Forward semantic: A compiler-assisted instruction fetch method for heavily pipelined processors

Pohua P. Chang, Wen-Mei W Hwu

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

A new instruction fetch method, forward semantic, is offered to enable the deeply pipelined processors to fetch one useful instruction every cycle. Forward semantic is an improved alternative to the delayed branching (with or without squashing), with five major advantages. Fist, no restriction is imposed on the type of instructions filling the branch slots, which allows a large number of slots to be filled. Second, no modification to the offsets and displacements is necessary when an instruction is copied to fill a branch slot, which simplifies the linker implementation. Third, an interrupted program can resume execution with a single program counter, eliminating the need for reloading the instruction pipeline before resuming execution. Fourth, programs compiled with N slots can execute on pipelines requiring K (K ≤ N) slots, which makes family architecture compatibility possible. Lastly, the filling of branch slots is totally transparent to code compaction and software interlocking schemes. These advantages combine to provide an efficient instruction fetch mechanism and to eliminate artificial penalties on branch cost. At the cost of 11% static code expansion, forward semantic achieves an instruction fetch cost of 1.2 cycles for pipelines requiring 10 slots for each taken branch. This level of instruction fetch efficiency has never been achieved before with conventional instruction fetch methods. The branch cost is dictated by the accuracy of the compile-time branch prediction rather than artificial limitations, such as data dependencies, which prevent the slots from being filled. These results are measured from the execution of real UNIX and CAD programs with complex control structures.

Original languageEnglish (US)
Title of host publicationProceedings of the Annual International Symposium on Microarchitecture, MICRO
EditorsVicki H. Allan
PublisherIEEE Computer Society
Pages188-198
Number of pages11
ISBN (Electronic)0897913248
DOIs
StatePublished - Aug 1 1989
Event22nd Annual Workshop on Microprogramming and Microarchitecture, MICRO 1989 - Dublin, Ireland
Duration: Aug 14 1989Aug 16 1989

Publication series

NameProceedings of the Annual International Symposium on Microarchitecture, MICRO
ISSN (Print)1072-4451

Other

Other22nd Annual Workshop on Microprogramming and Microarchitecture, MICRO 1989
CountryIreland
CityDublin
Period8/14/898/16/89

ASJC Scopus subject areas

  • Hardware and Architecture

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  • Cite this

    Chang, P. P., & Hwu, W-M. W. (1989). Forward semantic: A compiler-assisted instruction fetch method for heavily pipelined processors. In V. H. Allan (Ed.), Proceedings of the Annual International Symposium on Microarchitecture, MICRO (pp. 188-198). (Proceedings of the Annual International Symposium on Microarchitecture, MICRO). IEEE Computer Society. https://doi.org/10.1145/75362.75418