[codex] Refactor BTB branch and prediction payloads

[jensen-yan]

Summary

• separate branch slot data from BTB entry storage metadata
• extract BTB prediction-result and final-prediction selection helpers
• move final prediction attribution into explicit metadata
• group fetch prediction payload in a FetchPredictionSnapshot

Motivation

This is a readability/refactor patch to reduce coupling in the BTB BPU top-level data flow. The intent is to make branch-slot, BTB-entry, prediction-result, and FTQ/update payload ownership easier to reason about and easier to unit test.

Scope and impact

This is intended to be behavior-preserving. There is no intended performance impact; the branch is pushed as a slash-free *-align branch so the 0.3c perf workflow can check that.

Validation

• scons build/RISCV/cpu/pred/btb/test/final_prediction_selector.test.debug build/RISCV/cpu/pred/btb/test/prediction_result.test.debug build/RISCV/cpu/pred/btb/test/history_manager.test.debug build/RISCV/cpu/pred/btb/test/uras.test.debug build/RISCV/cpu/pred/btb/test/btb.test.debug build/RISCV/cpu/pred/btb/test/abtb.test.debug build/RISCV/cpu/pred/btb/test/tage.test.debug build/RISCV/cpu/pred/btb/test/mgsc.test.debug --unit-test -j100
• ./build/RISCV/cpu/pred/btb/test/final_prediction_selector.test.debug
• ./build/RISCV/cpu/pred/btb/test/prediction_result.test.debug
• ./build/RISCV/cpu/pred/btb/test/history_manager.test.debug
• ./build/RISCV/cpu/pred/btb/test/uras.test.debug
• ./build/RISCV/cpu/pred/btb/test/btb.test.debug
• ./build/RISCV/cpu/pred/btb/test/abtb.test.debug
• ./build/RISCV/cpu/pred/btb/test/tage.test.debug
• ./build/RISCV/cpu/pred/btb/test/mgsc.test.debug
• scons build/RISCV/gem5.opt --gold-linker -j100

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• Commit unit tests in branch bpu-branch-entry-refactor-align

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[github-actions]

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (xs-dev)	2.2906	-
This PR	2.2875	📉 -0.0030 (-0.13%)

✅ Difftest smoke test passed!

[github-actions]

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (xs-dev)	2.2906	-
This PR	2.2875	📉 -0.0030 (-0.13%)

✅ Difftest smoke test passed!

[github-actions]

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (xs-dev)	2.2906	-
This PR	2.2875	📉 -0.0030 (-0.13%)

✅ Difftest smoke test passed!

[XiangShanRobot]

[Generated by GEM5 Performance Robot]
commit: 88d4059
workflow: gem5 Align BTB Performance Test(0.3c)

Align BTB Performance

Overall Score

	PR	Master	Diff(%)
Score	18.91	18.90	+0.04 🟢

[Generated by GEM5 Performance Robot]
commit: 88d4059
workflow: gem5 Align BTB Performance Test(0.3c)

Align BTB Performance

Overall Score

	PR	Previous Commit	Diff(%)
Score	18.91	18.91	0.00