Index: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -983,6 +983,13 @@
                          LLVMContext::MD_access_group};
   combineMetadata(C, cpyLoad, KnownIDs, true);
 
+  // Merging alias scope metadata requires no new domains to be introduced
+  // otherwise it could lead to bad aliasing results. Taking a simpler approach
+  // here where unless alias scope remains the same we go conservative.
+  if (C->getMetadata(LLVMContext::MD_alias_scope) !=
+      cpyLoad->getMetadata(LLVMContext::MD_alias_scope))
+    C->setMetadata(LLVMContext::MD_alias_scope, nullptr);
+
   ++NumCallSlot;
   return true;
 }
Index: llvm/test/Transforms/MemCpyOpt/callslot_badaa.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/MemCpyOpt/callslot_badaa.ll
@@ -0,0 +1,39 @@
+; RUN: opt < %s -S -memcpyopt | FileCheck --match-full-lines %s
+
+; Make sure callslot optimization goes conservative when merging scopes with different domains
+; Merging here naively generates:
+;  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 %dst, i8* align 8 %src, i64 1, i1 false), !alias.scope !3
+;  call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %src), !noalias !0
+;   ...
+;  !0 = !{!1}
+;  !1 = distinct !{!1, !2, !"hello2: %a"}
+;  !2 = distinct !{!2, !"hello2"}
+;  !3 = !{!1, !4, !5, !"hello: %a"}
+;  !4 = distinct !{!4, !5, !"hello: %a"}
+;  !5 = distinct !{!5, !"hello"}
+; Which is incorrect because the lifetime.end of %src will now "noalias" the above memcpy.
+define i8 @test(i8 %input) {
+  %tmp = alloca i8
+  %dst = alloca i8
+  %src = alloca i8
+; Note we're matching the full line and looking for the lack of !alias.scope here
+; CHECK:   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 %dst, i8* align 8 %src, i64 1, i1 false)
+  call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull %src), !noalias !3
+  store i8 %input, i8* %src
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 %tmp, i8* align 8 %src, i64 1, i1 false), !alias.scope !0
+  call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %src), !noalias !3
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 %dst, i8* align 8 %tmp, i64 1, i1 false), !alias.scope !3
+  %ret_value = load i8, i8* %dst
+  ret i8 %ret_value
+}
+
+declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8*, i8*, i64, i1)
+
+!0 = !{!1}
+!1 = distinct !{!1, !2, !"hello: %a"}
+!2 = distinct !{!2, !"hello"}
+!3 = !{!4}
+!4 = distinct !{!4, !5, !"hello2: %a"}
+!5 = distinct !{!5, !"hello2"}