Pair tage 2 taken

[Yakkhini]

Summary by CodeRabbit

• New Features

  • Added PairTAGE two-block branch predictor with configurable phase/second-block behavior and optional integration into the decoupled BPU.
  • Added side-effect-free BTB lookup paths to support safe, non-mutating prediction queries.

• Improvements

  • Added prediction-metadata refresh hooks across BTB/TAGE predictors and extended fetch-target metadata for pair-aware predictions.
  • Improved const-correctness and expanded statistics for pair/second-block workflows.

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📝 Walkthrough

Walkthrough

Adds a new two-block PairTAGE BTB/TAGE predictor and integrates it into DecoupledBPUWithBTB; introduces TimedBaseBTBPredictor::refreshPredictionMeta, side-effect-free BTB lookup APIs and const-correct helpers across BTB predictors, extends FetchTarget with PairTAGE fields, and adds FTQ const accessors.

Changes

Cohort / File(s)	Summary
PairTAGE implementation src/cpu/pred/btb/pairtage.hh, src/cpu/pred/btb/pairtage.cc	New TimedBaseBTBPredictor subclass implementing two-block entries, folded-history meta (TageMeta), phase gating, first/second-block prediction assembly, refreshPredictionMeta, training/update, allocation, history recoveries, and statistics.
Predictor wiring & build src/cpu/pred/BranchPredictor.py, src/cpu/pred/SConscript	Adds PairTAGE SimObject/Params and exposes DecoupledBPUWithBTB.pairtage; registers pairtage source file in build.
Decoupled BPU integration & stats src/cpu/pred/btb/decoupled_bpred.cc, src/cpu/pred/btb/decoupled_bpred.hh, src/cpu/pred/btb/decoupled_bpred_stats.cc	Integrates PairTAGE into tick/enqueue/squash flows, per-thread pairPhase/state, createFetchTarget overloads, second-block teacher/student flow, history update API changes, and extensive PairTAGE-specific stats.
No-side-effect lookup & meta refresh (BTB family) src/cpu/pred/btb/abtb.{cc,hh}, mbtb.{cc,hh}, btb_tage.{cc,hh}, btb_ittage.{cc,hh}, btb_mgsc.{cc,hh}, btb_ubtb.{cc,hh}, microtage.{cc,hh}, ras.{cc,hh}	Adds const lookupNoSideEffect/processEntriesNoSideEffect variants and refreshPredictionMeta implementations to recompute FullBTBPrediction metadata without mutating predictor state or stats; tightens const-correctness of many helper methods.
Common BTB types & FetchTarget src/cpu/pred/btb/common.hh	Adds PairPhase enum; expands FetchTarget with pairPhase, pairtageUsed, pairtageSecondBlock; increases predMetas capacity (8→12); initializes FullBTBPrediction defaults.
Timed base predictor hook src/cpu/pred/btb/timed_base_pred.hh	Adds virtual refreshPredictionMeta(Addr, const boost::dynamic_bitset<>&, FullBTBPrediction&) hook (default no-op).
FTQ const accessors src/cpu/pred/btb/ftq.hh	Adds const overloads for front, back, and get to return const FetchTarget&.

Sequence Diagram(s)

sequenceDiagram
    participant Fetch as Fetch Unit
    participant DBPU as DecoupledBPUWithBTB
    participant PT as PairTAGE
    participant FTQ as FetchTargetQueue
    participant Hist as History Tracker

    Fetch->>DBPU: request prediction(startPC, history)
    DBPU->>PT: putPCHistory(startPC, history)
    PT->>PT: TAGE lookup -> build first-block FullBTBPrediction
    PT-->>DBPU: first-block FullBTBPrediction (pairtageUsed, pairPhase)
    DBPU->>FTQ: enqueue first FetchTarget (pairPhase)

    alt PairTAGE enabled && phase allowed && second-block candidate
        DBPU->>PT: refreshPredictionMeta(secondStartPC, history, pred)
        PT->>PT: lookupNoSideEffect -> second-block prediction/meta
        PT-->>DBPU: second-block info (pairtageSecondBlock)
        DBPU->>FTQ: conditionally enqueue second FetchTarget
    end

    Note over DBPU,PT: On resolution
    DBPU->>PT: trainFromActualPred(firstFetchTarget, optionalSecondPred)
    PT->>Hist: specUpdateHist / recoverHist / doUpdateHist

Loading

sequenceDiagram
    participant Caller as DecoupledBPU
    participant Predictor as BTB Predictor
    participant NoSE as NoSideEffect Lookup
    participant Meta as FullBTBPrediction

    Caller->>Predictor: refreshPredictionMeta(startAddr, history, pred)
    Predictor->>NoSE: lookupNoSideEffect(...) / processEntriesNoSideEffect(...)
    NoSE->>NoSE: tag match, select entries (no MRU/LRU/stats changes)
    NoSE-->>Meta: populate hit_entries / per-PC preds
    Meta-->>Caller: refreshed prediction metadata snapshot

Loading

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~70 minutes

Possibly related PRs

• cpu-o3: update tage index #627 — BTBTAGE const-correctness and prediction/refresh API changes overlap with TAGE interface and meta-refresh work.
• cpu-o3:Add S3 prediction update AheadBTB #629 — AheadBTB modifications touch no-side-effect lookup and ahead-path behavior introduced here.
• cpu-o3: remove simple functions in fetch #725 — DecoupledBPUWithBTB changes overlap with fetch/FTQ/second-block integration and state bookkeeping.

Suggested reviewers

• jensen-yan
• CJ362ff

"🐇
I hop on tags both even and odd,
Two blocks foretold, each stitch a nod.
I peek without a single side effect,
I learn when true and gently correct.
A rabbit's hop: predict, teach, connect."

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 18.39% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	❓ Inconclusive	The title 'Pair tage 2 taken' is vague and does not clearly convey the main change. It appears to reference PairTAGE but lacks specificity about what 'taken' means in this context, making it unclear to reviewers scanning commit history.	Consider a more descriptive title that clearly indicates the main functionality being added or modified, such as 'Add PairTAGE predictor with two-block prediction' or 'Implement PairTAGE for dual-block branch prediction.'

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch pair-tage-2-taken

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