chapter3-m5-ziavras

Ziavras execution time 9 cycles ti 25 opscycle vliw

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Unformatted text preview: BNE R1,R2,Loop 56-32 Assuming NO BRANCH DELAY NO BRANCH DELAY S. Ziavras Execution time: 9 cycles ti 2.5 ops/cycle VLIW Discussion • Original VLIW approach VLIW – Increased code size • Exploiting parallelism requires ambitiously parallelism requires ambitiously unrolling loops • Unused FUs wasted bits in instr. encoding Clever encodings needed. E.g.: Compress instrs. in memory; uncompress when fetched Only 1 large immediate field for any FU – Limitations of lockstep operation of lockstep operation • No hazard detection HW (original approach) Blocking caches, causing all FUs to stall S. Ziavras VLIW Discussion (2) • More recent VLIWs – FUs operate more independently – Compiler tries to avoid hazards at issue time ti – HW supports unsynchronized execution once instrs. are issued • Binary code compatibility: problem in early approach – Depends on #s & types of FUs, & unit latencies Solutions object-code translation or emulation Temper the strictness of approach (IA-64) S. Ziavras Extracting (& Enhancing) Loop Extracting (& Enhancing) Loop-Level Parallelism Example with independence between iterations For (i=1000; i>0; i=i-1) (i=1000; i>0; i=i x[i] = x[i] + s; • The compiler checks for loop dependences in early stages (before the machine-language code is produced) • This code doesn’t involve any loop-carried dependences S. Ziavras Extracting Loop-Level Parallelism (2) For (i=1; i<=100; i=i+1) { A[i+1] = A[i] + C[i]; /* S1 */ B[i+1] = B[i] + A[i+1]; /* S2 */ B[i] A[i+1] /* S2 */ } Dependences • Between 2 consecutive iterations of S1 (a loop-carried dependence) • Between 2 consecutive iterations of S2 (a loop-carried dependence) • Between S1 & S2 in the same iteration S. Ziavras Extracting Loop-Level Parallelism (3) For (i=1; i<=100; i=i+1) { (i A[i] = A[i] + B[i]; /* S1 */ B[i+1] C[i] D[i]; /* S2 */ B[i+1] = C[i] + D[i]; /* S2 */ } Loop-carried dependence S1 S2 S1’ S2’ Doesn’t prevent parallelism Parallel loop because of non-circular dependences Unroll 2 iterations iterations S1: A[i] = A[i] + B[i] S2: B[i+1] = C[i] + D[i]...
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This document was uploaded on 02/09/2014.

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