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Consider the following intermediate program in three address code

p = a - b
q = p * c
p = u * v
q = p + q

Which one of the following corresponds to a static single assignment form of the above code?

1. p1 = a  - b
q1 = p1 * c
p1 = u  * v
q1 = p1 + q1
2. p3 = a  - b
q4 = p3 * c
p4 = u  * v
q5 = p4 + q4
3. p1 = a  - b
q1 = p2 * c
p3 = u  * v
q2 = p4 + q3
4. p1 = a - b
q1 = p * c
p2 = u * v
q2 = p + q


In option B , in the last line, rightmost var should be q4 not q.

Correct me?

B IS CORRECT A IS WRONG

In option A – the 3rd expression is overridden when again p1 is used.

In option C – the 2nd expression is wrong as p2 is not initialized before but used.

In option D – the 2nd  expression is wrong as p is not initialized before but used.

Here each and every variable used are distinct. So the answer is (B).

by

### 1 comment

In compiler design, static single assignment form (often abbreviated as SSA form or simply SSA) is a property of an intermediate representation (IR), which requires that each variable is assigned exactly once, and every variable is defined before it is used. Thus option B only satisfies the above criteria.

Option D INCORRECT because in last line we are directly adding P+Q without moving it to register

option C INCORRECT because in last line Q3+P4 is done however they dont even contain any value

Option A INCORRECT because P1 has been assigned value twice

So OPTION (B) is CORRECT
OPTION B becoz in option A same variable is used p1 , q1 which creates ambiguity also bt in option B different variable is used .

### 1 comment

But in option B, last line q5= p4 + q

instead of it should be q5= p4+ q4 because q is pointing to value  of statement 2

Please correct me if i am wrong