PairTAGE PPA-Aware Model Implementation by Yakkhini · Pull Request #1080 · OpenXiangShan/GEM5

Yakkhini · 2026-08-26T06:53:23Z

Summary by CodeRabbit

New Features
- Enabled PairTAGE branch prediction enhancements, including second-block support, in applicable processor configurations.
- Improved fetch behavior for simultaneous multithreading (SMT) configurations.
- Reduced the memory footprint of branch prediction tables while preserving the updated prediction capabilities.
Bug Fixes
- Corrected two-fetch mode handling so fetch behavior now automatically reflects whether SMT is enabled.

coderabbitai · 2026-08-26T06:54:31Z

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src/cpu/pred/BranchPredictor.py

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📝 Walkthrough

Walkthrough

The KMHV3 configurations now set SMT-dependent two-fetch behavior and enable PairTAGE second-block support. PairTAGE now uses four 1024-entry tables instead of four 8192-entry tables.

Changes

KMHV3 configuration and PairTAGE updates

Layer / File(s)	Summary
SMT-dependent two-fetch mode `configs/example/idealkmhv3.py`, `configs/example/kmhv3.py`	Two-fetch mode is enabled when SMT is disabled.
PairTAGE configuration and table sizing `configs/example/idealkmhv3.py`, `configs/example/kmhv3.py`, `src/cpu/pred/BranchPredictor.py`	The `DecoupledBPUWithBTB` configurations enable PairTAGE and second-block support. `PairTAGE.tableSizes` changes to four 1024-entry tables.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: happy-lx, jensen-yan

Merge Risk: ⚪ Minimal · up to 94493

This PR changes PairTAGE simulator defaults and KMHV3 prediction configuration without introducing an actionable merge-blocking risk. It is merge-ready after normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 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.
Title check	✅ Passed	The title clearly identifies the main change: implementing and optimizing the PairTAGE model. It matches the PairTAGE configuration updates and reduced table sizes in the changeset.

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Commit unit tests in branch pairtage-reduce-tablesize

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github-actions · 2026-08-26T12:33:33Z

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (`xs-dev`)	`2.2146`	-
This PR	`2.2362`	📈 `+0.0216` (`+0.98%`)

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github-actions · 2026-08-26T13:55:37Z

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (`xs-dev`)	`2.2146`	-
This PR	`2.2362`	📈 `+0.0216` (`+0.98%`)

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github-actions · 2026-08-27T02:56:48Z

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Base (`xs-dev`)	`2.2146`	-
This PR	`2.2441`	📈 `+0.0295` (`+1.33%`)

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github-actions · 2026-08-31T09:23:28Z

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Base (`xs-dev`)	`2.2146`	-
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github-actions · 2026-09-01T07:23:04Z

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github-actions · 2026-09-02T08:00:30Z

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github-actions · 2026-09-02T08:33:44Z

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github-actions · 2026-09-03T10:36:43Z

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github-actions · 2026-09-04T08:53:15Z

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Base (`xs-dev`)	`2.2146`	-
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Change-Id: I2034bfe14355358e5f476a8fba94b1d804a97052

Change-Id: Ib62fc2af72ef4e55f4613e4111d130ccda48aeae

FYI, the performance decrease on SPEC06Int 0.3c is acceptable (0.95 -> 0.90) Change-Id: I8825ef0cc7c034988924982fce6a11ff31c868ad

Change-Id: I91a6bada92f82747140a3727986b41b0de68fc85

Change-Id: Ibdc0f78c6e943866b98346ea7a37f9d35579ce71

This is GEM5 specific support since rtl impl not store predBTBEntries in FTQ Entry. Change-Id: I273180825c3603d49c8c8a60dc386910b575e277

Change-Id: Ia286d58ef4ce6ac7bbd9dac9eb37395ade47c96d

Change-Id: I73e570db6fd3adb7ed5fac287c5dd19ff4d12b24

github-actions · 2026-09-14T08:04:25Z

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Branch	IPC	Change
Base (`xs-dev`)	`2.1869`	-
This PR	`2.2240`	📈 `+0.0371` (`+1.70%`)

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github-actions · 2026-09-14T08:51:32Z

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Base (`xs-dev`)	`2.1869`	-
This PR	`2.2240`	📈 `+0.0371` (`+1.70%`)

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Change-Id: Iacc5f1c74c6bdd13a546278119bd441ed3ab5239

github-actions · 2026-09-15T04:06:17Z

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Change-Id: Ie9a9ce4a43bb66ac77fa566da339542022970477

Change-Id: I5e0e2320537d008615792637ba4a738afeb4dc93

github-actions · 2026-09-15T13:35:34Z

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Yakkhini requested review from happy-lx and jueshiwenli as code owners August 26, 2026 06:53

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 50534c3 to a07fbd4 Compare August 26, 2026 13:07

Yakkhini changed the base branch from xs-dev to microtage-2-taken-fix August 26, 2026 13:27

jueshiwenli previously approved these changes Aug 26, 2026

View reviewed changes

Yakkhini dismissed jueshiwenli’s stale review via 01b554c August 27, 2026 02:45

Yakkhini added regression Run the full gem5 regression suite before merge perf and removed regression Run the full gem5 regression suite before merge labels Aug 27, 2026

Base automatically changed from microtage-2-taken-fix to xs-dev August 31, 2026 08:41

Yakkhini changed the title ~~PairTAGE Size Reduce & Algorithm Fine Tune~~ PairTAGE Practical Model Implementation Aug 31, 2026

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 01b554c to 46d55db Compare August 31, 2026 09:14

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 3b4e58a to 60c0aea Compare September 2, 2026 07:50

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 60c0aea to 572c326 Compare September 2, 2026 08:15

Yakkhini changed the title ~~PairTAGE Practical Model Implementation~~ PairTAGE PPA-Aware Model Implementation Sep 3, 2026

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 27c0d08 to 3c3f957 Compare September 10, 2026 08:35

Yakkhini requested a review from jensen-yan as a code owner September 10, 2026 08:35

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 3c3f957 to 033be4c Compare September 14, 2026 07:46

Yakkhini added 2 commits September 14, 2026 15:47

cpu-o3: enable pairtage & 2 taken

115ae0f

Change-Id: I2034bfe14355358e5f476a8fba94b1d804a97052

cpu-o3: enable 2fetch both on kmhv3 and idealkmhv3 config

9031244

Change-Id: Ib62fc2af72ef4e55f4613e4111d130ccda48aeae

Yakkhini added 6 commits September 14, 2026 15:47

cpu-o3: reduce pairtage tablesize 8k * 4 -> 4k * 4

72946e5

FYI, the performance decrease on SPEC06Int 0.3c is acceptable (0.95 -> 0.90) Change-Id: I8825ef0cc7c034988924982fce6a11ff31c868ad

cpu-o3: set-associativity configurable ubtb

21ed5c7

Change-Id: I91a6bada92f82747140a3727986b41b0de68fc85

cpu-o3: 64 set * 4 way ubtb as 2 taken second block checker

1fe5819

Change-Id: Ibdc0f78c6e943866b98346ea7a37f9d35579ce71

cpu-o3: merge mbtb entries into second block FTQ entry

70ccfad

This is GEM5 specific support since rtl impl not store predBTBEntries in FTQ Entry. Change-Id: I273180825c3603d49c8c8a60dc386910b575e277

cpu-o3: multi-slot ubtb

093b032

Change-Id: Ia286d58ef4ce6ac7bbd9dac9eb37395ade47c96d

cpu-o3: align block uBTB with xs-dev training API

6dd8181

Change-Id: I73e570db6fd3adb7ed5fac287c5dd19ff4d12b24

Yakkhini force-pushed the pairtage-reduce-tablesize branch from 033be4c to 6dd8181 Compare September 14, 2026 08:36

cpu-o3: ubtb mbtb consistency implementation

e62690b

Change-Id: Iacc5f1c74c6bdd13a546278119bd441ed3ab5239

Yakkhini added 2 commits September 15, 2026 17:14

cpu-o3: Add H2 MainTAGE lookup for second-block checker

1bb538f

Change-Id: Ie9a9ce4a43bb66ac77fa566da339542022970477

cpu-o3: Fix second-block MainTAGE training context

be83fa0

Change-Id: I5e0e2320537d008615792637ba4a738afeb4dc93

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