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The Light at the End of the Tunnel – Doesn’t 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
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The Light at the End of the Tunnel – Doesn’t 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 nop’s “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.
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.
Example of Impact of Successful Branch on the Pipeline EX/MEM–Mem–MEMWB stage Fig. 6.50 Illustrates the “conservative” approach where 3 instructions must be flushed. In this case decision for branch made in MEM stage.
Data path with the branch hazard feature Fig. 6.51 Note: the compare circuit (“=“ ) above is the selector of the PC MUX
