strncpy: pad the destination with NULs

And optimize for word aligned loads/stores

Signed-off-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

src/system/libroot/posix/string/strncpy.c

@@ -5,15 +5,56 @@
 
 #include <sys/types.h>
 #include <string.h>
+#include <SupportDefs.h>
 
 
-char *
-strncpy(char *dest, char const *src, size_t count)
+/* From Bit twiddling hacks:
+	http://graphics.stanford.edu/~seander/bithacks.html */
+#define LACKS_ZERO_BYTE(value) \
+	(((value - 0x01010101) & ~value & 0x80808080) == 0)
+
+
+char*
+strncpy(char* dest, const char* src, size_t count)
 {
-	char *tmp = dest;
+	char* tmp = dest;
 
-	while (count-- && (*dest++ = *src++) != '\0')
-		;
+	// Align destination buffer for four byte writes.
+	while (((addr_t)dest & 3) != 0 && count != 0) {
+		count--;
+		if ((*dest++ = *src++) == '\0') {
+			memset(dest, '\0', count);
+			return tmp;
+		}
+	}
+
+	if (count == 0)
+		return tmp;
+
+	if (((addr_t)src & 3) == 0) {
+		// If the source and destination are aligned, copy a word
+		// word at a time
+		uint32* alignedSrc = (uint32*)src;
+		uint32* alignedDest = (uint32*)dest;
+		size_t alignedCount = count / 4;
+		count -= alignedCount * 4;
+
+		for (; alignedCount != 0 && LACKS_ZERO_BYTE(*alignedSrc);
+				alignedCount--)
+			*alignedDest++ = *alignedSrc++;
+
+		count += alignedCount * 4;
+		src = (char*)alignedSrc;
+		dest = (char*)alignedDest;
+	}
+
+	// Deal with the remainder.
+	while (count-- != 0) {
+		if ((*dest++ = *src++) == '\0') {
+			memset(dest, '\0', count);
+			return tmp;
+		}
+	}
 
 	return tmp;
 }