GATE 2017 [Set-1]

Question 12

Consider the following intermediate program in three address code

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

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

A
B
C
D

Compiler-DesignStatic-single-assignmentVideo-Explanation

Question 12 Explanation:

Static Single Assignment is used for intermediate code in compiler design.

In Static Single Assignment form (SSA) each assignment to a variable should be specified with distinct names.
We use subscripts to distinguish each definition of variables.

In the given code segment, there are two assignments of the variable p
p = a-b
p = u*v
and two assignments of the variable q
q = p*c
q = p+q

So we use two variables p₁, p₂ for specifying distinct assignments of p and q₁, q₂ for
each assignment of q.
Static Single Assignment form(SSA) of the given code segment is:
p₁ = a-b
q₁ = p₁ * c
p₂ = u * v
q₂ = p₂ + q₁
Note:
As per options, given in GATE 2017 answer is B.

p₃ = a – b
q₄ = p₃ * c
p₄ = u * v
q₅ = p₄ + q₄

Correct Answer: B