Author Topic: Parsing issue with bitwise not "~" operator  (Read 2535 times)

garysdickinson

  • Expert Member
  • Posts: 482
  • Old PLC Coder
    • View Profile
Parsing issue with bitwise not "~" operator
« on: June 16, 2017, 02:45:51 PM »
The bitwise not operator, "~", is not handled very well by i-TRiLOGI.

In an arithmetic expression the bitwise not operator is only must be the first thing in the expression to be correctly parsed.  If "~" is not the first thing in the expression, then "~" and the expression that follows must by grouped with parenthesis (I'm guessing that the expression parser is recursive and this why the use of parenthesis makes the unary operator appear to be the first operator since the open parenthesis).

OK. This works:
Code: [Select]
B = ~A & &hff      ' bitwise not compiles
This fails:
Code: [Select]
B =  &hFF & ~A      ' Error:Unknown Keyword: "~"
This will compile:
Code: [Select]
B =  &hFF & (~A)      ' OK

"Bitwise not" is a unary operator and should work on the numeric expression that follows the ~. But the design of TBASIC either got the hierarchy of operators (operator precedence) wrong or was never designed to handle unwary operators correctly. Unary operators should be right below the parentheses in hierarchy and not lumped in with other binary operators (&,|,^).

The negate operator "-" has the same parsing behavior issue as "~".  The negate operator is, also, a unary operator and only works on the numeric argument that follows.  It's operator precedence should be up with the "~".

In both cases, I don't think that you can fix TBASIC, now.  It is way too late.  But you could document the behavior and suggest that the programmer force the evaluation order by using parenthesis around most usages of the unary operators "-" and "~".

The documentation on Bitwise logical operators:
iii) Bitwise Logical Operators: logical operations is perform bit-for-bit between two 16-bit integer data.

There are a major problems with the documentation:
1. All operations are 32 bit not 16 bit for current FMD, Fx, Nano-10 style PLCs.
2. The ~ operator is a unary (not binary) operator and does not "perform bit-for-bit between two 16-bit integer data [values]".  The ~ operator acts on the numeric expression that follows and the result of the operation will be 32 bit.
3. The negate, "-",  operator is a unary (not binary) operator and does not "perform bit-for-bit between two 16-bit integer data [values]".  The - operator acts on the numeric expression that follows the operator.
4. TBASIC's expression parser cannot handle 2 consecutive operators.  if the second operator is - or ~ you must enclose this operator and the expression to the operator's right in parenthesis inorder to get TBASIC to properly evaluate the expression.
5. Mixing up the terms, bitwise, logical and comparison operators.  I think of logical and comparison operators resulting in a value of 0 and not 0.  Bit wise operators result in a 32-bit value with each individual bit being some function of the value(s) and the operator involved.

Best regards,

Gary D*ckinson
« Last Edit: June 16, 2017, 05:58:42 PM by garysdickinson »