Browse Prior Art Database

Shortening the Recursive Branch Predictor

IP.com Disclosure Number: IPCOM000105827D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 85K

Publishing Venue

IBM

Related People

Ekanadham, K: AUTHOR [+3]

Abstract

A Joint Recursive Branch Predictor (JRBP) has the following set of properties:

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Shortening the Recursive Branch Predictor

      A Joint Recursive Branch Predictor (JRBP) has the following set
of properties:

o   It should continue with the prediction scheme if the prior
    prediction is correct.
o   It should wrap around so that a finite length predictor can
    continue to predict indefinitely as long as it is correct.
o   It need not synch with addresses of the branches, except at a
    error, because
    -   A recursive predictor of a branch is defined as a sequence of
        the actions of branches and can be used to predict the next
        action of the next branch in conjunction with a BHT which
        will supply the related target address.

        The ability to operate within a joint target set of a set of
        branches with the same predictor format used for a single
        branch is a result of the observation that each branch will
        specify a target that dictates the next branch that is
        encountered if the target is correct.  Just as joint
        distributions contain additional information than marginal
        distributions, the use of the joint recursive predictors of
        prespecified length can be superior to the operation of
        individual recursive predictors on the marginal sequences of
        individual branch actions.
o   The predictor needs to have at least one occurrence of each
    action of each branch within the loop so that the recovery
    mechanism can recover to the position specified by the correct
    action following an error.

      The generation of a JRBP is subject to the recovery option that
that predictor has available to it.  A means of comparing two
predictors that illustrates their equivalence on the same sequence
shows how predictors can be shortened.  Elimination of intervening B
-> B subsequences as being a manifestation of an error recovery from
an existing branch establishes a new basis for judging the candidate
sequences that arise from a sampling of consecutive B -> B
subsequences.  This provides a means of closing the process under
which test-sequences and recovery can be judged against the
historical sequences that occur.

      A means is required to close a logical loophole in the
candidate set of joint recursive branch predictors.  The problem is
that a given historical sequence can be better predicted with an
error recovery reentry into joint recursive predictor than with the
actual sequence. ...