Browse Prior Art Database

Aliasing Reduction in the Decoding History Tables

IP.com Disclosure Number: IPCOM000105079D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Emma, PG: AUTHOR [+5]

Abstract

Disclosed is a design that avoids aliasing in the DHT by only updating the DHT on conditional branches that miss in the BHT.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 75% of the total text.

Aliasing Reduction in the Decoding History Tables

      Disclosed is a design that avoids aliasing in the DHT by only
updating the DHT on conditional branches that miss in the BHT.

      Branch History Tables (BHTs) and Decode History Tables (DHTs)
are used to predict the outcome of branches.  It has been found that
combining these two mechanisms can provide some performance
advantages over using either of the mechanisms alone.

      The prediction algorithm used in a combined design is to allow
the BHT to predict any branch that it has information stored for.  In
the event that the BHT does not have information for a particular
branch, and the branch is a conditional branch, the DHT is used to
predict the outcome of the branch.  The DHT is acting as a BHT-miss
handler.  (It is assumed that the decoder can efficiently predict
unconditional branches, so that the DHT only handles conditional
branches.)

      Typically the DHT is updated on the execution of every
conditional branch.  Since the DHT is addressed with a hash of the
branch instruction address, hashing (aliasing) collisions often occur
in a DHT.  This can cause the undesireable result of overwriting
history of another branch (and providing an incorrect historical
prediction when the DHT is interrogated).

      To limit the amount of aliasing in the DHT in a combined
BHT-DHT design, the DHT should only be updated on conditional
branches that also miss in the BHT (not on every conditiona...