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 codes 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:
Algorithm: Partitioning three-address code into basic blocks.
Input: A sequence of three address instructions.
Process: Instructions from intermediate code which are leaders are determined. The following are the rules used for finding a leader:
- The first three-address instruction of the intermediate code is a leader.
- Instructions that are targets of unconditional or conditional jump/goto statements are leaders.
- Instructions that immediately follow unconditional or conditional jump/goto statements are considered leaders.
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 <= 10 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