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chapter3-m5-ziavras

# Ziavras extracting loop level parallelism 4 new code

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Unformatted text preview: S1’: A[i+1] = A[i+1] + B[i+1] S2’: B[i+2] = C[i+1] + D[i+1] S1 depends on S2 S2 doesn’t depend on S1 dependence is not circular is not circular S. Ziavras Extracting Loop-Level Parallelism (4) New code does not contain loop-carried dependence A[1] = A[1] + B[1]; A[1] B[1] For (i=1; i<=99; i=i+1) { B[i+1] = C[i] + D[i]; C[i] D[i]; A[i+1] = A[i+1] + B[i+1]; } B[101] = C[100] + D[100]; Iterations may be overlapped, provided statements in each iteration are kept in order S. Ziavras Extracting Loop-Level Parallelism (5) For (i=1; i<=100; i=i+1) { A[i] = B[i] + C[i] D[i] = A[i] * E[i] } /* S1 */ /* S2 */ The dependence S1 S2 is not a problem if S2 gets the value of A[i] from the reg. that receives the result of S1 (as long as the reg. is not overwritten by any other instr.; not here) S. Ziavras Extracting Loop-Level Parallelism (6) Recurrence: a variable is defined based on the value of that variable is defined based on the value of that variable in an earlier iteration • Detecting a recurrence is important for 2 reasons 1. Some architectures (e.g., vector computers) have special support for executing recurrences 2. Some recurrences can be the source of a reasonable recurrences can be the source of reasonable amount of parallelism Dependence distance of 5 distance of For (i=6; i<=100; i=i+1) { Y[i] = Y[i-5] + Y[i]; } The larger the dependence distance, the more potential parallelism can be obtained by loop unrolling S. Ziavras Improving Instruction Delivery Rate • • Current multiple-issue processors deliver 4-8 instrs. every clock cycle Techniques supporting multiple issues 1. Branch-target buffers 2. Integrated instr. fetch units 3. Predicting addresses for indirect branches S. Ziavras Branch-Target Buffers Branch-prediction cache that stores the predicted address for the next instr. after a branch is called S. Ziavras Steps in Handling an Instruction Assume: only taken branches stored in buffer S. Ziavras Penalties for Branch-Target Buffers Instr. in buffer Prediction Yes T Yes Actual branch Penalty cycles cycles T 0 T NT 2 (1 cycle...
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