What a Control Hazard Is
A Control Hazard arises from conditional branch instructions. In a pipelined processor, the next instruction must be fetched immediately, one cycle after the current one, but a branch's outcome, whether it is taken or not, is not known until it reaches a later pipeline stage. This creates a genuine uncertainty: which instruction should the processor fetch next?
Why This Problem Cannot Simply Be Ignored
Unlike a data hazard, where the needed value simply is not ready yet, a control hazard involves not knowing which instruction is even the correct one to fetch. Fetching blindly and guessing wrong means the processor has been doing pointless work on instructions that should never have executed.
Strategy One: Assume the Branch Is Not Taken
The simplest strategy is to always assume a branch will not be taken and continue fetching instructions sequentially. If the assumption turns out to be correct, no time is lost at all. If the branch is actually taken, however, the instructions that were fetched based on the wrong assumption must be discarded, an action called a Pipeline Flush.
Branch instruction: IF ID EX MEM WB
Next instr (guessed): IF ID [discarded if branch taken]
Correct target instr: IF [fetched late, after flush]Strategy Two: Predict Branch Direction
More sophisticated designs use Branch Prediction, where hardware tracks the past behavior of a branch and guesses its likely outcome based on history rather than always assuming the same fixed direction. A Dynamic Predictor can learn that a particular branch, such as one at the bottom of a loop, is usually taken, and predict accordingly, improving accuracy well beyond a fixed always-not-taken assumption.
Resolving the Branch as Early as Possible
Another way to reduce the cost of control hazards is simply to compute the branch outcome earlier in the pipeline rather than waiting until a later stage. Moving the comparison logic needed to resolve a branch to an earlier stage shortens the number of incorrectly fetched instructions that must be discarded when a misprediction occurs, directly reducing the average performance penalty.
The Cost of a Misprediction
Whenever the assumption or prediction turns out to be wrong, every instruction fetched based on that incorrect guess must be flushed from the pipeline, and the correct instruction must be fetched starting from scratch. The number of cycles lost in this situation is called the Branch Penalty, and it grows directly with how many pipeline stages occur before the branch outcome becomes known.
Why This Matters for Overall Performance
Programs with frequent, hard-to-predict branches suffer more from control hazards than those with predictable or infrequent branching. This is one of the reasons why compilers and processor designers invest heavily in accurate branch prediction, since even a modest improvement in prediction accuracy can produce a meaningful gain in overall program execution speed.