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

[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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ayalz wrote:

Scalar preheader could also/only be reached from runtime guards that bypass the vector region and its middle block? Would it be better to retrieve the middle block from its predecessor vector loop region, than from its successors scalar preheader and/or exit blocks? Is the term "middle block" well defined, when early exits are involved which currently splits it into two blocks, rather than having a single block with three successors?

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