Enable op fusion by default on A5

[Zhendong404]

Summary

• make --enable-op-fusion default to enabled on A5 and disabled on A3
• reject --enable-op-fusion=true on A3 instead of silently ignoring it
• keep --enable-op-fusion=false as the explicit opt-out on A5 and update docs/tests

Validation

• CCACHE_DISABLE=1 ninja -C build ptoas
• build/tools/ptoas/ptoas --pto-arch=a5 --pto-level=level2 --emit-pto-ir test/lit/tile_fusion/op_fusion_nonfused_control.pto -o -
• build/tools/ptoas/ptoas --pto-arch=a5 --pto-level=level2 --enable-op-fusion=false --emit-pto-ir test/lit/tile_fusion/op_fusion_nonfused_control.pto -o -
• build/tools/ptoas/ptoas --pto-arch=a5 --pto-level=level1 --enable-op-fusion=true test/lit/tile_fusion/op_fusion_cli_flags.pto -o /dev/null
• build/tools/ptoas/ptoas --pto-arch=a3 --enable-op-fusion=true test/lit/tile_fusion/op_fusion_cli_flags.pto -o /dev/null

[gemini-code-assist]

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[reedhecre]

Codex Review

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• PR: Enable op fusion by default on A5 #860 Enable op fusion by default on A5
• Author: Zhendong404
• Base/Head: main / enable-fusion-default
• Head SHA: c60bf0892526
• Trigger: 检测到新的 open PR
• Generated At: 2026-06-24T06:28:16Z
• Status: completed

Summary

PR #860 introduces a P1 regression: the new default-on A5 fusion path is only partially enabled for VPTO, so fused IR can be produced without the required post-lowering lifecycle passes.

Findings

1. P1 Default A5 fusion does not enable the VPTO post-lowering fusion lifecycle tools/ptoas/ptoas.cpp:1488

The PR changes the frontend gate to enable fusion by default on A5 via opFusionEnabled in compilePTOASModule, but the VPTO post-lowering lifecycle still keys off the raw CLI option object: enableA5VPTOPostLoweringFusionLifecycle = enableOpFusion && moduleArchAttr && moduleArchAttr.getValue() == "a5". With the new tri-state flag, the default A5 path leaves enableOpFusion unset, so FusionPlan/OpScheduling/PTOFusionRegionGen now run by default, but PTOLowLevelLoopFusion, predicate/load-store elision, and flattening are skipped. That contradicts the documented contract in this PR and leaves pto.fusion_region in the VPTO path without the cleanup pipeline that previously made fused IR consumable, which is a high-risk correctness/runtime regression for the new default mode.