptr.iv, i64 1
+  store i8 1, i8* %ptr.iv, align 1
+  br label %loop.header
+
+exit:
+  ret void
+}