Optimize mb_strcut for fixed-byte-length text encodings

On microbenchmarks run on my dev machine, mb_strcut is now ~50% faster
for fixed-byte-length text encodings like ASCII. (This is because the
previous code did an extra, unnecessary copy operation on the
resulting output string.)

Author: alexdowad

ext/mbstring/mbstring.c

@@ -2443,12 +2443,22 @@ PHP_FUNCTION(mb_strcut)
 
 	if (enc->cut) {
 		RETURN_STR(enc->cut(string.val, from, len, string.val + string.len));
-	} else {
-		ret = mbfl_strcut(&string, &result, from, len);
-		ZEND_ASSERT(ret != NULL);
-		RETVAL_STRINGL((char *)ret->val, ret->len); /* the string is already strdup()'ed */
-		efree(ret->val);
 	}
+
+	unsigned int char_len = string.encoding->flag & (MBFL_ENCTYPE_SBCS | MBFL_ENCTYPE_WCS2 | MBFL_ENCTYPE_WCS4);
+	if (char_len) {
+		/* Round `from` down to a multiple of `char_len`; works because `char_len` is a power of 2 */
+		from &= -char_len;
+		if (len > string.len - from) {
+			len = string.len - from;
+		}
+		RETURN_STR(zend_string_init_fast((const char*)(string.val + from), len & -char_len));
+	}
+
+	ret = mbfl_strcut(&string, &result, from, len);
+	ZEND_ASSERT(ret != NULL);
+	RETVAL_STRINGL((char *)ret->val, ret->len); /* the string is already strdup()'ed */
+	efree(ret->val);
 }
 /* }}} */
 

ext/mbstring/tests/mb_strcut.phpt

@@ -225,6 +225,33 @@ print "== UHC ==\n";
 
 print "Single byte 0x96: [" . bin2hex(mb_strcut("\x96", 1, 1280, "UHC")) . "]\n";
 
+print "== ASCII ==\n";
+
+print "Empty: [" . bin2hex(mb_strcut("ABC", 0, 0, "ASCII")) . "]\n";
+print "Empty: [" . bin2hex(mb_strcut("ABC", 1, 0, "ASCII")) . "]\n";
+print "Empty: [" . bin2hex(mb_strcut("ABC", 2, 0, "ASCII")) . "]\n";
+
+print "One char: [" . bin2hex(mb_strcut("ABC", 2, 1, "ASCII")) . "]\n";
+print "Two chars: [" . bin2hex(mb_strcut("ABC", 1, 2, "ASCII")) . "]\n";
+print "Two chars: [" . bin2hex(mb_strcut("ABC", 1, 3, "ASCII")) . "]\n";
+
+print "== UCS-2BE ==\n";
+
+print "Empty: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 0, 0, "UCS-2BE")) . "]\n";
+print "Empty: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 1, 0, "UCS-2BE")) . "]\n";
+print "Empty: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 2, 0, "UCS-2BE")) . "]\n";
+
+print "Empty: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 2, 1, "UCS-2BE")) . "]\n";
+print "One char: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 1, 2, "UCS-2BE")) . "]\n";
+print "Cut in middle of following char: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 1, 3, "UCS-2BE")) . "]\n";
+print "Two chars: [" . bin2hex(mb_strcut("\x00A\x00B\x00C", 1, 4, "UCS-2BE")) . "]\n";
+
+print "== UCS-4BE ==\n";
+
+print "From 1, Length 5: [" . bin2hex(mb_strcut("\x00\x00\x00\x41\x00\x00\x00\x42", 1, 5, "UCS-4BE")) . "]\n";
+print "From 1, Length 6: [" . bin2hex(mb_strcut("\x00\x00\x00\x41\x00\x00\x00\x42", 1, 6, "UCS-4BE")) . "]\n";
+print "From 1, Length 8: [" . bin2hex(mb_strcut("\x00\x00\x00\x41\x00\x00\x00\x42", 1, 8, "UCS-4BE")) . "]\n";
+
 ?>
 --EXPECT--
 == EUC-JP ==
@@ -382,3 +409,22 @@ Invalid byte 0xF5: []
 Double-byte char: []
 == UHC ==
 Single byte 0x96: [96]
+== ASCII ==
+Empty: []
+Empty: []
+Empty: []
+One char: [43]
+Two chars: [4243]
+Two chars: [4243]
+== UCS-2BE ==
+Empty: []
+Empty: []
+Empty: []
+Empty: []
+One char: [0041]
+Cut in middle of following char: [0041]
+Two chars: [00410042]
+== UCS-4BE ==
+From 1, Length 5: [00000041]
+From 1, Length 6: [00000041]
+From 1, Length 8: [0000004100000042]