diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1555,7 +1555,7 @@
     // FIXME: For multi-BB cases, we need to see reachability from it to
     // store.
     // Bailout if Dest may have any ModRef before Store.
-    if (UI->comesBefore(Store) && isModOrRefSet(Res))
+    if (DT->dominates(UI, Store) && isModOrRefSet(Res))
       return false;
     DestModRef |= BAA.getModRefInfo(UI, DestLoc);
 
diff --git a/llvm/test/Transforms/MemCpyOpt/cross-block.ll b/llvm/test/Transforms/MemCpyOpt/cross-block.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/MemCpyOpt/cross-block.ll
@@ -0,0 +1,40 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -passes=early-cse,memcpyopt -verify-memoryssa | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+%struct.nsRect = type { i32, i32, i32, i32 }
+
+define void @test_case(ptr %this, ptr %aFrame, i1 %call14) unnamed_addr {
+; CHECK-LABEL: @test_case(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[R:%.*]] = alloca [[STRUCT_NSRECT:%.*]], align 8
+; CHECK-NEXT:    [[REF_TMP18:%.*]] = alloca [[STRUCT_NSRECT]], align 8
+; CHECK-NEXT:    store i32 0, ptr [[R]], align 4
+; CHECK-NEXT:    br i1 [[CALL14:%.*]], label [[IF_THEN17:%.*]], label [[IF_END21:%.*]]
+; CHECK:       if.then17:
+; CHECK-NEXT:    call void @fwd_decl(ptr [[REF_TMP18]], ptr [[R]])
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[R]], ptr [[REF_TMP18]], i64 16, i1 false)
+; CHECK-NEXT:    br label [[IF_END21]]
+; CHECK:       if.end21:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %r = alloca %struct.nsRect
+  %ref.tmp18 = alloca %struct.nsRect
+  %x = getelementptr inbounds %struct.nsRect, ptr %r, i32 0, i32 0
+  store i32 0, ptr %x
+  br i1 %call14, label %if.then17, label %if.end21
+
+if.then17:
+  call void @fwd_decl(ptr %ref.tmp18, ptr %r)
+  call void @llvm.memcpy.p0.p0.i64(ptr %r, ptr %ref.tmp18, i64 16, i1 false)
+  br label %if.end21
+
+if.end21:
+  ret void
+}
+
+declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg)
+
+declare hidden void @fwd_decl(ptr, ptr )