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In-line penalty = 5 cycles |
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Target penalty = 4 cycles |
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Then we should guess the target path if |
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4P 5(1 - P) < 6P ® P > 5/7. |
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* Note: we assume that the target is fetchedbefore the loop arithmetic is completed (EX) to avoid EX conflict with other instructions. |
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If we guess in-line and go in-line, we have (remember, no conditional EX): |
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If we guess in-line and go to target, we have: |
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In-line penalty = 1 cycle |
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Target penalty = 7 cycles |
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If we guess target and go in line: |
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If we guess target and go to target: |
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In-line penalty = 6 cycles |
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Target penalty = 4 cycles |
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Then we should guess the target path if: |
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4P + 6(1 - P) < 7P + 1(1 - P) ® P < 5/8. |
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P = probability of branching. |
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Penalty = (taken penalty * P + in-line penalty * (1 - P)) * frequency of instruction. |
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Remember, the in-line strategy means if there is a choice, choose in-line. (There is no choice on an unconditional branch.) |
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Once we have computed the "best" static strategy, we compute the penalty based on that strategy. For example, for BCR, our analysis shows that we should guess in-line. The penalty for guessing in-line on BCR is the frequency of BCR times the sum of the expected in-line penalty (frequency ´ penalty) and the expected target penalty = (0 cycles ´ 0.46 + 6 cycles ´ 0.54) 0.015 =.049 cycles. |
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The in-line strategy is simply to fetch in-line on branch, except when the branch is unconditional. The only difference in strategy is for the loop control instructions, but the net change was only 0.04 CPI, or about 5%. |
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