diff --git a/libc/bionic/memmove_words.c b/libc/bionic/memmove_words.c
new file mode 100644
index 0000000..22058bc
--- /dev/null
+++ b/libc/bionic/memmove_words.c
@@ -0,0 +1,147 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <stdlib.h>
+#include <assert.h>
+
+/*
+ * Works like memmove(), except:
+ * - if all arguments are at least 32-bit aligned, we guarantee that we
+ *   will use operations that preserve atomicity of 32-bit values
+ * - if not, we guarantee atomicity of 16-bit values
+ *
+ * If all three arguments are not at least 16-bit aligned, the behavior
+ * of this function is undefined.  (We could remove this restriction by
+ * testing for unaligned values and punting to memmove(), but that's
+ * not currently useful.)
+ *
+ * TODO: add loop for 64-bit alignment
+ * TODO: use __builtin_prefetch
+ * TODO: write an ARM-optimized version
+ */
+void _memmove_words(void* dest, const void* src, size_t n)
+{
+    assert((((uintptr_t) dest | (uintptr_t) src | n) & 0x01) == 0);
+
+    char* d = (char*) dest;
+    const char* s = (const char*) src;
+    size_t copyCount;
+
+    /*
+     * If the source and destination pointers are the same, this is
+     * an expensive no-op.  Testing for an empty move now allows us
+     * to skip a check later.
+     */
+    if (n == 0 || d == s)
+        return;
+
+    /*
+     * Determine if the source and destination buffers will overlap if
+     * we copy data forward (i.e. *dest++ = *src++).
+     *
+     * It's okay if the destination buffer starts before the source and
+     * there is some overlap, because the reader is always ahead of the
+     * writer.
+     */
+    if (__builtin_expect((d < s) || ((size_t)(d - s) >= n), 1)) {
+        /*
+         * Copy forward.  We prefer 32-bit loads and stores even for 16-bit
+         * data, so sort that out.
+         */
+        if ((((uintptr_t) d | (uintptr_t) s) & 0x03) != 0) {
+            /*
+             * Not 32-bit aligned.  Two possibilities:
+             * (1) Congruent, we can align to 32-bit by copying one 16-bit val
+             * (2) Non-congruent, we can do one of:
+             *   a. copy whole buffer as a series of 16-bit values
+             *   b. load/store 32 bits, using shifts to ensure alignment
+             *   c. just copy the as 32-bit values and assume the CPU
+             *      will do a reasonable job
+             *
+             * We're currently using (a), which is suboptimal.
+             */
+            if ((((uintptr_t) d ^ (uintptr_t) s) & 0x03) != 0) {
+                copyCount = n;
+            } else {
+                copyCount = 2;
+            }
+            n -= copyCount;
+            copyCount /= sizeof(uint16_t);
+
+            while (copyCount--) {
+                *(uint16_t*)d = *(uint16_t*)s;
+                d += sizeof(uint16_t);
+                s += sizeof(uint16_t);
+            }
+        }
+
+        /*
+         * Copy 32-bit aligned words.
+         */
+        copyCount = n / sizeof(uint32_t);
+        while (copyCount--) {
+            *(uint32_t*)d = *(uint32_t*)s;
+            d += sizeof(uint32_t);
+            s += sizeof(uint32_t);
+        }
+
+        /*
+         * Check for leftovers.  Either we finished exactly, or we have
+         * one remaining 16-bit chunk.
+         */
+        if ((n & 0x02) != 0) {
+            *(uint16_t*)d = *(uint16_t*)s;
+        }
+    } else {
+        /*
+         * Copy backward, starting at the end.
+         */
+        d += n;
+        s += n;
+
+        if ((((uintptr_t) d | (uintptr_t) s) & 0x03) != 0) {
+            /* try for 32-bit alignment */
+            if ((((uintptr_t) d ^ (uintptr_t) s) & 0x03) != 0) {
+                copyCount = n;
+            } else {
+                copyCount = 2;
+            }
+            n -= copyCount;
+            copyCount /= sizeof(uint16_t);
+
+            while (copyCount--) {
+                d -= sizeof(uint16_t);
+                s -= sizeof(uint16_t);
+                *(uint16_t*)d = *(uint16_t*)s;
+            }
+        }
+
+        /* copy 32-bit aligned words */
+        copyCount = n / sizeof(uint32_t);
+        while (copyCount--) {
+            d -= sizeof(uint32_t);
+            s -= sizeof(uint32_t);
+            *(uint32_t*)d = *(uint32_t*)s;
+        }
+
+        /* copy leftovers */
+        if ((n & 0x02) != 0) {
+            d -= sizeof(uint16_t);
+            s -= sizeof(uint16_t);
+            *(uint16_t*)d = *(uint16_t*)s;
+        }
+    }
+}