CS 2210 – Project 3: Decaf Errors & IR
Due: Sunday, March 24, 2013 by 11:59pm
In this part of the project, we will add error handling to the parser and create a semantic analyzer. At the end of this phase, we will generate quadruples of three-address code as our intermediate representation.
JavaCUP and YACC/bison support a special “error” token that a production can go to. For example, we may have an error in a statement and thus we would add a production such as:
statement ::= … /* Other productions */
|
error SEMICOLON
;
The semicolon (and additional tokens afterward) is a point of synchronization
that gets the parser back on track by discarding erroneous input off of the
stack.
For your project, handle the following four error situations:
1. Statements synchronized by a semicolon
2. Variable declarations synchronized by a semicolon
3. Class and method bodies synchronized by a }
4. Formal and actual parameter lists synchronized by a )
For each parser error (regardless of using the error symbol), report the line and character position that the error occurred on:
Parse error at line %d, column %d
With the AST that the first part of the compiler generated, we can now do the name and type analysis for Decaf.
Name analysis discovers identifier names that are improperly defined.
We will handle two types of errors in this step:
1. Redefinition of an identifier in a given scope
2. Use of an undefined identifier
To do name analysis and later parts of the project, you will need to construct a symbol table. You should use either the tree of hashtables or the stack of hashtables to represent your nested scopes.
As you build the symbol table, or in a second pass, you may encounter the errors listed above.
If you discover more than one declaration of an identifier in a scope, output the message:
Multiply defined identifier %s at line %d, character %d
If you discover that an undefined identifier is used, output the message:
Use of undefined identifier %s at line %d, character %d
With AST and Symbol Table in hand, we next need to do type checking.
We will identify the following type errors:
1. Assignment to a class or method name
2. Assignment from a class or method name
3. Attempting to call a class or method name
4. Attempting to extend from a non-class
5. Attempting to inherit from a non-interface
6. Missing a function declaration specified by an inherited interface
7. Calling a method with the wrong number and/or types of arguments
8. Applying an operator to the wrong type
9. Using a non-boolean expression in an if or while condition
10. Type mismatch during assignment
If there is an attempt to assign to a class or method name, or the keyword this, output the message:
Invalid l-value, %s is a %s, at line %d, character %d
If there is an attempt to assign from a class or method name, output the message:
Invalid r-value: %s is a %s, at line %d, character %d
If an operator is applied to a class or method name, output the message:
Invalid operands for %s operator, at line %d, character %d
If an attempt to extend something other than a class is made, output:
Attempt to extend a non-class at line %d, character %d
If an attempt to inherit something other than an interface is made, output:
Attempt to inherit from a non-interface at line %d, character %d
If a class inherits an interface but does not implement a required function, output:
Class %s (line %d, character %d) does not implement method %s from interface %s
If an attempt to call something that isn’t a method (class, variable, etc.) is made, output:
Attempt to call a non-method at line %d, character %d
If a method is called with the wrong number of arguments:
Call of method %s does not match its declared number of arguments at line %d, character %d
If a method is called with the wrong type of argument:
Call of method %s does not match its declared signature at line %d, character %d
If you attempt to use an arithmetic or comparison operator with non-integer or double operands, or use Strings with an operator other than == or !=, output:
Invalid operand for operator %c at line %d, character %d
If you attempt to use a Boolean operator with non-boolean types, output:
Attempt to use boolean operator %s on non-boolean operands at line %d, character %d
If you attempt to use .length on any type other than an array, output:
Length property only applies to arrays, line %d, character %d
If the condition of an if or while statement does not evaluate to a boolean, output:
Non-boolean expression used as the condition of %s statement at line %d, character %d
If the right hand side of an assignment does not match the type of the left hand side*, output:
Type mismatch during assignment at line %d, character %d
* The exception to this is the subclass/base class and interface relationship. Any subclass or class that inherits from an interface may be assigned to a reference of the Interface or superclass type.
If you try to use this in a static method (main), output:
Illegal use of keyword ‘this’ in static method at line %d, character %d
Note:
· For each error, the character output should be the index of the first letter of the offending code
Make sure that there is a main() method that returns void and takes no parameters. In the absence of one, print an error:
Error. No main method.
We will treat Print(), ReadLine() and ReadInteger() as function calls that do not require definition. (You may wish to add them to your symbol table to make semantic analysis easier.) Print may only take one parameter of type string, int, or bool.
The last part of this phase will be to generate IR code. We will generate quadruples of three-address code.
These are the IR statements to use:
|
IR Operation |
Three-address code |
|
Assignment |
x := y op z |
|
Unary Assignment |
x := op y |
|
Copy |
x := y |
|
Unconditional Jump |
goto LABEL |
|
Conditional Jump |
iftrue x goto LABEL |
|
|
iffalse x goto LABEL |
|
Parameter |
param x |
|
Call |
x := call f, NUMPARAMS |
|
Return |
return y |
|
Indexed assignment |
x := y[i] |
|
|
y[i] := x |
|
New |
x := new TYPE |
|
New Array |
x := new TYPE, SIZE |
|
Length |
x := length y |
You will likely want the operands as pointers to entries in the symbol table. The keyword “this” is a local variable implicitly defined as the first parameter in each non-static method. When calling a non-static method, pass the left hand side of the dot operator as the value of parameter this.
For example:
c.f(a)
should be translated into:
param c
param a
call f, 2
Treat true and false as constants of your choosing.
Generate IR on a method-by-method basis. At the end of this phase, print out your IR using the above style, one IR statement per line. Make sure that the main() method is the first one you output.