Pipeline Hazards

1. EC 8552 UNIT III PIPELINE HAZARDS
Mr. C.KARTHIKEYAN ,
ASSISTANT PROFESSOR,
ECE, RMKCET

2. PIPELINE HAZARDS
The events which leads to Pipeline stall by preventing the next instruction to
execute are known as Hazards.
1.Structural Hazard
2.Data Hazard
3.Control Hazard

3. Pipelined Data Path with
Control Signals

4. Structural Hazard
IF ID EX MEM WB
IF ID EX MEM WB
lw $10,20($1)
IF ID EX MEM WB
IF ID EX MEM WB
1 2 3 4 5 6 7 8 9
CLOCK CYCLES
PROGRAM
INSTRUCTIONS
Instruction 1
IF ID EX MEM WB
IF ID EX MEM WB
Instruction 2
Instruction 3
Instruction 4
sub $11,$2,$3
add $12,$3,$4
lw $13,24($1)
add $14,$5,$6

5. Pipeline Stall

6. Von Neumann Vs Harvard Architecture
Pipeline Hardware Resources

7. DATA HAZARD
Data hazards occur when the pipeline get stalled because one step must wait for
another to complete.
Data Dependency
IF ID EX MEM WB
IF ID EX MEM WB
1 2 3 4 5 6 7 8
add $s0, $t0, $t1
sub $t2, $s0, $t3

8. Types of Data Hazard
There are three situations in which a data hazard can occur
Read After Write (RAW)
a true dependency
Write After Read (WAR)
an anti-dependency
write after write (WAW)
an output dependency
add $s0, $t0, $t1
sub $t2, $s0, $t3
add $s1, $t2, $t1
sub $t2, $s0, $t3
add $s1, $t2, $t1
sub $s1, $s0, $t3

9. How is to overcome Data Hazard?
1. Forwarding / Bypassing

12. Forwarding Unit to
overcome pipeline stall

13. Load-use Data hazard
lw $s0, 20($t1)
sub $t2, $s0, $t3
IF ID EX MEM WB
IF ID EX MEM WB
1 2 3 4 5 6 7 8
2. Pipeline Stall / Bubble
lw $s0, 20($t1)
NOP
sub $t2, $s0, $t3
NOP

14. 3. Re-ordering the code
a = b + e;
c = b + f;
lw $t1, 0($t0)
lw $t2, 4($t0)
add $t3, $t1,$t2
sw $t3, 12($t0)
lw $t4, 8($t0)
add $t5, $t1,$t4
sw $t5, 16($t0)
lw $t1, 0($t0)
lw $t2, 4($t0)
add $t3, $t1,$t2
sw $t3, 12($t0)
lw $t4, 8($t0)
add $t5, $t1,$t4
sw $t5, 16($t0)
lw $t1, 0($t0)
lw $t2, 4($t0)
lw $t4, 8($t0)
add $t3, $t1,$t2
sw $t3, 12($t0)
add $t5, $t1,$t4
sw $t5, 16($t0)

15. Computer Organization and Design by David A Patterson – Page no. 361
I1: or r1,r2,r3
I2: or r2,r1,r4
I3: or r1,r1,r2
1. Indicate dependences and their type
Comparing I1 & I2
a. RAW on r1 – True Dependency
b. WAR on r2 – Anti - Dependency

16. Computer Organization and Design by David A Patterson – Page no. 361
I1: or r1,r2,r3
I2: or r2,r1,r4
I3: or r1,r1,r2
1. Indicate dependences and their type
Comparing I1 & I3
a. RAW on r1 – True Dependency
b. WAW on r1 – Output Dependency

17. Computer Organization and Design by David A Patterson – Page no. 361
I1: or r1,r2,r3
I2: or r2,r1,r4
I3: or r1,r1,r2
1. Indicate dependences and their type
Comparing I2 & I3
a. RAW on r2 – True Dependency
b. WAR on r1 – Anti- Dependency

18. Computer Organization and Design by David A Patterson – Page no. 361
I1: or r1,r2,r3
I2: or r2,r1,r4
I3: or r1,r1,r2
2. Assume there is no forwarding in this pipelined
processor. Indicate hazards and add NOP instructions
to eliminate them.
or r1,r2,r3
NOP
NOP
or r2,r1,r4
NOP
NOP
NOP
NOP
or r1,r1,r2
IF ID EX MEM WB
IF ID EX MEM WB
1 2 3 4 5 6 7 8 9 10 11
IF ID EX MEM WB

19. Computer Organization and Design by David A Patterson – Page no. 361
I1: or r1,r2,r3
I2: or r2,r1,r4
I3: or r1,r1,r2
3. Assume there is full forwarding. Indicate hazards and add
NOP instructions to eliminate them.
or r1,r2,r3
or r2,r1,r4
or r1,r1,r2
IF ID EX MEM WB
IF ID EX MEM WB
1 2 3 4 5 6 7
IF ID EX MEM WB

20. Control Hazard / Branch Hazard
control hazard arising from the need to make a decision based on the results
of one instruction while others are executing
beq $6,$7,7
sub $11,$2,$3
add $12,$3,$4
IF ID EX MEM WB
IF ID EX MEM
1 2 3 4 5 6 7
IF ID EX

21. Control Hazard / Branch Hazard
control hazard arising from the need to make a decision based on the results
of one instruction while others are executing
A control hazard is when we need to find destination of a branch, and can’t
fetch any new instruction until we know that destination
A branch is either
Taken : PC + 4 + Immediate address * 4
Not Taken : PC + 4

22. Penalty is 3 Clock Cycle

23. How to overcome Control Hazard?
1. Stall – Stop loading the instructions until the branch target is available
2. Prediction – Assume whether branch is taken or not and continue
fetching the instructions (Static Branch and Dynamic Branch Prediction)
3. Delayed Branch

24. Assume Branch is Not Taken
Static Prediction
FLUSH

25. Reducing the Delay of Branches
sub $11,$2,$3
beq $6,$7,7
add $12,$3,$4
IF ID EX MEM WB
IF ID EX MEM
1 2 3 4 5 6 7
IF ID EX
WB
MEM WB
Complication Factors
1. New forwarding unit is required
2. Data dependency from previous instruction

26. Dynamic Branch Prediction
Prediction of branches at run time using run time information
1.1-bit Prediction scheme
2.2-bit Prediction scheme

27. Dynamic Branch Prediction
1-bit Prediction scheme
Branch Prediction Buffer / Branch History Table (BHT)
- A small memory indexed by the lower portion of the address of the branch instruction
Prediction Bit
1 if Branch is taken previously
0 if Branch is not taken previously

28. Dynamic Branch Prediction
Disadvantage of 1-bit Prediction scheme
Even if a branch is almost taken always, there is a possibility to
predict it incorrectly twice, when the branch is not taken

29. Dynamic Branch Prediction
2-bit Prediction scheme
The prediction is changed
iff the branch is wrongly
predicted twice

30. Advanced Prediction Schemes
Correlation Predictor Tournament Predictor
A branch predictor that
combines local behavior of a
particular branch and global
information about the behavior
of some recent number of
executed branches.
A branch predictor with
multiple predictions for each
branch and a selection
mechanism that chooses
which predictor to enable for
a given branch