[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[via llvm-commits]

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@@ -3254,6 +3255,14 @@ class VPBasicBlock : public VPBlockBase {
   /// Returns true if the block is exiting it's parent region.
   bool isExiting() const;
 
+  /// Checks if the block is a loop header block in the plain CFG; that is, it
+  /// has exactly 2 predecessors (preheader and latch), where the block
+  /// dominates the latch and the preheader dominates the block. If it is a
+  /// header block, returns a pair with the corresponding preheader and latch
+  /// blocks. Otherwise return std::nullopt.
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ayalz wrote:

Sounds more like getPreheaderAndLatch()? Return value of "isHeader()" is intuitively boolean.

https://github.com/llvm/llvm-project/pull/129402