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Tage way tuning align #803

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70d0ca3
cpu-o3: change tage T2 way to 4
Mar 19, 2026
f9e128f
cpu-o3: tage T3 table change to 4
Mar 19, 2026
8bdbac9
cpu-o3: tage T1 table 4way
Mar 20, 2026
c113f1c
cpu-o3: tage 8way 8k
Mar 20, 2026
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4 changes: 2 additions & 2 deletions src/cpu/pred/BranchPredictor.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1044,15 +1044,15 @@ class BTBTAGE(TimedBaseBTBPredictor):
enableSC = Param.Bool(False, "Enable SC or not") # TODO: BTBTAGE doesn't support SC
updateOnRead = Param.Bool(True, "Enable update on read, no need to save tage meta in FTQ")
numPredictors = Param.Unsigned(8, "Number of TAGE predictors")
tableSizes = VectorParam.Unsigned([2048, 2048, 2048, 2048, 2048, 2048, 2048,2048],"the TAGE T0~Tn length")
tableSizes = VectorParam.Unsigned([8192] * 8, "the TAGE T0~Tn length")
TTagBitSizes = VectorParam.Unsigned([13] * 8, "the T0~Tn entry's tag bit size")
TTagPcShifts = VectorParam.Unsigned([1] * 8, "when the T0~Tn entry's tag generating, PC right shift")
blockSize = 32 # tage index function uses 32B aligned block address

histLengths = VectorParam.Unsigned([4, 9, 17, 29, 56, 109, 211,397],"the BTB TAGE T0~Tn history length")
maxHistLen = Param.Unsigned(970, "The length of history passed from DBP")
numTablesToAlloc = Param.Unsigned(1,"The number of table to allocated each time")
numWays = VectorParam.Unsigned([2] * 8,"the T0~Tn number of ways per set")
numWays = VectorParam.Unsigned([8] * 8, "the T0~Tn number of ways per set")
Comment on lines +1047 to +1055
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@coderabbitai coderabbitai bot Mar 20, 2026

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⚠️ Potential issue | 🟠 Major

🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Verify that BTBTAGE capacity is derived as tableSizes * numWays,
# and locate the default consumers of BTBTAGE().
# Expected:
# - btb_tage.cc shows outer resize by tableSizes[i] and inner resize by way count.
# - BranchPredictor.py shows BTBTAGEUpperBound inheriting BTBTAGE and
#   DecoupledBPUWithBTB constructing BTBTAGE() directly.

rg -n -C2 'tageTable\[i\]\.resize\(tableSizes\[i\]\)|tageTable\[i\]\[j\]\.resize\(ways\)|getNumWays\(|class BTBTAGEUpperBound|Param\.BTBTAGE\(BTBTAGE\(\)' \
  src/cpu/pred/btb/btb_tage.cc src/cpu/pred/BranchPredictor.py

Repository: OpenXiangShan/GEM5

Length of output: 3100

Verify this doesn't unintentionally increase BTBTAGE capacity by 16x.

Line 1047 sets the number of indices (tableSizes: [8192] * 8), and line 1055 sets the associativity (numWays: [8] * 8). In src/cpu/pred/btb/btb_tage.cc:116-119, these are multiplied during allocation: each table now allocates 8192 × 8 = 65,536 entries instead of the previous 2048 × 2 = 4,096 entries—a 16x increase per table.

This base class change propagates to all default BTBTAGE() instantiations, including DecoupledBPUWithBTB (line 1202) and BTBTAGEUpperBound (line 1063). If the intended target is an 8-way 8K-entry configuration, reduce tableSizes to [1024] * 8. Otherwise, isolate this geometry into a dedicated subclass instead of modifying the base default.

🧰 Tools
🪛 Ruff (0.15.6)

[error] 1047-1047: VectorParam may be undefined, or defined from star imports

(F405)

[error] 1048-1048: VectorParam may be undefined, or defined from star imports

(F405)

[error] 1049-1049: VectorParam may be undefined, or defined from star imports

(F405)

[error] 1052-1052: VectorParam may be undefined, or defined from star imports

(F405)

[error] 1053-1053: Param may be undefined, or defined from star imports

(F405)

[error] 1054-1054: Param may be undefined, or defined from star imports

(F405)

[error] 1055-1055: VectorParam may be undefined, or defined from star imports

(F405)

🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@src/cpu/pred/BranchPredictor.py` around lines 1047 - 1055, The change to the
base params (tableSizes and numWays in BranchPredictor.py) unintentionally
increases BTBTAGE capacity; either restore conservative defaults on the base
class or isolate the larger geometry into a dedicated subclass: if your intent
is an 8-way ×8K-entry BTBTAGE, set tableSizes = VectorParam.Unsigned([1024] * 8,
...) while keeping numWays = VectorParam.Unsigned([8] * 8, ...); otherwise
revert tableSizes and numWays in the base BranchPredictor default to the
previous smaller values (or create a new BTBTAGE-specific subclass used by
BTBTAGE(), DecoupledBPUWithBTB and BTBTAGEUpperBound that overrides
tableSizes/numWays) so the global default does not inflate all BTB instances.

maxBranchPositions = Param.Unsigned(32, "Maximum branch positions per 64-byte block")
useAltOnNaSize = Param.Unsigned(128, "Size of the useAltOnNa table")
useAltOnNaWidth = Param.Unsigned(7, "Width of the useAltOnNa table")
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