diff --git a/lib/SILPasses/LoadStoreOpts.cpp b/lib/SILPasses/LoadStoreOpts.cpp
index 832eccc5c39..3957de2f904 100644
--- a/lib/SILPasses/LoadStoreOpts.cpp
+++ b/lib/SILPasses/LoadStoreOpts.cpp
@@ -409,6 +409,15 @@ forwardAddressValueToUncheckedAddrToLoad(SILValue Address,
 
   // If the output is trivial, we have a trivial bit cast.
   if (OutputIsTrivial) {
+
+    // The structs could have different size. We have code in the stdlib that
+    // casts pointers to differently sized integer types. This code prevents
+    // that we bitcast the values.
+    SILType InputTy = UADCI->getOperand().getType();
+    if (OutputTy.getStructOrBoundGenericStruct() &&
+        InputTy.getStructOrBoundGenericStruct())
+      return SILValue();
+
     CastValue =  B.createUncheckedTrivialBitCast(UADCI->getLoc(), StoredValue,
                                                  OutputTy.getObjectType());
   } else {
diff --git a/lib/SILPasses/SILCombine.cpp b/lib/SILPasses/SILCombine.cpp
index 89533714921..1de11129b11 100644
--- a/lib/SILPasses/SILCombine.cpp
+++ b/lib/SILPasses/SILCombine.cpp
@@ -1506,6 +1506,12 @@ visitPointerToAddressInst(PointerToAddressInst *PTAI) {
   return nullptr;
 }
 
+static bool areBothStructs(SILType Ty1, SILType Ty2) {
+  if (Ty1.getStructOrBoundGenericStruct() && Ty2.getStructOrBoundGenericStruct())
+    return true;
+  else return false;
+}
+
 SILInstruction *
 SILCombiner::visitUncheckedAddrCastInst(UncheckedAddrCastInst *UADCI) {
   SILModule &Mod = UADCI->getModule();
@@ -1546,6 +1552,13 @@ SILCombiner::visitUncheckedAddrCastInst(UncheckedAddrCastInst *UADCI) {
   if (InputIsTrivial && !OutputIsTrivial)
     return nullptr;
 
+  // The structs could have different size. We have code in the stdlib that
+  // casts pointers to differently sized integer types. This code prevents that
+  // we bitcast the values.
+  if (InputTy.getStructOrBoundGenericStruct() &&
+      OutputTy.getStructOrBoundGenericStruct())
+    return nullptr;
+
   // For each user U of the unchecked_addr_cast...
   for (auto U : UADCI->getUses())
     // Check if it is load. If it is not a load, bail...
diff --git a/test/SILPasses/globalloadstoreopts.sil b/test/SILPasses/globalloadstoreopts.sil
index 4236611eb22..85c86b9c4ae 100644
--- a/test/SILPasses/globalloadstoreopts.sil
+++ b/test/SILPasses/globalloadstoreopts.sil
@@ -638,3 +638,23 @@ bb2:
   %9999 = tuple()
   return %9999 : $()
 }
+
+struct C {
+  var i : Builtin.Int16
+}
+
+// Don't bitcast differently sized structs.
+// CHECK-LABEL: sil @store_to_load_forward_unchecked_addr_cast_different_sized_struct
+// CHECK-NOT: unchecked_trivial_bit_cast
+// CHECK: return
+
+sil @store_to_load_forward_unchecked_addr_cast_different_sized_struct : $@thin (A) -> () {
+bb0(%0 : $A):
+  %1 = alloc_stack $A
+  store %0 to %1#1 : $*A
+  %2 = unchecked_addr_cast %1#1 : $*A to $*C
+  %3 = load %2 : $*C
+  dealloc_stack %1#0 : $*@local_storage A
+  %9999 = tuple()
+  return %9999 : $()
+}
diff --git a/test/SILPasses/sil_combine.sil b/test/SILPasses/sil_combine.sil
index 2cf365aa4c9..63c8f496740 100644
--- a/test/SILPasses/sil_combine.sil
+++ b/test/SILPasses/sil_combine.sil
@@ -1704,3 +1704,25 @@ bb0(%0 : $Builtin.NativeObject):
   return %3 : $Builtin.NativeObject
 }
 
+struct FakeInt16 {
+  var val : Builtin.Int16
+}
+
+struct FakeInt32 {
+  var val : Builtin.Int32
+}
+
+// Don't promote this to an unchecked_trivial_bit_cast.
+
+// CHECK-LABEL: sil @unchecked_addr_cast_of_different_size_structs
+// CHECK-NOT: unchecked_trivial_bit_cast
+// CHECK: return
+
+sil @unchecked_addr_cast_of_different_size_structs : $@thin (@inout FakeInt32) -> Builtin.Int16 {
+bb0(%0 : $*FakeInt32):
+  %1 = unchecked_addr_cast %0 : $*FakeInt32 to $*FakeInt16
+  %2 = load %1 : $*FakeInt16
+  %3 = struct_extract %2 : $FakeInt16, #FakeInt16.val
+  return %3 : $Builtin.Int16
+}
+