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The content of this blog is my personal opinion only. Although I am an employee - currently of Imagination Technologies's MIPS group, in the past of other companies such as Intellectual Ventures, Intel, AMD, Motorola, and Gould - I reveal this only so that the reader may account for any possible bias I may have towards my employer's products. The statements I make here in no way represent my employer's position, nor am I authorized to speak on behalf of my employer. In fact, this posting may not even represent my personal opinion, since occasionally I play devil's advocate.

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Wednesday, February 09, 2011

Skipahead Multithreading

http://semipublic.comp-arch.net/wiki/SkMT

[[Skipahead multithreading (SkMT)]] is a form of [[speculative multithreading (SpMT)]]
characterized by "skipping ahead" at certain points in
[[non-speculative or less speculative versus more speculative|non-speculative or less speculative execution to more speculative execution]].

Typically these "certain points" in thecode are places
where there is a well characterized [[control independence or convergence]] point:
* the instruction after a CALL instruction
* end of loop
* later iterations of loop
* IF convergence

I, Andy Glew, coined the term [[SkMT]]
when it became evident that the term [[SpMT]],
which was itself coined by Antonio Gonzales and promoted by me,
was more generic.
I.e. you can imagine creating speculative threads
that do not really skip that far ahead,
but which, e.g. execute past a place where execution would bee blocked,
either an in-order blockage, or where an OOO window would be full.
See [[non-skipahead speculative multithreading]].

    Oooo.... I just created a new term: [[slip-ahead multithreading]]. It rather nicely encapsulates what I just described, and is consistent with published project names such as [[slipstream]].

In much the same way, I had earlier used the term [[implicit multithreading (IMT)]],
and replaced it by [[speculative multithreading (SpMT)]],
which I am now (after, 10 years ago, 2000) specializing to [[skipahead multithreading (SkMT)]].

The term [[skipahead]] is intended to be contrasted with [[lookahead]],
a term which was once used to characterize all [[out-of-order (OOO)]] execution.

    Look-ahead processors. Robert M. Keller, Princeton. ACM Computing Surveys, Vol 7, Issue 4, Dec 1975. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.88.5118&rep=rep1&type=pdf

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