GATE CS 2016 Set 1 — Question 29
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Compiler Design → Intermediate Code Generation → Three-Address Code
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Question
Consider the following code segment.The minimum number of total variables required to convert the above code segment to static single assignment form is __________.
x = u - t;
y = x * v;
x = y + w;
y = t - z;
y = x * y;
Correct answer
10 to 10
Solution
Static Single Assignment (SSA) form requires that every variable is assigned exactly once. We rename the variables on the left-hand side (LHS) to ensure this property and update the uses on the right-hand side (RHS) to refer to the most recent definition.Let's convert the code line by line:
1.
x = u - t; x1 = u - t; (First assignment to x)2.
y = x * v; y1 = x1 * v; (First assignment to y, uses x1)3.
x = y + w; x2 = y1 + w; (Second assignment to x, uses y1)4.
y = t - z; y2 = t - z; (Second assignment to y)5.
Now, let's count the total distinct variables used in the SSA form:Input variables (read but not written in this block):y = x * y; y3 = x2 * y2; (Third assignment to y, uses x2 and y2)1.
u2.
t3.
v4.
w5.
New SSA variables (defined in the block):z6.
x17.
y18.
x29.
y210.
Total variables = 5 (inputs) + 5 (definitions) = 10.y3Continue learning with Success Tracker
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