Enable -cce-vf-aa-between-iters by default for A5 CCEC builds

[yanghaoran29]

Summary

• Add -mllvm -cce-vf-aa-between-iters=true to the default CCEC flags for A5 hardware.
• This option enables alias analysis across loop iterations. With it on, AA is stricter and directly affects BiSheng VF fusion scale and correctness.
• Context: without this option, BiSheng fusion can misbehave in some cases; fusion still needs to run under the stricter AA setting. BiSheng plans to make this always-on later; simpler enables it by default on A5 CCEC now to align with that direction.
• a5sim is unchanged: KernelCompiler leaves ccec unset and uses Gxx15Toolchain, so this LLVM option does not apply there.
• Board check: DeepSeek V4 qkv_proj_rope.py (decode, -p a5) fails with VF fusion alone (AICore / scheduler stall); adding this option makes it PASS. Turning fusion off also PASSes.

Test plan

• Confirm CCECToolchain(platform="a5").get_compile_flags() includes -mllvm / -cce-vf-aa-between-iters=true
• Confirm CCECToolchain(platform="a2a3") and a5sim kernel path do not add this option
• A5 board: qkv_proj_rope.py -p a5 --mode decode PASSes

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📝 Walkthrough

Walkthrough

Changes

A5 compiler flag configuration

Layer / File(s)	Summary
Compile flag assembly and A5 option simpler_setup/toolchain.py	CCECToolchain.get_compile_flags() builds a mutable flag list and adds the vf-aa-between-iters LLVM option for a5 and a5sim platforms.

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

Poem

I’m a rabbit with flags in my hat,
Adding A5’s LLVM tweak just like that.
Loops now peer through each iteration,
With alias analysis in formation.
Hop, compile, and away we go!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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 matches the main change: enabling the VF AA flag by default for A5 CCEC builds.
Description check	✅ Passed	The description is directly related and accurately explains the A5 CCEC flag change and its motivation.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@simpler_setup/toolchain.py`:
- Around line 192-197: Update the platform check in the toolchain flag builder
so it targets only real A5 hardware, removing the ineffective a5sim branch from
CCECToolchain. If a5sim support is required, instead add the option through the
host G++ simulation flag path used when self.ccec is unset; otherwise document
that the setting applies only to A5 hardware.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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📥 Commits

Reviewing files that changed from the base of the PR and between 717d703 and 2ac1b81.

📒 Files selected for processing (1)

• simpler_setup/toolchain.py

[yanghaoran29]

Addressed CodeRabbit feedback: narrowed the flag to A5 hardware only (platform == "a5"). a5sim uses Gxx15Toolchain and does not go through CCECToolchain.