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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