[llvm] [VPlan] Document current status. (PR #85689)

[via llvm-commits]

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@@ -19,6 +19,49 @@ candidates. It serves for both optimizing candidates including estimating their
 cost reliably, and for performing their final translation into IR. This
 facilitates dealing with multiple vectorization candidates.
 
+Current Status
+==============
+VPlan is currently used to drive code-generation in LoopVectorize. VPlans are
+constructed after all cost-based and most legality-related decisions have been
+taken. As def-use chains between recipes are now fully modeled in VPlan,
+VPlan-based analyses and transformations are used to simplify and modularize
+the vectorization process [10]_. Those include transformations to
+
+1. Legalize the initial VPlan, e.g. by introducing specializedrecipes for
+   reductions and interleave groups.
+
+2. Optimize the legalized VPlan, e.g. by removing redundant recipes or
+   introducing active-lane-masks.
+   .
+3. Apply unroll- and vectorization-factor specific optimizations, e.g. removing
+   the branch to exit the vector loop based on VF & UF.
+
+Refer to :numref:`fig-vplan-transform-pipeline` for a overview of the current
----------------
ayalz wrote:

```suggestion
Refer to :numref:`fig-vplan-transform-pipeline` for an overview of the current
```


https://github.com/llvm/llvm-project/pull/85689