Data Dependences

An instruction is data dependent on instruction if there is a path from to and

Where:

• : set of read data by instruction
• : set of written data by instruction

Classification of data dependences #

1:   add R1,R2,R3 ;; R1 = R2 + R3
2:   sub R4,R5,R6
3:   add R6,R1,R8
4:   xor R4,R1,R11

True dependences (write … read) #

• In example from instruction 1 to instruction 3.
• 1 writes to R1 while 3 reads from R1

Anti dependences (read … write) #

• In example from 2 to 3

• 2 reads from R6 while 3 writes to R6

• Name depencency (can be avoided by using different registers)

Output dependences (write … write) #

• In example from 2 to 4

• 2 and 4 both write to R4

• Name depencency (can be avoided by using different registers)