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Cliquez pour modifier le style du titre du masque,Cliquez pour modifier les styles de texte du masque,Deuxime niveau,Troisime niveau,Quatrime niveau,Cinquime niveau,*,The O-GEHL branch predictor,Andr Seznec,Caps Team,Irisa,The,O-GEHL,branch predictor,O,ptimized,GE,ometric,H,istory,L,ength,Andr Seznec,IRISA/INRIA/HIPEAC,1,What is classic,Global history based:,Yeh and Patt 91, Pan and So 91,Use of multiple history lengths:,McFarling 93, Evers et al. 96,Use an adder tree instead of a meta-predictor,Vintan and Iridon 99, Jimnez and Lin 01,2,L(4),L(3),L(2),L(1),L(0),TO,T1,T2,T3,T4,Multiple history length neural predictor,3,GEometric History Length,predictor,The set of history lengths forms a,geometric,series,0, 2, 4, 8, 16, 32, 64, 128,What is important:,L(i)-L(i-1) is drastically increasing,4,Updating the predictor,Update on misprediction and under a threshold,8-bit counters and perceptron update threshold (29),Would not have qualified for CBP-1,5,Dynamic update threshold fitting,On an O-GEHL predictor, best threshold depends on,the application,the predictor size,the counter width,By chance for the best fixed threshold,updates on mispredictions ,updates on correct predictions,Monitor both numbers,and adapt the update threshold,8 components 8 bits counter would qualify for CBP-1,6,Counter width on O-GEHL predictors,8 bits are just overkilling,4 bits are sufficient,Mixing 5 bits for short histories and 4 bits for long histories is slightly better ,3 bits are not so bad !,7,Adaptative history length fitting,(inspired by Juan et al 98),( applications: L(7) 150,),Let us adapt some history lengths to the behavior of each application,8 tables:,T2: L(2) and L(8),T4: L(4) and L(9),T6: L(6) and L(10),8,Adaptative history length fitting,(2),Intuition:,if high degree of aliasing on T7, stick with short history,Implementation:,monitoring of aliasing on updates on T7 through a tag bit and a counter,Simple is sufficient:,Flipping from short to long histories and vice-versa,9,L(4),L(3) or L(6),L(2),L(1) or L(5),L(0),TO,T1,T2,T3,T4,Tag bits,10,Information to be hashed,Address + conditional branch history:,path confusion on short histories,Address + path:,Direct hashing leads to path confusion,Represent,all,branches in branch history,Use also path history ( 1 bit per branch, limited to 16 bits),11,Configuration for CBP,8 tables:,2 Kentries except T1, 1Kentries,5 bit counters for T0 and T1, 4 bit counters otherwise,1 Kbits of one bit tags associated with T7,10K + 5K + 6x8K + 1K = 64K,L(1) =3 and L(10)= 200,0,3,5,8,12,19,31,49,75,125,200,12,Hashing 200+ bits for indexing !,Need to compute 11 bits indexes :,Full hashing is unrealistic,Just regularly pick at most 33 bits in:,address+branch,history +path history,A single 3-entry exclusive-OR stage,13,A case for the OGEHL predictor (1),High accuracy,Robustness to variations of history lengths choices:,L(1) in 2,6, L(10) in 125,300,misp. rate 1.04 x reference misp. rate,Geometric series: not a bad formula !,best geometric L(1)=3, L(10)=223, REF-0.02 misp/KI,best overall,0, 2, 4, 9, 12, 18, 31, 54, 114, 145, 266 REF-0.04 misp/KI,14,A case for the OGEHL predictor (2),Reduce counter width by 1 bit: 49 Kbits,would have been a finalist,64 Kbits 4 components OGEHL predictor,would have been a finalist,50 Kbits 4 components OGEHL predictor (3-bit),would have been a finalist,768 Kbits 12 components OGEHL predictor,2.25 misp/KI,15,A case for the O-GEHL predictor (3),O-GEHL predictor uses only global information,Can be ahead pipelined,Prediction computation logic complexity is low,(The End),16,
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