Step4 Instructions

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Project Step 4 — Part 1: Expressions

Your goal in part 1 is to generate code for expressions. To do this, we will build semantic

actions that generate code in an intermediate representation (IR) for assignment statements

and expressions, and then translate that intermediate representation to assembly code.

We recommend that you do this in three steps, as it will make it easier to debug your code,

but you can also choose to do it in two steps (step 1 is optional):

1. Generate an abstract syntax tree (AST) for the code in your function.

2. Convert the AST into a sequence of IR Nodes that implement your function using three

address code.

3. Traverse your sequence of IR Nodes to generate assembly code.

We will discuss each of these steps next. (Note: in this step 4, we will only have one function

in our program, main. You can assume that all variables are defined globally. There will not

be any additional variables defined in main().)

Abstract Syntax Tree

An Abstract Syntax Tree is essentially, a cleaned-up form of your parse tree that more

straightforwardly captures the structure of expressions, control constructs, etc. in your

program. For many compilers, the AST is the intermediate representation, though we will

further convert the AST into another intermediate representation.

What is the difference between a parse tree and an AST?

Parse trees capture all of the little details necessary to implement your grammar. This means

that it often contains extraneous information beyond what is necessary to capture the details

of a piece of code (e.g., there are nodes for tokens like ";", and nodes for all of the sub-

constructs we used to correctly implement order of operations). ASTs, in contrast, contain

exactly the information needed to capture the meaning of an expression, including being

structured to preserve order of operations.

For example, consider the parse tree for a + b * c:

Complicated, huh? Here's an abstract syntax tree that captures the same thing:

Much simpler! We aren't preserving anything except the bare minimum needed to describe

the expression (note that we included the type of each of the variables in the program -- we

can get that information from our symbol table!).

Building an AST

Note that the information in the AST is associated with various nodes in the parse tree. We

can use semantic actions, just as we did in Step 3, to pass information "up" the parse tree to

build up the AST. Instead of passing information about a declaration, we can instead pass

partially constructed abstract syntax tree nodes (you may want to define a class or structure

called ASTNode that can be the "return type" for the relevant constructs). For example:

§ add_op : generate an AddExpr AST node that has two children (that you leave uninitialized) and

keeps track of the operator (+ or -).

§ expr_prefix: this will have three AST Nodes passed up from its sub rules: one

from expr_prefix (which may be NULL, but otherwise will be an AddExpr node missing its right

child), one passed up from factor (which will be a complete AST Node with all its fields filled

in), and one passed up from add_op (which will be an AddExpr node that has neither its left nor

its right child filled in).

1. If expr_prefix is NULL, make the add_op node's left child the node from factor and

return up the add_op node (note that it won't have its right child filled in!)

2. If expr_prefix isn't NULL, note that it will be missing its right child. Make

the factor node its right child, then make the expr_prefix node the add_op node's left

child, which you pass up.

This basic idea: creating AST nodes when you have the information for a new node, then

filling in various fields of the node as you work your way up the parse tree, will let you

eventually create an AST for all the statements in the function.

Hint: you should also create an AST node to capture lists of statements; each element of the

list will point to an AST node for a single assign_stmt.

IR: 3 Address Code

The next step in our compilation process is to generate 3 Address Code (3AC), which is our

intermediate representation. 3AC is an intermediate representation where each instruction

has at most two source operands and one destination operand. Unlike assembly code, 3AC

does not have any notion of registers. Instead, the key to 3AC is to generate temporaries --

variables that are used to hold the intermediate results of computations. For example, the

3AC for d := a + b * c (where all variables are integers) will be:

MULTI b c $T1

ADDI a $T1 $T2

STOREI $T2 d

Generating 3AC

Generating 3AC is straightforward from an AST. We can perform a post-order walk of the

tree, passing up increasingly longer sequences of IR code called CodeObjects. Each code

object retains three pieces of information:

1. A sequence of IR Nodes (a structure representing a single 3AC instruction) that holds

the code for this part of the AST (i.e., that implements this part of the expression)

2. An indication of where the "result" of the IR code is being stored (think: the name of

the temporary or variable where the result of the expression is stored)

3. An indication of the type of the result (INT or FLOAT)

Then, when we encounter something like an AddExpr Node, we can generate code for the

overall expression as follows:

1. Create a new CodeObject whose code list is all the code from the left child of

the AddExpr followed by all the code for the right child.

2. Use the result fields of the left and right CodeObjects to create a new 3AC instruction

performing the add, storing the result in a new temporary. Add this new instruction to

the end of your code list

3. Indicate in your CodeObject the temporary where the result is stored, and its type.

4. Return the new CodeObject up the AST as part of your post-order walk.

Hint: the CodeObject for a simple variable won't have any 3AC code associated with it. Instead,

mark the variable itself as the "temporary" the result is stored in.

Hint: You may find it useful to write a helper function to generate "fresh" temporaries.

Then, when you get to the top of the AST, you will have a single CodeObject that contains all

of the IR code for the entire main function.

Note: We are generating code by performing a post-order walk of the AST. You can also

generate code using this strategy by performing a post-order walk of the parse-tree (which is

why you can optionally skip building the AST).

3AC instructions

Here are the 3AC instructions you should use:

ADDI OP1 OP2 RESULT (Integer add; RESULT = OP1 + OP2)

SUBI OP1 OP2 RESULT (Integer sub; RESULT = OP1 - OP2)

MULTI OP1 OP2 RESULT (Integer mul; RESULT = OP1 * OP2)

DIVI OP1 OP2 RESULT (Integer div; RESULT = OP1 / OP2)

ADDF OP1 OP2 RESULT (Floating point add; RESULT = OP1 + OP2)

SUBF OP1 OP2 RESULT (Floating point sub; RESULT = OP1 - OP2)

MULTF OP1 OP2 RESULT (Floating point mul; RESULT = OP1 * OP2)

DIVF OP1 OP2 RESULT (Floating point div; RESULT = OP1 / OP2)

STOREI OP1 RESULT (Integer store; store OP1 in RESULT)

STOREF OP1 RESULT (Floating point store; store OP1 in RESULT)

READI RESULT (Read integer from console; store in RESULT)

READF RESULT (Read float from console; store in RESULT)

WRITEI OP1 (Write integer OP1 to console)

WRITEF OP1 (Write float OP1 to console)

WRITES OP1 (Write string OP1 to console)

Generating Assembly

Once you have your IR, your final task is to generate assembly code. In this class, we will be

using an assembly instruction set called Tiny. The tiny simulator is meant to work as a

simplified version of a real machine. It works by executing a stream of assembly instructions.

The “tinyNew.C” file is the source code for the tiny simulator. You can compile the source

code using the following command: g++ -o tiny tinyNew.C

See the tinyDoc.txt for details about the instruction set.

This task is fairly straightforward: iterate over the list of 3AC you generated in the previous

step and convert each individual instruction into the necessary Tiny code (note that Tiny

instructions reuse one of the source operands as the destination, so you may need to

generate multiple Tiny instructions for each 3AC instruction).

We will be using a version of Tiny emulator that supports 1000 registers, so you can more or

less directly translate each temporary you generate into a register (i.e. you don’t have to worry

about efficient register allocation).

What you need to do

In this part, you will be generating assembly code for assignment statements, expressions,

and READ and WRITE commands. Use the steps outlined above to generate Tiny code. Your

code should output a list of tiny code (which will be displayed as Non-graded Output) that we

will then run through the Tiny emulator to make sure you generated the right result (which will

be displayed as Graded Output).

For debugging purposes, it may also be helpful to emit your list of IR code. You can precede

a statement with a ; to turn it into a comment that our simulator will not interpret.

Handling errors

All the inputs we will give you in this step will be valid programs. We will also ensure that all

expressions are type safe: a given expression will operate on either INTs or FLOATs, but not

a mix, and all assignment statements will assign INT results to variables that are declared as

INTs (and respectively for FLOATs).

Grading

In this step, we will only grade your compiler on the correctness of the generated Tiny code.

We will run your generated code through the Tiny simulator and check to make sure that you

produce the same result as our code. When we say result, we mean the outputs of any WRITE

statements in the program. Any testcase that requires console input (sys read) will receive

random inputs and result(s) will be calculated based on these random inputs.

We will not check to see if you generate exactly the same Tiny code that we. In other words,

we only care if your generated Tiny code works correctly. You may generate slightly different

Tiny code than we did.

Project Step 4 — Part 2: Control Structures

This step builds on Part 1. Now that we are able to generate code for lists of statements, what

happens if those lists of statements are embedded in control structures? (IF statements and

FOR loops)?

(Note: as in part 1, we will only have one function in our program, main. You can assume that

all variables are defined globally. There will not be any additional variables defined in main())

ASTs for Control Structures

ASTs for control structures are, intuitively, simple: each control structure will have several

children (3 in the case of an IF statement, etc.) that are themselves ASTs (ASTs for statement

lists in the case of the bodies of IF statements and WHILE loops, ASTs for conditional

expressions in the case of the conditions in the IF statements and WHILE loops, etc.).

Because you already have working code for building an AST for statement lists (and can

readily adapt your code for binary expressions to build ASTs for conditional expressions), all

you have to do is create semantic actions for the control structures that "stitch together" the

existing ASTs.

Generating 3AC for Control Structures

Generating 3AC for control structures builds directly on your code for part 1, which is able to

generate code for lists of statements. This means that when you are generating code for an

IF AST node, you know that the 3AC for the three children already exists. All that is left is to

put them together in the correct order and insert any necessary labels and jumps.

There are two things that you need to pay attention to when putting together 3AC:

Generating Labels

At various points in your code, you will need to insert labels and jumps to allow control to

transfer from one part of your code to another. You will need to make sure that you can

generate unique labels every time (since your code will not work properly if there are multiple

labels with the same name).

The 3AC you will generate for labels looks like:

LABEL STRING

Where STRING is whatever name you decide to give to your label

Generating Jumps

Unconditional jumps (like you might use to jump over an ELSE block) are easy:

JUMP STRING

Where STRING is the label you want to jump to.

Conditional jumps are a little bit tricky in our 3AC (and in Tiny): you need to generate the right

kind of jump:

GT OP1 OP2 LABEL (Jump to Label if OP1 > OP2)

GE OP1 OP2 LABEL (Jump to Label if OP1 >= OP2)

LT OP1 OP2 LABEL (Jump to Label if OP1 < OP2)

LE OP1 OP2 LABEL (Jump to Label if OP1 <= OP2)

NE OP1 OP2 LABEL (Jump to Label if OP1 != OP2)

EQ OP1 OP2 LABEL (Jump to Label if OP1 == OP2)

To generate the right kind of jump for a conditional expression, you will need to inspect the

AST node for the comparison operation and use that information to select the right 3AC

instruction.

Note: the 3AC does not preserve type information about what kind of comparison you are

doing, but the Tiny code for jumps does; you may want to extend either your 3AC or your

data structure to keep track of this information.

What you need to do

In this part, you will be generating assembly code for IF statements and WHILE loops. Use

the steps outlined above to generate Tiny code. Your compiler should output a list of tiny code

that we will then run through the Tiny simulator to make sure you generated the right result.