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HW Versus SW Speculation Mechanisms

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4.6 HW Versus SW Speculation Mechanisms

• To speculate extensively, we must be able to disambiguate memory reference easy for HW (Tomasulo)

• HW speculation works better when control flow is unpredictable, and when HW branch prediction is superior to SW branch prediction done at compiler time– Misprediction rate 16%/10% for 4 major integer SPEC92

SW/HW• HW speculation maintains a completely precise exception

model for SI• HW speculation does not require compensation or

bookkeeping code, needed by ambitious SW speculation

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HW Versus SW Speculation Mechanisms (Cont.)

• HW speculation with dynamic scheduling does not require different code sequences to achieve good performance for different implementation of an architecture

• HW speculation require complex and additional HW resources

• Some designers have tried to combine the dynamic and compiler-based approaches to achieve the best of each