Describe primary structure-preserving transformations on basic blocks

Describe primary structure-preserving transformations on basic blocks

Primary structure-preserving transformations on basic blocks-

• Common sub-expression elimination 
• Dead-code elimination
• Renaming of temporary variable
• Interchanging of two independent-adjacent statements.

Common sub-expression elimination:  Consider the basic block-

a : = b+c        c : = b+c

b : = a-d         d : = a-d

The 2nd and fourth statements compute same expression namely b + c - d. So, the transform basic block is as-

a : = b+c     c : = b+c

b : = a-d     d : = b

Dead-Code Elimination :   Let us a variable x is dead, that is, never subsequently used.  at the point where the statement x : = y + z appears in a basic block. Then this statement may be safely removed without changing the value of the basic block.

Renaming of temporary variable:   Let us a statement t : = a+b where t is a temporary. If we change this statement t to u, then the value of basic block is not changed.

Interchange of statement:  Let us consider the two adjacent statements-

t_{1 : = x+y}

t_{2 : = a+b}

We can interchange the two statement without effecting the value of the block if neithern a nor b is t_{1 & neither x nor y is t2.} 

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