[SOLVED] Java compiler

Modify the recursive descent compiler for the language PL0 (provided on the course web page with this assignment) to add askipstatement, a multiple assignment statement, and acase(switch) statement.
Please ensure you follow the course Piazza bulletin board for any updates and further information on the assignment.
You shouldonlymodify the files that must be submitted (see below). You shouldnotmodify any other files because we will be testing your implementation using the existing other files with your submitted files.
Please donotreformat the files because we would like to just print the differences between the originals and the versions you hand in. Please keep the length of lines in your files below 120 characters, so that we can print them sensibly. Either avoid using tabs or set your tabs stops to 4 spaces (this is the default for Eclipse) so that your files will print sensibly.
Your implementation should be in Java 1.8. Set the Eclipse preferences for the Java compiler to 1.8 (or if not in Eclipse use the -source 1.8 option to the Java compiler). The compiler will compile under Java 1.6 and that level is sufficient for doing your assignment.
Read the fine print in detail before starting! When you have finished implementing the assignment, come back and re-read the fine print again.
Overview
The addition of askipstatement allows one to write
if x < 0 then x := -x else skipto assign x its absolute value.The multiple assignment statement allows set of assignments to take place simultaneously, conceptually. The trick is that all the expressions are evaluated before any of the assignments takes place, for example, the following multiple assignmentswapsthe values of x and y:x := y | y := xThe following assignment rotates the values of x, y and z:x := y | y := z | z := xThe followingcasestatement assigns to y exactly once.case i ofwhen 2: y := 1when 3: y := xwhen 5: y := x*xdefault y := 42endSyntax ChangesThe following lexical tokens have been added to the scanner module of PL0 already.BAR”|”KW_CASE”case”KW_OF”of”KW_WHEN”when”KW_DEFAULT”default”KW_SKIP”skip”Keywords are case sensitive.The non-terminalStatementin the grammar for PL0 is modified to the following Extended Backus-Naur Form (EBNF) grammar rules.StatementWhileStatement | IfStatement | CallStatement |Assignment | ReadStatement | WriteStatement | CompoundStatement |SkipStatement | CaseStatementwhereSkipStatementKW_SKIPAssignmentSingleAssign { BAR SingleAssign }SingleAssignLValue ASSIGN ConditionCaseStatementKW_CASE Condition KW_OF { CaseBranch } [ KW_DEFAULT StatementList ] KW_ENDCaseBranchKW_WHEN Constant COLON StatementListNote that the branches of thecasestatement contain statement lists rather than just a single statement.Static Semantic RestrictionsSkip statementThere aren’t any static semantic restrictions on theskipstatement, that is, it is well formed for any symbol table context.syms WFStatement(skip)Multiple assignmentAll of the variables on the left side of a multiple assignment must be distinct. The type of each condition (expression) on the right side of an assignment must be assignment compatible with (coercible to) the base type of the corresponding variable to which it is assigned. i, j [1..n] i j vi vj i [1..n] (syms vi: ref(Ti)) && (syms ei: Ti)syms WFStatement(v1:= e1| v2:= e2| … | vn:= en)Case statementThe type of the expression (condition) for thecasestatement must be a scalar type (it could be int or boolean or a subrange thereof) and all labels of case branches must be compatible with that type and must be elements of the type (see containsElement in Type.java). The labels must be constant expressions, and the value of each label must be unique within a singlecasestatement. The bodies of all the case branches must be well formed.The rule for well formedness of acasestatement is given below; within it T must be a scalar type and for a scalar type values(T) is the set of values in the type T, e.g. values(boolean) = {false,true} and values([-2..2]) = {-2,-1,0,1,2}.syms e : Tsyms WFStatement(ss) j [1..n] syms WFStatement(ssj) && syms cj: T && (cjevj vj values(T)) i,j [1..n] i j && cievi && cjevj vi vjsyms WFStatement(case e of when c1: ss1when c2: ss2… when cn: ssndefault ss end)Dynamic SemanticsSkip statementTheskipstatement does nothing. The less it does the better.Multiple assignmentFor a multiple assignment, all the expressions in all the assignments are evaluated before any of the variables are assigned, and then all of the assignments take place. Note that it doesn’t matter in what order the simple assignments are done because the left sides are all distinct and the expressions have all been evaluated using the values of the variables before any assignments take place (and the language does not have any side effects).Case statementThecasestatement evaluates its (scalar) expression and then selects the branch for which the value of the expression is equal to the label on that branch and executes the corresponding statement. If the value of the expression is not equal to any of the labels of any of the branches then thedefaultbranch is executed. If there is nodefaultbranch, the programs stops using instruction STOP with an exit code of StackMachine.CASE_LABEL_MISSING.On completion of the execution of the appropriate branch within acasestatement, the next statement executed is the one following the wholecasestatement. That is, it isNOTlike a switch statement in which execution of one branch flows into the next branch unless abreakstatement is placed at the end of the first branch — a language feature that is the source of many a bug.Object CodeThe PL0 compiler generates code for the Stack Machine. A description of the stack machine is available inInst.pdf. See also the classStackMachinefor details of the instruction set.The code generation is done in classCodeGeneratorby making calls to methods in classCodethat stores instructions in a code sequence. There are also utility methods withinCodethat may be of use for implementing the code generation.The code generation for theskipstatement is trivial.The code generated for the multiple assignment does not need to make use of additional temporary variables, but it can (should) use the stack. It can be done quite simply (once you work out how) with the same number of instructions as the equivalent single assignments.The code generation for thecasestatement can be implemented via a branch table, i.e., it uses a branch that branches to one of a sequence of branches all stored one after the other (or a table of addresses). Which branch the first branch branches to is determined dynamically by the value of the case expression. The selected branch then transfers control (branches) to the code for the corresponding statement list for that case. The code generation needs to handle values of the expression that are outside the range of values of the labels in thecasestatement as well as gaps in thecasestatement. The code generation for thecasestatement is quite intricate. You should draw up a plan of attack before diving into code. Most of the code for the branches can be generated and saved and then the code for the case statement can be assembled using the knowledge of the sizes of all the branches, etc.There are methods withinCodethat may be of use for implementing the code generation. In particular,genJumpIfFalseandgenJumpAlwaysgenerate branches to a given offset. These two methods also return the index into the Code array of theLOAD_CONinstruction that loads the offset ready for the branch. This allows one to later patch the offset usingupdateLoadConinCode. But beware that the index of an instruction in the code array does not correspond to its offset in the generated machine code because theLOAD_CONinstruction takes two words.There are two basic approaches to generating the code for the case statement. One generates all the fragments of the code, except for jumps, and then assembles it and adds the jumps once the size of all the fragments is known. Working out the exact offsets for the jumps requires some careful planning. The other approach generates all the code, including jumps, but if the destination of a jump is unknown at the time it is generated, a dummy value is used which is later patched (usingupdateLoadCon) to the correct offset as soon as that is known, i.e. patch all the references to an address in the code when the code generation has reached that point.FilesAll sources for the assignment 1 PL0 compiler are available on the web page as a1-compiler.zip. Please be sure to use the version for assignment 1 and not the one used for the tutorials. There are differences (like the lexical tokens you need for this assignment are only defined in the assignment version). There is a file calleda1-README.htmlthat contains a brief description of the files.Seecompiler.htmlfor details on running the compiler and its options.SubmissionYou need to submit the following list of individual files, not a .zip file. Note that file names are case-sensitive. You must only modify the files Parser.java StaticChecker.java CodeGenerator.java StatementNode.java StatementVisitor.java StatementTransform.javafor evaluation and marking. You can submit your assignment multiple times, but only the last copy submitted will be retained for marking.Please keep the length of lines in your files below 120 characters, so that we can print them sensibly. You should avoid using tabs or set your tabs stops to 4 spaces so that when we print them (with tab stops set to 4 spaces) they will print sensibly. Don’t forget to remove any code generating debugging output before submission.