Recovery Mechanisms

Recovery Mechanisms

• In case of compiler speculation, additional instructions are inserted to check the accuracy of the speculation and provide a recovary routine to use when the speculation is incorrect.

• In case of hardware speculation, the processor usually use buffers to temporary store the speculative results. If the speculation is correct, the instructions are executed by allowing the contents of the buffers to be written to the registers or memory. If the speculation is incorrect, the special hardware is used to flush the buffers and re‒execute the correct instruction sequence.

• In some cases, speculating instructions may introduce exceptions that were not expected. For example, if memory related instruction is executed with a wrong address when the speculation is incorrect, exception occurs.

• In compiler‒based speculation, such exceptions are avoided by adding special speculation support that ignores needless exceptions.

• In hardware‒based speculation, exceptions are simply buffered until it is clear that the instruction causing them is no longer speculative and is ready to complete; at that point the exception is raised and normal exception handling proceeds.

Review Question

1. Explain the recovery mechanisms used when speculation is incorrect.