lect08-branch-prediction

lect08-branch-prediction - LectureOverview BranchPrediction...

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ECE/CS 752:Advanced Computer Architecture I 1 Branch Prediction Prof. Mikko H. Lipasti University of Wisconsin Madison Lecture notes based on notes by John P. Shen Updated by Mikko Lipasti Lecture Overview Program control flow Implicit sequential control flow Disruptions of sequential control flow Branch Prediction Branch instruction processing Branch instruction speculation Key historical studies on branch prediction UCB Study [Lee and Smith, 1984] IBM Study [Nair, 1992] Branch prediction implementation (PPC 604) BTAC and BHT design Fetch Address Generation Program Control Flow Implicit Sequential Control Flow Static Program Representation Control Flow Graph (CFG ) Nodes = basic blocks Edges = Control flow transfers Physical Program Layout Mapping of CFG to linear program memory Implied sequential control flow Dynamic Program Execution Traversal of the CFG nodes and edges (e.g. loops) Traversal dictated by branch conditions Dynamic Control Flow Deviates from sequential control flow Disrupts sequential fetching Can stall IF stage and reduce I fetch bandwidth Program Control Flow Dynamic traversal of static CFG Mapping CFG to linear memory (a) (b) Disruption of Sequential Control Flow Instruction/Decode Buffer Fetch Dispatch Buffer Decode Dispatch Reservation Reorder/ Store Buffer Complete Retire Stations Issue Execute Finish Completion Buffer Branch Branch Prediction Target address generation Target Speculation Access register: PC, General purpose register, Link register Perform calculation: +/ offset, autoincrement, autodecrement Condition resolution Condition speculation Access register: Condition code register, General purpose register Perform calculation: Comparison of data register(s)
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ECE/CS 752:Advanced Computer Architecture I 2 Target Address Generation Decode Buffer Fetch Dispatch Buffer Decode Reservation Dispatch PC- rel.
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