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

[Florian Hahn via llvm-commits]

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@@ -3529,12 +3529,18 @@ 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.
   VPBasicBlock *getMiddleBlock() {
-    return cast<VPBasicBlock>(getScalarPreheader()->getPredecessors().front());
+    if (!getScalarPreheader()->getPredecessors().empty())
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fhahn wrote:

Done, thanks.

The middle block selects between the scalar preheader and the exit block from the latch, which is the same with and w/o early exits.

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