CS 302 Homework 3 - due 3:30 p.m., Wed., May 19, 1999

Generating Intermediate Code for PCAT

Working individually or in teams of two, write an intermediate code generator for (a subset of) the PCAT language. As before, exclude anything to do with real numbers, and the LOOP, and EXIT statements. Also exclude anything to do with records. The abstract syntax for this subset of PCAT is described in /u/cs302acc/3/ast.txt.

The intermediate code representation you will generate is similar in style to the SPARC assembly code, but with major simplifications, particularly in the treatment of variable addressing (addressing by name is supported), operand types (there is essentially only one type), and procedure call. In addition, nested procedure and variable declarations are included in the output format. Details of the intermediate code are given below.

Your intermediate code generator should be an executable irgen that reads an .ast file from stdin and produces an intermediate code (.ir) file on stdout.

To ease the implementation task, you may make the following simplifying assumptions:

Array bounds violations should be handled by generating code to call a specified runtime library function that prints ``Array bounds violation'' to stderr and exits immediately.

As usual, a ``correct'' generator is available in /u/cs302acc/3/irgen. In addition, an interpreter that executes .ir files is available in /u/cs302acc/3/irinterp. It is not necessary that the code you generate be identical to what my irgen generates, but it must behave the same way as my code does when fed to irinterp.

Intermediate Code

The intermediate code consists of declarations and code sequences, once for each procedure body and variable initialization, as well as one for the main program. Each code sequence is a sequence of instructions, each with a SPARC-like operator and 0-3 operands. We assume a SPARC-like load/store architecture, with an unlimited number of temporary registers available. A full grammar for the intermediate representation is on-line in /u/cs302acc/3/ir.gram. The operators are shown in Table 1.

Table: Intermediate code operators.
ld [a1],r2 load from memory address a1 into register r2
st r1,[a2] store r1 into memory at address a2
call procedure_name do procedure call
return   return from procedure
br label unconditional branch
bg label branch if last compare said greater
bl label branch if last compare said less
be label branch if last compare said equal
bge label branch if last compare said greater or equal
ble label branch if last compare said less or equal
bne label branch if last compare said not equal
cmp op1,op2 compare operands op1 and op2
mov op1,r2 move operand op1 to register r2
neg op1,r2 register r2 := - op1
add op1,op2,r3 register r3 := op1 + op2
sub op1,op2,r3 register r3 := op1 - op2
umul op1,op2,r3 register r3 := op1 * op2
udiv op1,op2,r3 register r3 := op1 / op2(integer division)
inc r1 register r1 := r1 + 1
dec r1 register r1 := r1 - 1

Registers (r) are:

Addresses (a) are: Operands (op) are: Labels are written as Ln for some integer n.

Note that source operands are considerably more general than on SPARC. In particular, variables can be referenced by name without the need for explicit addressing. The intermediate code is essentially typeless: all operands represent integers or addresses, and are assumed to have size 1.

Branches and calls can be to any label without regard for its offset from the current pc. There are no delay slots, and thus no annul forms. As on SPARC, conditional jumps are formed from a cmp and an appropriate branch instruction.

Procedure calls use the following idiom: first, the actual values of procedure arguments 1, 2, $\ldots$are stored into the memory locations represented by $a0, $a1, $\ldots$. Then a call instruction is executed. Within the procedure, the formal parameters can be accessed by name, just like local variables. Procedures return by executing a return instruction. If the procedure returns a value, it moves that value to special register %i0 before returning. The calling procedure can fetch the returned value from special register %o0. Note that this protocol requires that nested procedure calls are performed before any arguments are stored into the $a locations.

IO and heap memory allocation are performed by issuing ordinary procedure calls to the special built-in procedures PCAT$read_int (which returns an integer result value), PCAT$write_int (which takes an integer argument), PCAT$write_string (which takes the address of a string as argument), PCAT$write_newline, PCAT$bounds_error (which takes no arguments, issues the message Array bounds violation to stderr, and exits), and PCAT$alloc (which takes an integer size argument and returns the address of the allocated storage).

Label and temporary names should be unique within a procedure (including the initialization code for the procedure's locals).


Use the files /u/cs302acc/2/{ast.c,ast.h,parse_ast.y} to handle AST trees.

Definitions and supporting code for generating operands and emitting code are in /u/cs302acc/3/ir.[ch]. Code can be emitted one line at a time as it is generated; there is no need to store it up. A skeleton for a working generator is in irgen0.c; feel free to use this as the basis for your generator if you wish.

It is not necessary to use a symbol table when generating intermediate code. This causes problems only when generating code for the boolean constants TRUE and FALSE; since there is no symbol table, there is no easy way to see if these have been redefined, and no uniform way to handle the fact that they are constants rather than variables. So don't worry about the former, and expect the latter to lead to messy code: irgen0.c shows one approach.

Use control flow form for booleans by default; when a value is needed, use a full-word integer. You'll need to write code to convert from control flow form to value form when storing a boolean; the converse code, to generate control flow from a value, is already in irgen0.c.

Arrays should be represented by pointers to contiguous heap-allocated memory; the first word can represent the size of the array, with the contents starting at the second word.

Submitting the Program

Prepare a makefile that builds your generator and produces an executable called irgen. Submit your program by mailing a shar ``bundle'' containing all the relevant files to cs302acc@cs.pdx.edu.

Andrew P. Tolmach