Data Dependency

It indicates to the situation in which two or more instructions share same data. The instructions in a program are able to be arranged based on relationship of data dependency. It means how two segments or instructions are data dependent on each other. The subsequent types of data dependencies are identified:

i)  Output dependence: When output of the two instructions I₁ and I₂ overlap on same data, the instructions are said output dependent.  

ii) I/O dependence: When read and write operations by two instructions are invoked on the same file, it's the condition of I/O dependence.

iii) Flow dependence: If instruction I₂ follows I₁ and output of I₁ becomes input of I₂, then I₂ is said to be flow dependent on I₁

iv) Anti dependence: When instruction I₂ follows instruction I₁ such that output of instruction I₂ overlaps with the input of instruction I₁ on the same data.

Think about the subsequent program instructions:

I₁: a = b

I₂: c = a + d

I₃: a = c

In this program segment instructions I₁ and I₂ are Flow dependent since variable a is produced by instruction I₁ as output and used by instruction I₂ as input. Instructions I₂ and I₃ are Anti dependent since variable a is produced by I₃ but used by I₂ and in sequence I₂ comes first. I₃ is flow dependent on I₂ due to variable c. Instructions I₃ and Instruction I₁ are Output dependent since variable a is produced by both instructions.