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Reducing Bad OSC Dependencies Using Distance Counter

IP.com Disclosure Number: IPCOM000234804D
Publication Date: 2014-Feb-07
Document File: 1 page(s) / 28K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that applies a novel algorithm to reduce bad Operand-Store-Compare (OSC) dependencies using distance information between the store and load instructions that caused the OSC hazards.

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Reducing Bad OSC Dependencies Using Distance Counter

An Operand-Store-Compare (OSC) history table (OHT) is used to predict and avoid OSC hazards in out-of-order microprocessors. The existing method does not work

well for workloads having many intermingled store-load pairs. This is because bad dependencies are created when a load marked as likely-victim is forced to be dependent on all stores marked as likely-perpetrators in the issue queue, even though the load is actually dependent on only one or none of the store. Bad dependencies prevent the load from being issued until all stores with forced dependencies (i.e. stores are depended upon) complete execution. In turn, the number of instructions being executed in parallel is limited, which is detrimental to

performance. Performance degradation by bad dependencies can be greater than performance benefit by avoiding OSC hazards (e.g., pipeline flushes or rejects) with

good dependencies. Larger issue/execution windows used in modern microprocessors cause more bad dependencies.

The solution is to reduce bad OSC dependencies using distance information between the store and load instructions that caused the OSC hazards. The novel algorithm follows, in brief:

1. Load Store Unit (LSU) calculates the distance between the store and the load that caused the OSC hazard, and then records the distance in the Store Queue (STQ) entry of the store

2. The distance is stored into the OHT when the store that caused the OSC hazard is...