[llvm] [VPlan] Add initial CFG simplification, removing BranchOnCond true. (PR #106748)

[Florian Hahn via llvm-commits]

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@@ -3527,12 +3527,20 @@ class VPlan {
 
   /// Returns the 'middle' block of the plan, that is the block that selects
   /// whether to execute the scalar tail loop or the exit block from the loop
-  /// latch.
-  const VPBasicBlock *getMiddleBlock() const {
-    return cast<VPBasicBlock>(getScalarPreheader()->getPredecessors().front());
-  }
+  /// latch. If the scalar tail loop or exit block are known to always execute,
+  /// the middle block may branch directly to the block.
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fhahn wrote:

Updated the comment.

> Note that if the middle block branches unconditionally to exit block (or scalar preheader block), the two blocks may subsequently be merged, causing RegionSucc to have no successors (or be the scalar preheader itself).

Yep, for now, we don't merge VPIRBBs into other blocks.

https://github.com/llvm/llvm-project/pull/106748