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Multi-Method Dispatch Using Multiple Row Displacement

Multi-Method Dispatch Using Multiple Row Displacement. Candy Pang, Wade Holst, Yuri Leontiev, and Duane Szafron ECOOP’99. Presented by: Irene Cheng Date: 7 November 2002.  1. AxA AxB BxA AxC BxB CxA BxC CxB

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Multi-Method Dispatch Using Multiple Row Displacement

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  1. Multi-Method Dispatch Using Multiple Row Displacement Candy Pang, Wade Holst, Yuri Leontiev, and Duane Szafron ECOOP’99 Presented by: Irene Cheng Date: 7 November 2002 1 AxA AxB BxA AxC BxB CxA BxC CxB CxC 3 2 4

  2. Presentation Topics • Review of Terminology • Review of Row Displacement Dispatch • for Single-Receiver • Multiple Row Displacement (MRD) • Optimizations • Performance Results and Conclusion

  3. Review of Terminology • Single-receiver dispatch uses the dynamic type of a receiver object and the method name to determine which method to execute at • run-time, • e.g. aPerson.id(); • Multi-method dispatch uses the dynamic types of the arguments and the method name to determine the method to execute, • e.g. shape.intersect( rectangle, circle );

  4. Review of Terminology (cont) • What is a call-site ? • In single-receiver languages - viewed as a • message sent to the receiver object. • e.g. • In multi-method languages - viewed as the • execution of a behavior on a set of arguments. • e.g. • The run-time determination of the method to • invoke at a call-site is called method dispatch. aPerson.id() shape.intersect( rectangle, circle )

  5. Review of Terminology (cont) • Dispatch strategy: • Cache-based (global or local) • Table-based - • Pre-determine the method for every possible call-site, • and record these methods in a table. • When a method is defined, each argument has a specific • static type. However, at a call-site, the dynamic type of each • argument can either be the static type, or any of its subtypes, • e.g. Person aPerson; if (…) aPerson = new Person(); else aPerson = new Student(); aPerson.id( ); Person (id1) Student (id2) id1 id2

  6. Product-Type Graph (for 2 arguments) A product-type graph hierarchy contains all the possible call-sites 1 AxA AxB BxA AxC BxB CxA BxC CxB CxC A B C 3 2 4 The underlying Inheritance Hierarchy, H The 2-arity product-type graph, H2

  7. Inheritance Conflicts General Definition A conflict occurs when a product-type can see 2 different method definitions by looking up different paths in the induced product-type graph. 1 AxC AxD BxC AxE BxD ExC BxE ExD ExE • Relaxation • Let  = { P1 … Pn } and P < P1 … Pn, the methods in Pi and Pj do not conflict in P if : • i.e. 1 <= i, j, k <= n • Pi < Pj or • Pj < Pi or • { Pk  | Pk Pi Pk Pj } 2 2 2 2

  8. Review of single-receiver Row displacement Dispatch (RD) 4 Example: E anE = new E(); anE.(); 1 + 4 = 5 D:: A0 D3  B1 C2, E4

  9. MRD Dispatch Table for method  with 2 arguments AA AB BA AC BB CA AD BC CB DA AEBD CC DB EA BE CD DC EB CE DD EC DEED EE 1 2 3

  10. MRD Dispatch Table for method  with 2 arguments AA AB BA AC BB CA ADBC CB DA AEBD CC DB EA BE CD DC EB CE DD EC DE ED EE 1 2

  11. Data Structure per Behavior - Array of pointers to arrays • ( Level-0 array ) L0 - indexed by the 1st argument type • ( Level-1 array ) L1 - indexed by the 2nd argument type and • contains method addresses A0 B1 C2 D3 E4 A0 B1 C2 D3 E4

  12. Compressing the Data Structure for  M - Global Master Array I0 - Global Index Array B - Global Behavior Array

  13. Compressing the Data Structure for  and  Example: Dispatch a call-site (anE, aD) M[ I0[5 + 4 = 9] + 3 = 14] M - Global Master Array I0 - Global Index Array B - Global Behavior Array

  14. MRD is designed for n-dimensional dispatch tables 1 BDB BDE BEB BEE EDB EDE EEB EEE 2 DBD DBE DED DEE EBD EBE EED EEE

  15. BD DB EE M [I1 [I0 [ B[] +4] +3] +1] = M [I1 [I0 [ 7+4] +3] +1] = M [I1 [ 5+3] +1] = M[ 15+1] Example: dispatch a call-site  (anE, aD, aB)

  16. Optimizations Single I - One Global Index Array, I, to store all L0 to Lk-2 arrays 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 I0 0 0 1 1 5 8 11 - - 11 I1 - - 14 15 15 14 14 - 15 19 - 8- 913    L0 B 0 5 17 M [ I0 [ I0 [ B[] +4] +3] +1] = M [ I0 [ I0 [ 17+4] +3] +1] = M [ I0 [ 13+3] +1] = M [ 15+1] Example:  (anE, aD, aB)

  17. Optimizations (cont) Row-matching instead of row-shifting (additional 10-14% ) Before displacement Row-shifting Row-matching Row-matching cannot be used in single-receiver RD because different rows contain different behavior, and thus different method addresses.

  18. Optimizations (cont) Byte vs. Word Storage (MRD-B) • M is the most memory consuming data structure • (duplicate 4-byte method address) • Use method-map per behavior • (each method address is stored only once) • Only 1 byte (max. 256 methods) is used in M to store the index • of the corresponding method in the method-map. • Size of M is reduced to 1/4 but extra redirection time at dispatch.

  19. Timing results Noop - dummy function to time the overhead incurred MRD - multiple row displacement MRD-B - MRD using byte instead of word for Master array CNT - Compressed N-Dimensional Tables SRP - Single Receiver Projections LUA - Lookup Automata

  20. Memory Utilization LUA - Lookup Automata MRD - multiple row displacement MRD-B - MRD using byte instead of word for Master array CNT - Compressed N-Dimensional Tables SRP - Single Receiver Projections

  21. Conclusion • A new multi-method dispatch technique is introduced to • compress an n-dimensional table by row displacement. • The first time a comparison of multi-method techniques • has appeared in the literature. • Experimental results shows that when comparing with other • table-based multi-method techniques, MRD has the • fastest dispatch time and • the second smallest per-call-site code size.

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