Question

What problems occur in a pipelined CPU architecture when conditional branches are encountered? Name and describe ways in which the effects of conditional branches can be reduced or eliminated.

Answer 1

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A branching in a pipelined CPU architecture often involves a hazard. Unless the processor can give effect to the branch in a single time cycle, the pipeline will continue fetching instructions sequentially. Such instructions cannot be allowed to take effect because the programmer has diverted control to another part of the program.

The problem with a conditional branch is even more serious. The processor may or may not branch, depending on a calculation that has not yet occurred. Various processors may stall, may attempt branch prediction, and may be able to begin to execute two different program sequences (eager execution), both assuming the branch is and is not taken, discarding all work that pertains to the incorrect guess.^[a]

A processor with an implementation of branch prediction that usually makes correct predictions can minimize the performance penalty from branching. However, if branches are predicted poorly, it may create more work for the processor, such as flushing from the pipeline the incorrect code path that has begun execution before resuming execution at the correct location.

Programs written for a pipelined processor deliberately avoid branching to minimize possible loss of speed. For example, the programmer can handle the usual case with sequential execution and branch only on detecting unusual cases. The programmer can also measure how often particular branches are actually executed and gain insight with which to optimize the code. In some cases, a programmer can handle both the usual case and unusual case with branch-free code.

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