Managing Pipeline Flushes on Misprediction

In this lesson, you will learn how the processor safely discards incorrect instructions from the pipeline when a branch prediction turns out to be wrong.

The Problem: Speculative Execution

In earlier lessons, we learned that to keep the pipeline full, processors make "guesses" about which direction a branch will go. We call this speculative execution. If our branch predictor guesses correctly, the pipeline flows smoothly. However, if the prediction is wrong, the instructions currently sitting in the earlier stages of the pipeline (Fetch, Decode, and Execute) are "polluted"—they are the wrong instructions and must never be allowed to update the state of the machine (like writing to a register or memory).

Implementing the Flush

When the branch resolution logic (usually in the Execute stage) discovers a misprediction, it must trigger a "flush" or "pipeline clear." To do this, we modify our control logic to reset the control signals of the instructions behind the branch.

Imagine a 5-stage pipeline. If the branch is resolved at stage 3, the instructions in stages 1 and 2 are invalid. We clear them by asserting a synchronous clear signal on the pipeline registers:

// Logic triggered upon branch misprediction detection
if (branch_mispredicted) begin
    // Clear instructions in Fetch and Decode stages
    instruction_register_fetch <= 0; 
    instruction_register_decode <= 0;
    
    // Flush control signals to prevent unwanted writes
    control_signals_decode <= 0;
end

By forcing these registers to 0 (or a NOP instruction—"No Operation"), we effectively turn those invalid instructions into "bubbles." These bubbles travel through the pipeline without performing any meaningful work, preventing them from corrupting the Register File or Data Memory.

Performance Impact

While flushing is necessary for correctness, it comes at a cost. Every time we flush, we lose the work performed during those clock cycles, effectively wasting time. This is why high-performance architecture is so obsessed with improving branch prediction accuracy; a perfect predictor means you never have to flush, keeping your throughput at its theoretical maximum.

Key Takeaway: Managing a pipeline flush requires the control unit to force subsequent pipeline registers to a NOP state upon detecting a misprediction, effectively clearing invalid instructions before they commit any architectural changes.