Design tradeoffs for the Alpha EV8 Conditional Branch Predictor André Seznec, IRISA/INRIA Stephen Felix, Intel Venkata Krishnan, Stargen Inc Yiannakis Sazeides, University of Cyprus

Alpha EV8 (cancelled june 2001) SMT: 4 threads wide-issue superscalar processor: 8-way issue Single process performance is the goal Multithreaded performance is a bonus 5-10 % overhead for SMT

Challenges on the EV8 conditional branch predictor High accuracy is needed: 14 cycles minimum miss penalty Up to 16 predictions per cycle: from two non-contiguous fetch blocks! Various implementation constraints: master the number of physical memory arrays use of single-ported memory cells timing constraints

instruction fetch blocks on EV8 br not taken br taken not

Alpha EV8 front-end pipeline Fetches up to two, 8-instruction blocks per cycle from the I-cache: a block ends either on an aligned 8-instruction end or on a taken control flow up to 16 conditional branches fetched and predicted per cycle Next two block addresses must be predicted in a single cycle: critical path: use of a line predictor backed with a complex PC address generator: conditional branch predictor, RAS, jump predictor ..

PC address generation pipeline C and D A and B Y and Z Cycle 1 Cycle 2 Cycle 3 Line prediction is completed Prediction table read is completed PC address generation is completed

EV8 predictor: (derived from) (2Bc-gskew) e-gskew

2Bc-gskew: degrees of freedom partial update policy on correct predictions, only updates correct components: do not destroy other predictions better accuracy ! On correct predictions: prediction bit is only read hysteresis bit is only written USE OF DISTINCT PREDICTION AND HYSTERESIS ARRAYS !! No reason for same size for hysteresis and prediction arrays

EV8 predictor: leveraging degrees of freedom Smaller bimodal table Different history lengths

Dealing with implementation constraints

Issues on global history Blocks A and B Blocks Y and Z Blocks C and D Branch infos from C, B and A are not valid to predict D! On each cycle, upto 16 branch are predicted: 0 to 16 bits to be inserted in the history vector !?

Block compressed history lghist Incorporate at most one bit in the history per fetch block: 0, 1 or 2 bits to be incorporated in history vector per cycle Which bit ? Direction of the last conditional branch in the block previous ones are not taken XORed with position (1st half/ 2nd half) in the block more uniform distribution of the history vectors

instruction fetch blocks on EV8 br taken 1 is inserted br taken not 0 is inserted

The EV8 branch predictor information vector History information is not available on the three previous blocks A, B, and C but, addresses are available !! Information vector to index the predictor: 1. Instruction address 2. Lghist (3-blocks-old history + path) 3. Path info on the last three blocks

Using single-ported memory arrays The challenge: 16 predictions to be performed per cycle from two non-contiguous blocks ! 8 updates per cycle: for two non-contiguous blocks ! But single-ported arrays are highly desirable :-)

Bank-interleaved or double-ported branch predictor ? Reads of predictions for two 8-instructions blocks: double-porting: memory cells twice as large losing half of the entries ? bank-interleaving: need for arbitration longer critical electrical path losing throughput short loops fitting in a single 8-instruction block !? ????????

Conflict free interleaved bank predictor Key idea: Force adjacent predictions to lie in distinct banks Bank for A is determined by Y and Z 4-way interleaved: if (y6,y5)== Bz then Ba =(y6,y5+1) else Ba = (y6,y5)

Conflict free bank-interleaved predictor (2) Conflicts are avoided by construction Bank number is computed one cycle ahead not on the critical path Single ported bank-interleaved memory arrays !

« Logical view » vs real implementation 4 tables * 4 banks * 2 (pred. +hyst.): 32 memory arrays Indexing functions are computed, then arrays are accessed 4 banks * 2 (pred. + hyst.) 4 tables in a single array 8 memory arrays No time to lose: start access and compute part of the index in //

Reading the branch prediction tables Bank selection 1 out of 4 Meta G0 G1 BIM Wordline selection 1 out 64 Column selection: 8 out of 256 Unshuffle: 8 to 8

Reading the branch prediction tables (2) Span over 5 cycle phases: Cycle -1: bank number computation bank selection Cycle 0: phase 0: wordline selection phase 1: column selection Cycle 1: phase 0: unshuffle permutation

Constraints for indices composition Strong: Wordline bits: immediate availability common to the four logical tables Medium: Column bits a single 2-entry XOR gate Weak: Unshuffle bits: near complete freedom, a full tree of XOR gates if needed

Designing the indexing functions (1) 6 wordline bits Must be available at the beginning of the cycle: block address bits 3-block old lghist bits path bits Tradeoff: address bits for emphasizing bimodal component behavior lghist bits are more uniformly distributed 4 lghist bits + 2 address bits

Designing the indexing functions (2) Column selection and unshuffle Favor independance of the four indexing functions: if two (address,history) pairs conflict on a table then try to avoid repeating the conflict on an other table Guarantee that for a single address, two histories that differ by only one or two bits will not map on the same entry Favor usage of the whole table: lghist bits are more uniformly distributed than address bits XORing 2 lghist bits for column bits a XOR tree with up to 11 bits for unshuffle

EV8 branch predictor configuration 208 Kbits for prediction and 144 Kbits for hysteresis «BIM»: 16 K + 16 K, 4 lghist bits (+ 3-block path) G0: 64 K + 32 K, 13 lghist bits G1: 64 K + 64 K, 21 lghist bits Meta: 64 K + 32 K, 17 lghist bits 4 prediction banks and 4 hysteresis banks

Performance evaluation Sorry, SPEC 95 :-)

Benchmarks characteristics Highly optimized SPECint 95: much more not-taken than taken ratio lghist/ghist length: from 1.12 to 1.59 from 8.9 to 16.2 branches per 100 instructions

2Bc-gskew vs other global history predictors

Quality of information vector

Reducing some table sizes no significant impact

Quality of indexing functions

Conclusion Design of a real branch predictor leads to challenges ignored in most academic studies: 3-block old history vector inability to maintain a complete history simultaneous accesses to the predictor minimization of the number of memory arrays timing constraints on the indexing functions We overcame these difficulties and adapted a state of the art academic branch predictor to real world constraints.

Summary of the contributions Efficient information vector can be built with mixing path and compressed history: don’t focus on the info vector, use what is convenient! Use of different table sizes, history lengths in the predictor. Sharing of hysteresis bits Conflict free parallel access scheme for the predictor Engineering of indexing functions

Acknowledgements To the whole EV8 design team Special mention to: Ta-chung Chang, George Chrysos, John Edmondson, Joel Emer, Tryggve Fossum, Glenn Giacalone, Balakrishnan Iyer, Manickavelu Balasubramanian, Harish Patil, George Tien and James Vash.