While lower supply voltage is effective for energy reduction, it suffers performance loss. To mitigate the loss, we propose to execute only the part, which does not have any influence on execution speed, with low-speed and low-voltage. We are investigating a multithreaded execution, named Contrail Architecture, which divides an instruction stream into two streams using trace-level value prediction. One is the speculation stream, which is the main part of a program and is applied value predictions. The other is the verification stream, which verifies the predictions. The energy consumption is reduced by the decrease in the execution time in the speculation stream and by the low-speed execution in the verification stream. This paper evaluates its potential in energy efficiency. Categories and Subject Descriptors