Tag: basic blocks and flow graphs

Miscellaneous Problems in Compiler Design

Three Address Code | DAGs | Basic Blocks & Flow Graphs-

 

Before you go through this article, make sure that you have gone through the previous articles on-

 

In this article, we will solve Miscellaneous Problems based on these Concepts.

 

PRACTICE PROBLEMS BASED ON ABOVE CONCEPTS-

 

Problem-01:

 

Generate three address code for the following code-

 

c = 0

do

{

if (a < b) then

x++;

else

x–;

c++;

} while (c < 5)

 

Solution-

 

Three address code for the given code is-

  1. c = 0
  2. if (a < b) goto (4)
  3. goto (7)
  4. T1 = x + 1
  5. x = T1
  6. goto (9)
  7. T2 = x – 1
  8. x = T2
  9. T3 = c + 1
  10. c = T3
  11. if (c < 5) goto (2)

 

Problem-02:

 

Generate three address code for the following code-

 

while (A < C and B > D) do

if A = 1 then C = C + 1

else

while A <= D

do A = A + B

 

Solution-

 

Three address code for the given code is-

  1. if (A < C) goto (3)
  2. goto (15)
  3. if (B > D) goto (5)
  4. goto (15)
  5. if (A = 1) goto (7)
  6. goto (10)
  7. T1 = c + 1
  8. c = T1
  9. goto (1)
  10. if (A <= D) goto (12)
  11. goto (1)
  12. T2 = A + B
  13. A = T2
  14. goto (10)

 

Problem-03:

 

Generate three address code for the following code-

 

switch (ch)

{

case 1 : c = a + b;

break;

case 2 : c = a – b;

break;

}

 

Solution-

 

Three address code for the given code is-

 

if ch = 1 goto L1

if ch = 2 goto L2

L1:

T1 = a + b

c = T1

goto Last

L2:

T1 = a – b

c = T2

goto Last

Last:

 

Problem-04:

 

Construct a DAG for the following three address code-

  1. a = b + c
  2. t1 = a x a
  3. b = t1 + a
  4. c = t1 x b
  5. t2 = c + b
  6. a = t2 + t2

 

Solution-

 

Directed acyclic graph for the given three address code is-

 

 

Problem-05:

 

Consider the following code-

 

prod = 0 ;

i = 1 ;

do

{

prod = prod + a[ i ] x b[ i ] ;

i = i + 1 ;

} while (i <= 10) ;

 

  1. Compute the three address code.
  2. Compute the basic blocks and draw the flow graph.

 

Solution-

 

Part-01:

 

Three address code for the given code is-

 

prod = 0

i = 1

T1 = 4 x i

T2 = a[T1]

T3 = 4 x i

T4 = b[T3]

T5 = T2 x T4

T6 = T5 + prod

prod = T6

T7 = i + 1

i = T7

if (i <= 10) goto (3)

 

Part-02:

 

Step-01:

 

We identify the leader statements as-

  • prod = 0 is a leader because first statement is a leader.
  • T1 = 4 x i is a leader because target of conditional or unconditional goto is a leader.

 

Step-02:

 

The above generated three address code can be partitioned into 2 basic blocks as-

 

 

Step-03:

 

The flow graph is-

 

 

To gain better understanding about these Miscellaneous Problems,

Watch this Video Lecture

 

Download Handwritten Notes Here-

 

Next Article- Code Optimization

 

Get more notes and other study material of Compiler Design.

Watch video lectures by visiting our YouTube channel LearnVidFun.

Basic Blocks and Flow Graphs | Examples

Basic Blocks-

 

Basic block is a set of statements that always executes in a sequence one after the other.

 

The characteristics of basic blocks are-

  • They do not contain any kind of jump statements in them.
  • There is no possibility of branching or getting halt in the middle.
  • All the statements execute in the same order they appear.
  • They do not lose lose the flow control of the program.

 

Example Of Basic Block-

 

Three Address Code for the expression a = b + c + d is-

 

 

Here,

  • All the statements execute in a sequence one after the other.
  • Thus, they form a basic block.

 

Also Read- Three Address Code

 

Example Of Not A Basic Block-

 

Three Address Code for the expression If A<B then 1 else 0 is-

 

 

Here,

  • The statements do not execute in a sequence one after the other.
  • Thus, they do not form a basic block.

 

Partitioning Intermediate Code Into Basic Blocks-

 

Any given code can be partitioned into basic blocks using the following rules-

 

Rule-01: Determining Leaders-

 

Following statements of the code are called as Leaders

  • First statement of the code.
  • Statement that is a target of the conditional or unconditional goto statement.
  • Statement that appears immediately after a goto statement.

 

Rule-02: Determining Basic Blocks-

 

  • All the statements that follow the leader (including the leader) till the next leader appears form one basic block.
  • The first statement of the code is called as the first leader.
  • The block containing the first leader is called as Initial block.

 

Flow Graphs-

 

A flow graph is a directed graph with flow control information added to the basic blocks.

 

  • The basic blocks serve as nodes of the flow graph.
  • There is a directed edge from block B1 to block B2 if B2 appears immediately after B1 in the code.

 

PRACTICE PROBLEMS BASED ON BASIC BLOCKS & FLOW GRAPHS-

 

Problem-01:

 

Compute the basic blocks for the given three address statements-

 

(1) PROD = 0

(2) I = 1

(3) T2 = addr(A) – 4

(4) T4 = addr(B) – 4

(5) T1 = 4 x I

(6) T3 = T2[T1]

(7) T5 = T4[T1]

(8) T6 = T3 x T5

(9) PROD = PROD + T6

(10) I = I + 1

(11) IF I <=20 GOTO (5)

 

Solution-

 

We have-

  • PROD = 0 is a leader since first statement of the code is a leader.
  • T1 = 4 x I is a leader since target of the conditional goto statement is a leader.

 

Now, the given code can be partitioned into two basic blocks as-

 

 

Problem-02:

 

Draw a flow graph for the three address statements given in problem-01.

 

Solution-

 

  • Firstly, we compute the basic blocks (already done above).
  • Secondly, we assign the flow control information.

 

The required flow graph is-

 

 

Also Read- More Problems On Basic Blocks & Flow Graphs

 

To gain better understanding about Basic Blocks and Flow Graphs,

Watch this Video Lecture

 

Download Handwritten Notes Here-

 

 

Next Article- Directed Acyclic Graphs

 

Get more notes and other study material of Compiler Design.

Watch video lectures by visiting our YouTube channel LearnVidFun.