Index: llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -468,8 +468,14 @@
   Value *StorePtr = SI->getPointerOperand();
 
   // Reject stores that are so large that they overflow an unsigned.
-  uint64_t SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
-  if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
+  // When storing out scalable vectors we cannot tell if the store size would
+  // overflow an unsigned. We can potentially revisit this if there is a way
+  // to query the maximum value of vscale. We'd also have to deal with the
+  // non-constant stride too, since the code below currently only works for
+  // constant strides.
+  TypeSize SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
+  if (SizeInBits.isScalable() || (SizeInBits.getFixedSize() & 7) ||
+      (SizeInBits.getFixedSize() >> 32) != 0)
     return LegalStoreKind::None;
 
   // See if the pointer expression is an AddRec like {base,+,1} on the current
Index: llvm/test/Transforms/LoopIdiom/memcpy-vectors.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopIdiom/memcpy-vectors.ll
@@ -0,0 +1,53 @@
+; RUN: opt -loop-idiom -S <%s 2>%t | FileCheck %s
+; RUN: FileCheck --check-prefix=WARN --allow-empty %s <%t
+
+; If this check fails please read test/CodeGen/AArch64/README for instructions on how to resolve it.
+; WARN-NOT: warning
+
+define void @memcpy_fixed_vec(i64* noalias %a, i64* noalias %b) local_unnamed_addr #1 {
+; CHECK-LABEL: @memcpy_fixed_vec(
+; CHECK: entry:
+; CHECK: memcpy
+; CHECK: vector.body
+entry:
+  br label %vector.body
+
+vector.body:                                      ; preds = %vector.body, %entry
+  %index = phi i64 [ 0, %entry ], [ %index.next, %vector.body ]
+  %0 = getelementptr inbounds i64, i64* %a, i64 %index
+  %1 = bitcast i64* %0 to <2 x i64>*
+  %wide.load = load <2 x i64>, <2 x i64>* %1, align 8
+  %2 = getelementptr inbounds i64, i64* %b, i64 %index
+  %3 = bitcast i64* %2 to <2 x i64>*
+  store <2 x i64> %wide.load, <2 x i64>* %3, align 8
+  %index.next = add nuw nsw i64 %index, 2
+  %4 = icmp eq i64 %index.next, 1024
+  br i1 %4, label %for.cond.cleanup, label %vector.body
+
+for.cond.cleanup:                                 ; preds = %vector.body
+  ret void
+}
+
+define void @memcpy_scalable_vec(i64* noalias %a, i64* noalias %b) local_unnamed_addr #1 {
+; CHECK-LABEL: @memcpy_scalable_vec(
+; CHECK: entry:
+; CHECK-NOT: memcpy
+; CHECK: vector.body
+entry:
+  br label %vector.body
+
+vector.body:                                      ; preds = %vector.body, %entry
+  %index = phi i64 [ 0, %entry ], [ %index.next, %vector.body ]
+  %0 = bitcast i64* %a to <vscale x 2 x i64>*
+  %1 = getelementptr inbounds <vscale x 2 x i64>, <vscale x 2 x i64>* %0, i64 %index
+  %wide.load = load <vscale x 2 x i64>, <vscale x 2 x i64>* %1, align 16
+  %2 = bitcast i64* %b to <vscale x 2 x i64>*
+  %3 = getelementptr inbounds <vscale x 2 x i64>, <vscale x 2 x i64>* %2, i64 %index
+  store <vscale x 2 x i64> %wide.load, <vscale x 2 x i64>* %3, align 16
+  %index.next = add nuw nsw i64 %index, 1
+  %4 = icmp eq i64 %index.next, 1024
+  br i1 %4, label %for.cond.cleanup, label %vector.body
+
+for.cond.cleanup:                                 ; preds = %vector.body
+  ret void
+}