diff --git a/src/cmd/compile/internal/ssa/deadstore.go b/src/cmd/compile/internal/ssa/deadstore.go
index 5129c17..59ac1b9 100644
--- a/src/cmd/compile/internal/ssa/deadstore.go
+++ b/src/cmd/compile/internal/ssa/deadstore.go
@@ -14,8 +14,7 @@
 	defer f.retSparseSet(loadUse)
 	storeUse := f.newSparseSet(f.NumValues())
 	defer f.retSparseSet(storeUse)
-	shadowed := f.newSparseSet(f.NumValues())
-	defer f.retSparseSet(shadowed)
+	shadowed := newSparseMap(f.NumValues()) // TODO: cache
 	for _, b := range f.Blocks {
 		// Find all the stores in this block. Categorize their uses:
 		//  loadUse contains stores which are used by a subsequent load.
@@ -81,17 +80,18 @@
 			shadowed.clear()
 		}
 		if v.Op == OpStore || v.Op == OpZero {
-			if shadowed.contains(v.Args[0].ID) {
+			sz := v.AuxInt
+			if shadowedSize := int64(shadowed.get(v.Args[0].ID)); shadowedSize != -1 && shadowedSize >= sz {
 				// Modify store into a copy
 				if v.Op == OpStore {
 					// store addr value mem
 					v.SetArgs1(v.Args[2])
 				} else {
 					// zero addr mem
-					sz := v.Args[0].Type.ElemType().Size()
-					if v.AuxInt != sz {
+					typesz := v.Args[0].Type.ElemType().Size()
+					if sz != typesz {
 						f.Fatalf("mismatched zero/store sizes: %d and %d [%s]",
-							v.AuxInt, sz, v.LongString())
+							sz, typesz, v.LongString())
 					}
 					v.SetArgs1(v.Args[1])
 				}
@@ -99,7 +99,10 @@
 				v.AuxInt = 0
 				v.Op = OpCopy
 			} else {
-				shadowed.add(v.Args[0].ID)
+				if sz > 0x7fffffff { // work around sparseMap's int32 value type
+					sz = 0x7fffffff
+				}
+				shadowed.set(v.Args[0].ID, int32(sz))
 			}
 		}
 		// walk to previous store
diff --git a/src/cmd/compile/internal/ssa/deadstore_test.go b/src/cmd/compile/internal/ssa/deadstore_test.go
index c38f1cd..beb6de0 100644
--- a/src/cmd/compile/internal/ssa/deadstore_test.go
+++ b/src/cmd/compile/internal/ssa/deadstore_test.go
@@ -8,7 +8,7 @@
 
 func TestDeadStore(t *testing.T) {
 	c := testConfig(t)
-	elemType := &TypeImpl{Size_: 8, Name: "testtype"}
+	elemType := &TypeImpl{Size_: 1, Name: "testtype"}
 	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr", Elem_: elemType} // dummy for testing
 	fun := Fun(c, "entry",
 		Bloc("entry",
@@ -18,7 +18,7 @@
 			Valu("addr1", OpAddr, ptrType, 0, nil, "sb"),
 			Valu("addr2", OpAddr, ptrType, 0, nil, "sb"),
 			Valu("addr3", OpAddr, ptrType, 0, nil, "sb"),
-			Valu("zero1", OpZero, TypeMem, 8, nil, "addr3", "start"),
+			Valu("zero1", OpZero, TypeMem, 1, nil, "addr3", "start"),
 			Valu("store1", OpStore, TypeMem, 1, nil, "addr1", "v", "zero1"),
 			Valu("store2", OpStore, TypeMem, 1, nil, "addr2", "v", "store1"),
 			Valu("store3", OpStore, TypeMem, 1, nil, "addr1", "v", "store2"),
@@ -95,3 +95,32 @@
 		t.Errorf("store %s incorrectly removed", v)
 	}
 }
+
+func TestDeadStoreUnsafe(t *testing.T) {
+	// Make sure a narrow store can't shadow a wider one. The test above
+	// covers the case of two different types, but unsafe pointer casting
+	// can get to a point where the size is changed but type unchanged.
+	c := testConfig(t)
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("start", OpInitMem, TypeMem, 0, nil),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Valu("v", OpConstBool, TypeBool, 1, nil),
+			Valu("addr1", OpAddr, ptrType, 0, nil, "sb"),
+			Valu("store1", OpStore, TypeMem, 8, nil, "addr1", "v", "start"),  // store 8 bytes
+			Valu("store2", OpStore, TypeMem, 1, nil, "addr1", "v", "store1"), // store 1 byte
+			Goto("exit")),
+		Bloc("exit",
+			Exit("store2")))
+
+	CheckFunc(fun.f)
+	cse(fun.f)
+	dse(fun.f)
+	CheckFunc(fun.f)
+
+	v := fun.values["store1"]
+	if v.Op == OpCopy {
+		t.Errorf("store %s incorrectly removed", v)
+	}
+}