Skip to content
Related Articles
Get the best out of our app
GeeksforGeeks App
Open App

Related Articles

Basic Blocks in Compiler Design

Improve Article
Save Article
Like Article
Improve Article
Save Article
Like Article

Basic Block is a straight line code sequence that has no branches in and out branches except to the entry and at the end respectively. Basic Block is a set of statements that always executes one after other, in a sequence. 

The first task is to partition a sequence of three-address code into basic blocks. A new basic block is begun with the first instruction and instructions are added until a jump or a label is met. In the absence of a jump, control moves further consecutively from one instruction to another. The idea is standardized in the algorithm below: 

Partitioning three-address code into basic blocks. 

Input: A sequence of three address instructions. 

Process: Instructions from intermediate code which are leaders are determined. Following are the rules used for finding a leader: 

  1. The first three-address instruction of the intermediate code is a leader. 
  2. Instructions that are targets of unconditional or conditional jump/goto statements are leaders. 
  3. Instructions that immediately follow unconditional or conditional jump/goto statements are considered leaders. 

For each leader thus determined its basic block contains itself and all instructions up to excluding the next leader. 

Example 1: 

The following sequence of three-address statements forms a basic block:

t1 := a*a

t2 := a*b

t3 := 2*t2

t4 := t1+t3

t5 := b*b

t6 := t4 +t5

A three address statement x:= y+z is said to define x and to use y and z. A name in a basic block is said to be live at a given point if its value is used after that point in the program, perhaps in another basic block.

Example 2: 
Intermediate code to set a 10*10 matrix to an identity matrix: 

1)  i=1        //Leader 1 (First statement)
2)  j=1             //Leader 2 (Target of 11th statement)
3)  t1 = 10 * i     //Leader 3 (Target of 9th statement) 
4)  t2 = t1 + j
5)  t3 = 8 * t2
6)  t4 = t3 - 88
7)  a[t4] = 0.0
8)  j = j + 1
9)  if j <= goto (3)       
10) i = i + 1                    //Leader 4 (Immediately following Conditional goto statement)
11) if i <= 10 goto (2)
12) i = 1                        //Leader 5 (Immediately following Conditional goto statement)
13) t5 = i - 1                   //Leader 6 (Target of 17th statement) 
14) t6 = 88 * t5
15) a[t6] = 1.0
16) i = i + 1
17) if i <= 10 goto (13) 

The given algorithm is used to convert a matrix into identity matrix i.e. a matrix with all diagonal elements 1 and all other elements as 0. 

Steps (3)-(6) are used to make elements 0, step (14) is used to make an element 1. These steps are used recursively by goto statements. 

There are 6 Basic Blocks in the above code : 
B1) Statement 1 
B2) Statement 2 
B3) Statement 3-9 
B4) Statement 10-11 
B5) Statement 12 
B6) Statement 13-17

My Personal Notes arrow_drop_up
Last Updated : 15 Jun, 2022
Like Article
Save Article
Similar Reads