--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -132,6 +132,7 @@ static int utf_ptr2cells_len __ARGS((cha
 static int dbcs_char2cells __ARGS((int c));
 static int dbcs_ptr2cells_len __ARGS((char_u *p, int size));
 static int dbcs_ptr2char __ARGS((char_u *p));
+static int utf_safe_read_char_adv __ARGS((char_u **s, size_t *n));
 
 /*
  * Lookup table to quickly get the length in bytes of a UTF-8 character from
@@ -1701,6 +1702,66 @@ utf_ptr2char(p)
 }
 
 /*
+ * Convert a UTF-8 byte sequence to a wide character.
+ * String is assumed to be terminated by NUL or after "n" bytes, whichever
+ * comes first.
+ * The function is safe in the sense that it never accesses memory beyond the
+ * first "n" bytes of "s".
+ *
+ * On success, returns decoded codepoint, advances "s" to the beginning of
+ * next character and decreases "n" accordingly.
+ *
+ * If end of string was reached, returns 0 and, if "n" > 0, advances "s" past
+ * NUL byte.
+ *
+ * If byte sequence is illegal or incomplete, returns -1 and does not advance
+ * "s".
+ */
+    static int
+utf_safe_read_char_adv(s, n)
+    char_u      **s;
+    size_t      *n;
+{
+    int		c, k;
+
+    if (*n == 0) /* end of buffer */
+	return 0;
+
+    k = utf8len_tab_zero[**s];
+
+    if (k == 1)
+    {
+	/* ASCII character or NUL */
+	(*n)--;
+	return *(*s)++;
+    }
+
+    if ((size_t)k <= *n)
+    {
+	/* We have a multibyte sequence and it isn't truncated by buffer
+	 * limits so utf_ptr2char() is safe to use. Or the first byte is
+	 * illegal (k=0), and it's also safe to use utf_ptr2char(). */
+	c = utf_ptr2char(*s);
+
+	/* On failure, utf_ptr2char() returns the first byte, so here we
+	 * check equality with the first byte. The only non-ASCII character
+	 * which equals the first byte of its own UTF-8 representation is
+	 * U+00C3 (UTF-8: 0xC3 0x83), so need to check that special case too.
+	 * It's safe even if n=1, else we would have k=2 > n. */
+	if (c != (int)(**s) || (c == 0xC3 && (*s)[1] == 0x83))
+	{
+	    /* byte sequence was successfully decoded */
+	    *s += k;
+	    *n -= k;
+	    return c;
+	}
+    }
+
+    /* byte sequence is incomplete or illegal */
+    return -1;
+}
+
+/*
  * Get character at **pp and advance *pp to the next character.
  * Note: composing characters are skipped!
  */
@@ -2667,7 +2728,8 @@ static convertStruct foldCase[] =
 	{0x10400,0x10427,1,40}
 };
 
-static int utf_convert(int a, convertStruct table[], int tableSize);
+static int utf_convert __ARGS((int a, convertStruct table[], int tableSize));
+static int utf_strnicmp __ARGS((char_u *s1, char_u *s2, size_t n1, size_t n2));
 
 /*
  * Generic conversion function for case operations.
@@ -3079,6 +3141,80 @@ utf_isupper(a)
     return (utf_tolower(a) != a);
 }
 
+    static int
+utf_strnicmp(s1, s2, n1, n2)
+    char_u      *s1, *s2;
+    size_t      n1, n2;
+{
+    int		c1, c2, cdiff;
+    char_u	buffer[6];
+
+    for (;;)
+    {
+	c1 = utf_safe_read_char_adv(&s1, &n1);
+	c2 = utf_safe_read_char_adv(&s2, &n2);
+
+	if (c1 <= 0 || c2 <= 0)
+	    break;
+
+	if (c1 == c2)
+	    continue;
+
+	cdiff = utf_fold(c1) - utf_fold(c2);
+	if (cdiff != 0)
+	    return cdiff;
+    }
+
+    /* some string ended or has an incomplete/illegal character sequence */
+
+    if (c1 == 0 || c2 == 0)
+    {
+	/* some string ended. shorter string is smaller */
+	if (c1 == 0 && c2 == 0)
+	    return 0;
+	return c1 == 0 ? -1 : 1;
+    }
+
+    /* Continue with bytewise comparison to produce some result that
+     * would make comparison operations involving this function transitive.
+     *
+     * If only one string had an error, comparison should be made with
+     * folded version of the other string. In this case it is enough
+     * to fold just one character to determine the result of comparison. */
+
+    if (c1 != -1 && c2 == -1)
+    {
+	n1 = utf_char2bytes(utf_fold(c1), buffer);
+	s1 = buffer;
+    }
+    else if (c2 != -1 && c1 == -1)
+    {
+	n2 = utf_char2bytes(utf_fold(c2), buffer);
+	s2 = buffer;
+    }
+
+    while (n1 > 0 && n2 > 0 && *s1 != NUL && *s2 != NUL)
+    {
+	cdiff = (int)(*s1) - (int)(*s2);
+	if (cdiff != 0)
+	    return cdiff;
+
+	s1++;
+	s2++;
+	n1--;
+	n2--;
+    }
+
+    if (n1 > 0 && *s1 == NUL)
+	n1 = 0;
+    if (n2 > 0 && *s2 == NUL)
+	n2 = 0;
+
+    if (n1 == 0 && n2 == 0)
+	return 0;
+    return n1 == 0 ? -1 : 1;
+}
+
 /*
  * Version of strnicmp() that handles multi-byte characters.
  * Needed for Big5, Sjift-JIS and UTF-8 encoding.  Other DBCS encodings can
@@ -3092,49 +3228,21 @@ mb_strnicmp(s1, s2, nn)
     char_u	*s1, *s2;
     size_t	nn;
 {
-    int		i, j, l;
+    int		i, l;
     int		cdiff;
-    int		incomplete = FALSE;
     int		n = (int)nn;
 
-    for (i = 0; i < n; i += l)
+    if (enc_utf8)
     {
-	if (s1[i] == NUL && s2[i] == NUL)   /* both strings end */
-	    return 0;
-	if (enc_utf8)
+	return utf_strnicmp(s1, s2, nn, nn);
+    }
+    else
+    {
+	for (i = 0; i < n; i += l)
 	{
-	    l = utf_byte2len(s1[i]);
-	    if (l > n - i)
-	    {
-		l = n - i;		    /* incomplete character */
-		incomplete = TRUE;
-	    }
-	    /* Check directly first, it's faster. */
-	    for (j = 0; j < l; ++j)
-	    {
-		if (s1[i + j] != s2[i + j])
-		    break;
-		if (s1[i + j] == 0)
-		    /* Both stings have the same bytes but are incomplete or
-		     * have illegal bytes, accept them as equal. */
-		    l = j;
-	    }
-	    if (j < l)
-	    {
-		/* If one of the two characters is incomplete return -1. */
-		if (incomplete || i + utf_byte2len(s2[i]) > n)
-		    return -1;
-		/* Don't case-fold illegal bytes or truncated characters. */
-		if (utf_ptr2len(s1 + i) < l || utf_ptr2len(s2 + i) < l)
-		    return -1;
-		cdiff = utf_fold(utf_ptr2char(s1 + i))
-					     - utf_fold(utf_ptr2char(s2 + i));
-		if (cdiff != 0)
-		    return cdiff;
-	    }
-	}
-	else
-	{
+	    if (s1[i] == NUL && s2[i] == NUL)	/* both strings end */
+		return 0;
+
 	    l = (*mb_ptr2len)(s1 + i);
 	    if (l <= 1)
 	    {