Basic Blocks in Compiler Design

Basic Block is a straight line code sequence which has no branches in and out branches except to the entry and at the end respectively. Basic Block is a set of statements which 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 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 leader:

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

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

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 0 and all other elements as 1. Steps (3)-(6) are used to make elements 0 and 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

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :


Please write to us at to report any issue with the above content.