456589.-Compiler-Design-Code-Generation (1).ppt

COMPILER DESIGN
BCA 5th
Semester 2020
Topic: Code Generation
Sakhi Bandyopadhyay
Department of Computer Science and BCA
Kharagpur College

Introduction
• The final phase of a compiler is code generator
• It receives an intermediate representation (IR) with supplementary
information in symbol table
• Produces a semantically equivalent target program
• Code generator main tasks:
• Instruction selection
• Register allocation and assignment
• Insrtuction ordering
Front end Code optimizer
Code
Generator

Issues in the Design of Code Generator
• The most important criterion is that it produces correct code
• Input to the code generator
• IR + Symbol table
• We assume front end produces low-level IR, i.e. values of names in it can be
directly manipulated by the machine instructions.
• Syntactic and semantic errors have been already detected
• The target program
• Common target architectures are: RISC, CISC and Stack based machines
• In this chapter we use a very simple RISC-like computer with addition of
some CISC-like addressing modes

complexity of mapping
• the level of the IR
• the nature of the instruction-set architecture
• the desired quality of the generated code.
x=y+z
LD R0, y
ADD R0, R0, z
ST x, R0
a=b+c
d=a+e
LD R0, b
ADD R0, R0, c
ST a, R0
LD R0, a
ADD R0, R0, e
ST d, R0

Register allocation
• Two subproblems
• Register allocation: selecting the set of variables that will reside in
registers at each point in the program
• Resister assignment: selecting specific register that a variable reside in
• Complications imposed by the hardware architecture
• Example: register pairs for multiplication and division
t=a+b
t=t*c
T=t/d
t=a+b
t=t+c
T=t/d
L R1, a
A R1, b
M R0, c
D R0, d
ST R1, t
L R0, a
A R0, b
M R0, c
SRDA R0, 32
D R0, d
ST R1, t

A simple target machine model
• Load operations: LD r,x and LD r1, r2
• Store operations: ST x,r
• Computation operations: OP dst, src1, src2
• Unconditional jumps: BR L
• Conditional jumps: Bcond r, L like BLTZ r, L

Target program for a sample call and return

Stack Allocation
Return to caller
in Callee: BR *0(SP)
in caller: SUB SP, SP, #caller.recordsize
Branch to called procedure

Target code for stack allocation

Basic blocks and flow graphs
• Partition the intermediate code into basic blocks
• The flow of control can only enter the basic block through the first
instruction in the block. That is, there are no jumps into the middle of the
block.
• Control will leave the block without halting or branching, except possibly at
the last instruction in the block.
• The basic blocks become the nodes of a flow graph

Flow graph based on Basic Blocks

DAG representation of basic blocks
• There is a node in the DAG for each of the initial values of the
variables appearing in the basic block.
• There is a node N associated with each statement s within the block.
The children of N are those nodes corresponding to statements that
are the last definitions, prior to s, of the operands used by s.
• Node N is labeled by the operator applied at s, and also attached to
N is the list of variables for which it is the last definition within the
block.
• Certain nodes are designated output nodes. These are the nodes
whose variables are live on exit from the block.

array accesses in a DAG
• An assignment from an array, like x = a [i], is represented by
creating a node with operator =[] and two children representing
the initial value of the array, a0 in this case, and the index i.
Variable x becomes a label of this new node.
• An assignment to an array, like a [j] = y, is represented by a new
node with operator []= and three children representing a0, j and y.
There is no variable labeling this node. What is different is that the
creation of this node kills all currently constructed nodes whose
value depends on a0. A node that has been killed cannot receive
any more labels; that is, it cannot become a common
subexpression.

DAG for a sequence of array assignments

Rules for reconstructing the basic block
from a DAG
• The order of instructions must respect the order of nodes in the DAG.
That is, we cannot compute a node's value until we have computed a
value for each of its children.
• Assignments to an array must follow all previous assignments to, or
evaluations from, the same array, according to the order of these
instructions in the original basic block.
• Evaluations of array elements must follow any previous (according to
the original block) assignments to the same array. The only permutation
allowed is that two evaluations from the same array may be done in
either order, as long as neither crosses over an assignment to that array.
• Any use of a variable must follow all previous (according to the original
block) procedure calls or indirect assignments through a pointer.
• Any procedure call or indirect assignment through a pointer must follow
all previous (according to the original block) evaluations of any variable.

Code sequence using global register
assignment

Intermediate-code tree for a[i]=b+1

Syntax-directed translation scheme

Optimal three-register code using only two
registers

Syntax tree for (a-b)+c*(d/e) with cost
vector at each node

minimum cost of evaluating the root
with two registers available
• Compute the left subtree with two registers available into
register R0, compute the right subtree with one register
available into register R1, and use the instruction ADD R0, R0,
R1 to compute the root. This sequence has cost 2+5+1=8.
• Compute the right subtree with two registers available into R l ,
compute the left subtree with one register available into R0,
and use the instruction ADD R0, R0, R1. This sequence has cost
4+2+1=7.
• Compute the right subtree into memory location M, compute
the left subtree with two registers available into register RO,
and use the instruction ADD R0, R0, M. This sequence has cost
5+2+1=8.

456589.-Compiler-Design-Code-Generation (1).ppt

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