SoftSig: Software-Exposed Hardware Signatures for Code Analysis and Optimizations

UIUC – ASPLOS 2008

by Evangelos Vlachos

Motivation

Runtime Disambiguation of Sets of Addresses Multiple Hardware watch-points Inter-thread dependencies (e.g., TLS,

TM) Compile-time analysis too limited Previous solutions

Compare address to an associative structure

Operate on sets of addresses (signatures)

Propose

Hardware signatures Perform multiple operations at a time Too simple software interface so far (TM)

Expose hw signatures to software Software flexibility: decides on

Memory accesses to collect Memory accesses to disambiguate against

Software Register File (SFR) & Sophisticated ISA

Background

1)

2) Memoization Don’t compute same result again –

Remember it

Examples

Design Guidelines

G1: Minimize SR accesses and copies SR size = 1Kbit Context switch – discard SRs Never spill SR to stack

Design Guidelines

G1: Minimize SR accesses and copies G2: Manage the SRF through

dynamic allocation Limited number of SRs Hard-to-predict lifetimes

Design Guidelines

G1: Minimize SR accesses and copies G2: Manage the SRF through

dynamic allocation G3: Imprecision should never

compromise correctness Software that uses SR must be able to

overcome false positives

Design Guidelines

G1: Minimize SR accesses and copies G2: Manage the SRF through dynamic

allocation G3: Imprecision should never

compromise correctness G4: Manage imprecision to provide

the most efficiency Shorter ranges & filter some of the

addresses

ISA extensions

SoftSig Architecture

Collection & Local Disambiguation

bcollect or bdisamb.loc Notify LSQ to send addresses to SPM If no conflict, the instruction can retire ecollect and edisamb.loc

Stop collecting and disambiguating addresses

Remote Disambiguation

When is an address disambiguated? ICD = In-flight

Conflict Detector What about

cache displacements?

Example: Memoization Framework

Identify redundant calls1. Remember inputs &

outputs2. Collect implicit inputs &

outputs3. Check to see if implicit

in/out get modified4. Don’t perform next call if

no conflict is found – Memoized

Example: Memoization Framework

Prologue: Avoid Function call? Compare explicit in/out with

the ones memoized See if there was a conflict

Setup: Cannot avoid it Remember in/out Allocate SR

Epilogue Finish setting up

Evaluation

Evaluation