Assignment: Translation

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In this project students implement a translator that takes the Abstract Syntax Tree (AST) output from the semantic analysis pass in Project 4 and converts it into an Intermediate Representation (IR) tree.


The AST representations of the Xinu external calls must be mapped to IR nodes that reference the corresponding underlying runtime functions. Care must be taken at this point to choose runtime function names that will not conflict with legitimate source language method names; we follow longstanding tradition and append an underscore to the runtime function names. The mapping of I/O and threading functions is self-explanatory, but also at this point we need to add in a dynamic memory allocation function to support the source language new operator for instantiating new objects and arrays. In addition, if the compiler is to support Java-like runtime checking for null pointers and array bounds, corresponding runtime error handlers must be mapped. The figure below gives a brief description of each part of the MiniJava to Xinu Compatability Layer, but the full c file is also available.

Changes must also be made to support the synchronized key word. First monitors must be added to Xinu. The new function in the compatibility layer must acquire a monitor from the O/S and associate it with the new object. Also, _lock and _unlock functions must be added to the compatibility layer which map to the monitor lock and unlock functions added to Xinu. To achieve Java-like thread synchronization the compiler must wrap synchronized method bodies with calls to the compatibility functions _lock and _unlock. It is important that the compiler ensures that the lock action precedes the evaluation of any part of the method body including evaluation of the right hand side of local variable declarations. Similarly, the unlock action must come after any part of the method body including the evaluation of the return expression.

MiniJava to Xinu Compatibility Layer

Xinu function Purpose
int _readint(void) Parse in integer input
syscall _printint(int i) Print an integer
syscall _print(char *s) Print a string literal
syscall _println(void) Print a carriage return
syscall _threadCreate(int *t) Spawn a new thread of execution, running t's run method
syscall _yield(void) Yield the processor
syscall _sleep(int time) Sleep time number of milliseconds
syscall _lock(int *obj) Lock the monitor associated with object obj
syscall _unlock(int *obj) Unlock the monitor associated with object obj
int *_new(int n, int f) Allocate array or allocate object and associate a monitor with that object
void _BADPTR(void) Null pointer exception
void _BADSUB(void) Bounds exception