[llvm] [VPlan] Add additional guiding principles to docs. (PR #85688)

[Nikolay Panchenko via llvm-commits]

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@@ -88,6 +88,33 @@ The design of VPlan follows several high-level guidelines:
    detection and formation involves searching for and optimizing instruction
    patterns.
 
+8. The adoption of VPlan components should be done as a gradual, always-on
+   refactoring to retain quality and integrate continuously.
+
+9. Gradually refactor into multiple VPlan-to-VPlan transforms to reduce
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npanchen wrote:

Can you please elaborate "reduce complexity" part ?  Is it just complexity in terms of building and executing VPlan or something else ?
Essentially, I'm missing what is **must** and what's **nice-to** in VPlan. At least my understanding that all transformations in `VPlanTransforms`, except for `VPInstructionsToVPRecipes`, are optional.


https://github.com/llvm/llvm-project/pull/85688