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 calls 1. Remember inputs & outputs 2. Collect implicit inputs & outputs 3. Check to see if implicit in/out get modified 4. 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