Question

Consider the following sequence of instructions:

I1   sub f4, f4, f0

I2    div f3, f4, f0

I3    add f2, f5, f9

I4       add f0, f2, f3

I5 mult f9, f2, f3

Answer the following three questions.

A. List all the dependencies among the instructions.

B. Name the hazard (WAW, WAR, RAW) for each dependency.

C. Discuss which dependencies can be removed through register renaming.

Answer 1

BEFORE answering lets see what are WAW,WAR,RAW

Data hazards do occur when instructions those exhibit data dependence, modify data in different stages of a pipeline. Hazard cause delays in the pipeline. There are mainly 3 types of data hazards:

1) RAW (Read after Write)
2) WAR (Write after Read) [Anti-Data dependency]
3) WAW (Write after Write) [Output data dependency]

Let there be two instructions I and J, such that J follow I. so,

• RAW hazard occurs when instruction J tries to read data before instruction I writes it.
  Eg:
  I: R8 <- R1 + R3
  J: R4 <- R8 + R3
• WAR hazard occurs when instruction J tries to write data before instruction I reads it.
  Eg:
  I: R8 <- R1 + R3
  J: R3 <- R4 + R5
• WAW hazard occurs when instruction J tries to write output before instruction I writes it.
  Eg:
  I: R9 <- R1 + R3
  J: R9 <- R4 + R5

WAR and WAW hazards occur during the out-of-order execution of the instructions

now lets see the dependencies above

I1   sub f4, f4, f0   

f4 <- f4 - f0

I2    div f3, f4, f0

f3 <- f4 / f0

I3    add f2, f5, f9

f2 <- f5 + f9

I4       add f0, f2, f3

f0 <- f2 + f3

I5 mult f9, f2, f3

f9 <- f2 * f3

I1 and I2 have a RAW dependency , what is I2 read f4 before I1 writes in f4

I1 and I4 have WAR dependency, if I4 writes in f0 before I1 reads it

I2 and I4 have WAR dependency

I2 and I5 have RAW depedency, if I5 reads f3 before I2 writes on it

I3 and I4 have RAW dependency

I3 and I5 have RAW dependency,

We don not have any WAW dependency, as no 2 instructions are writing simultaneoulsy on same register.

So option A, B have answered above, lets see option C

Register renaming is done to avoid WAR and WAW data hazards.

So here, we can see,dependencies having WAR hazards are

I1 and I4

I2 and I4

HOPE I HELPED YOU.