Add alternative memcpy implementation

core/klibc.c

@@ -16,14 +16,66 @@ int memcmp(const void *aptr, const void *bptr, size_t size)
     return 0;
 }
 
-void *memcpy(void *dst, const void *src, size_t n)
+// Inspirated by https://interrupt.memfault.com/blog/memcpy-newlib-nano
+/* Nonzero if either X or Y is not aligned on a "long" boundary.  */
+#define UNALIGNED(X, Y) \
+  (((long)X & (sizeof (long) - 1)) | ((long)Y & (sizeof (long) - 1)))
+
+/* How many bytes are copied each iteration of the 4X unrolled loop.  */
+#define BIGBLOCKSIZE    (sizeof (long) << 2)
+
+/* How many bytes are copied each iteration of the word copy loop.  */
+#define LITTLEBLOCKSIZE (sizeof (long))
+
+/* Threshhold for punting to the byte copier.  */
+#define TOO_SMALL(LEN)  ((LEN) < BIGBLOCKSIZE)
+
+void *memcpy(void *dst0, const void *src0, size_t len0)
 {
-    char *dstChar       = dst;
-    const char *srcChar = src;
-    for (size_t i = 0; i < n; i++) {
+#if 0
+    char *dstChar       = dst0;
+    const char *srcChar = src0;
+    for (size_t i = 0; i < len0; i++) {
         *(dstChar++) = *(srcChar++);
     }
-    return dst;
+    return dst0;
+#else
+    char *dst       = dst0;
+    const char *src = src0;
+    long *aligned_dst;
+    const long *aligned_src;
+
+    /* If the size is small, or either SRC or DST is unaligned,
+       then punt into the byte copy loop.  This should be rare.  */
+    if (!TOO_SMALL(len0) && !UNALIGNED(src, dst)) {
+        aligned_dst = (long *)dst;
+        aligned_src = (long *)src;
+
+        /* Copy 4X long words at a time if possible.  */
+        while (len0 >= BIGBLOCKSIZE) {
+            *aligned_dst++ = *aligned_src++;
+            *aligned_dst++ = *aligned_src++;
+            *aligned_dst++ = *aligned_src++;
+            *aligned_dst++ = *aligned_src++;
+            len0 -= BIGBLOCKSIZE;
+        }
+
+        /* Copy one long word at a time if possible.  */
+        while (len0 >= LITTLEBLOCKSIZE) {
+            *aligned_dst++ = *aligned_src++;
+            len0 -= LITTLEBLOCKSIZE;
+        }
+
+        /* Pick up any residual with a byte copier.  */
+        dst = (char *)aligned_dst;
+        src = (char *)aligned_src;
+    }
+
+    while (len0--)
+        *dst++ = *src++;
+
+    return dst0;
+#endif
 }
 
 void *memset(void *src, int c, size_t n)