MySQL Bugs: #60199: unnecessary string length computing for my

Bug #60199	unnecessary string length computing for my_ismbchar()
Submitted:	22 Feb 2011 8:50	Modified:	18 Jan 2018 13:17
Reporter:	Guoliang Jin	Email Updates:
Status:	Won't fix	Impact on me:	None
Category:	MySQL Server: Charsets	Severity:	S5 (Performance)
Version:	5.1.55, 5.1.62	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
While studying bug 15811, I found several cases may be handled similarly:

1. mysql-5.0.91/sql/item_func.cc:2668:    register uint32 n=0, l=my_ismbchar(res->charset(),str,str+res->length());

2. mysql-5.0.91/sql/item_strfunc.cc:767:      if ((l= my_ismbchar(res->charset(),ptr,end)))
other uses of "end" can also be eliminated

3. mysql-5.0.91/sql/item_strfunc.cc:881:        if ((l=my_ismbchar(res->charset(), ptr,strend))) ptr+=l;
the other use of "strend" can also be eliminated since the use of "end" can also be eliminated

4. mysql-5.0.91/sql/sql_parse.cc:6683:        (l = my_ismbchar(system_charset_info, name, strend)))
"strend" is calculated by "char *strend=name+(uint) strlen(name);" and not used elsewhere

5. mysql-5.0.91/client/mysql.cc:2035:        (length= my_ismbchar(charset_info, pos, end_of_line)))
"end_of_line" is calculated by "char *end_of_line=line+(uint) strlen(line);" and not used elsewhere

6. mysql-5.0.91/libmysqld/libmysql.c:1648:    if (use_mb_flag && (l = my_ismbchar( mysql->charset, name , end ) ) )
"end" is calculated by "for (end=name; *end ; end++) ;" and not used elsewhere

All these calculations can be eliminated and replaced by charset->mbmaxlen

How to repeat:
By code reviewing

I noticed support for 5.0 is stopped, so I update corresponding code in 5.1.55:

1. mysql-5.1.55/sql/item_func.cc:2690:
    register uint32 n=0, l=my_ismbchar(res->charset(),str,str+res->length());
calling length()

2. mysql-5.1.55/sql/item_strfunc.cc:824:
      if ((l= my_ismbchar(res->charset(),ptr,end)))
end calculated by calling length()
other uses of "end" can also be eliminated

3. mysql-5.1.55/sql/item_strfunc.cc:946:
        if ((l=my_ismbchar(res->charset(), ptr,strend))) ptr+=l;
end calculated by calling length()
the other use of "strend" can also be eliminated since the use of "end" can also be eliminated

4. mysql-5.1.55/sql/table.cc:2741:
      int len=my_ismbchar(system_charset_info, name, end);
"strend" is calculated by "char *strend=name+(uint) strlen(name);" and not used
elsewhere

5. mysql-5.1.55/client/mysql.cc:2096:
        (length= my_ismbchar(charset_info, pos, end_of_line)))
"end_of_line" is calculated by "char *end_of_line=line+(uint) strlen(line);" and not used elsewhere

6. mysql-5.1.55/libmysql/libmysql.c:1649:
    if (use_mb_flag && (l = my_ismbchar( mysql->charset, name , end ) ) )
"end" is calculated by "for (end=name; *end ; end++) ;" and not used elsewhere

The problem here is that my_ismbchar() does not require the actual length, so calling length() will waste time and it should use mbmaxlen instead.

Thank you for the problem report. Verified by code review. Same code is still used in 5.1.62.

[11 Jan 2018 18:37] Xing Z Zhang

Bytes passed in might be truncated. It is not safe to use just cs->mbmaxlen
instead of actual string lenth.