The Light at the End of the Tunnel

1
The Light at the End of the Tunnel Doesnt Mean
You Died! Part 3 - Branch Hazards Easier!
3/24/04

• This is a control hazard as opposed to data
  hazard we just examined
• Also called a branch hazard
• A successful conditional branch invalidates
  follow-on instructions
• Rather than stalling for each branch to see it is
  successful or not, assume it is not successful
• Branch prediction guess that the branch is NOT
  taken
• If guess is correct, the we proceed full speed
• If guess is wrong (branch taken), then the
  following instructions that are being fetched or
  decoded must be discarded.
• The over head for successful branches take place
  only ½ of the time.
• Discarding instructions
• Done much the same as in the data hazard stall
  case
• But must now change 3 instructions in the IF, ID,
  and EX stages when the branch reaches the MEM
  stage
• In the case of data hazard stalls we let nops
  percolate from the EX stage by inserting zeros
  for controls in the ID/EX register.
• In this case we would have to flush the 3
  following instruction dead in their tracks. In
  the next slide we see how to reduce this to one
  instruction.

2
The Final Data Path Tweak Reducing the Delay
of Successful Branches

• Move branch execution up earlier in the pipeline
  - See fig 6.30, p. 470 for the way it was.
• Fewer instructions to be flushed.
• Move branch decision to ID stage (was in MEM
  stage)
• Check equality of registers in ID stage by
  taking XOR of registers and then ANDing of
  results.
• The information for the target address
  calculation is available in the IF/ID register
  (in ID stage) move the branch address adder
  from EX stage to ID stage.
• Now on successful branch, there is only one
  instruction to be flushed, the one currently
  being fetched.
• Instruction in IF stage is flushed by adding a
  new control line IF.Flush
• Zeros out the the instruction field in the IF/ID
  register effectively making it a nop.

3
Example of Impact of Successful Branch on the
Pipeline
EX/MEMMemMEMWB ? stage ?
Fig. 6.50
Illustrates the conservative approach where 3
instructions must be flushed. In this case
decision for branch made in MEM stage.
4
Data path with the branch hazard feature
Fig. 6.51
Note the compare circuit ( ) above is the
selector of the PC MUX