buzzword
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buzzword [2014/02/03 19:20] rachata |
buzzword [2014/02/05 19:20] rachata |
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| + | ===== Lecture 10 (2/5 Wed.) ===== | ||
| + | |||
| + | * Branch prediction accuracy | ||
| + | * Why are they very important? | ||
| + | * Differences between 99% accuracy and 98% accuracy | ||
| + | * Cost of a misprediction when the pipeline is veryd eep | ||
| + | * Value prediction | ||
| + | * Global branch correlation | ||
| + | * Some branches are correlated | ||
| + | * Local branch correlation | ||
| + | * Some branches can depend on the result of past branches | ||
| + | * Pattern history table | ||
| + | * Record global taken/not taken results. | ||
| + | * Cost vs. accuracy (What to record, do you record PC? Just taken/not taken info.?) | ||
| + | * One-level branch predictor | ||
| + | * What information are used | ||
| + | * Two-level branch prediction | ||
| + | * What entries do you keep in the glocal history? | ||
| + | * What entries do you keep in the local history? | ||
| + | * How many table? | ||
| + | * Cost when training a table | ||
| + | * What are the purposes of each table? | ||
| + | * Potential problems of a two-level history | ||
| + | * GShare predictor | ||
| + | * Global history predictor is hashed with the PC | ||
| + | * Store both GHP and PC in one combined information | ||
| + | * How do you use the information? Why does the XOR result still usable? | ||
| + | * Slides (page 16-18) for a good overview of one- and two-level predictors | ||
| + | * Warmup cost of the branch predictor | ||
| + | * Hybrid solution? Fast warmup is used first, then switch to the slower one. | ||
| + | * Tournament predictor (Alpha 21264) | ||
| + | * Other types of branch predictor | ||
| + | * Using machine learning? | ||
| + | * Geometric history length | ||
| + | * Look at branches far behind (but using geometric step) | ||
| + | * Predicated execution - eliminate branches | ||
| + | * What are the tradeoffs | ||
| + | * What is the block is big (can lead to execution a lot of useless work) | ||
| + | * Allows easier code optimization | ||
| + | * From the compiler PoV, predicated execution combine multiple basic blocks into one bigger basic block | ||
| + | * Reduce control dependences | ||
| + | * Need ISA support | ||
| + | * Wish branches | ||
| + | * Compiler generate both predicated and non-predicated codes | ||
| + | * HW design which one to use | ||
| + | * Use branch prediction on an easy to predict code | ||
| + | * Use predicated execution on a hard to predict code | ||
| + | * Compiler can be more aggressive in optimimzing the code | ||
| + | * What are the tradeoffs (slide# 47) | ||
| + | * Multi-path execution | ||
| + | * Execute both paths | ||
| + | * Can lead to wasted work | ||
buzzword.txt · Last modified: 2015/04/27 18:20 by rachata
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