[llvm] [VPlan] Account for early-exit dispatch blocks when updating LI. (PR #185618)

[David Sherwood via llvm-commits]

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@@ -390,22 +390,40 @@ BasicBlock *VPBasicBlock::createEmptyBasicBlock(VPTransformState &State) {
   return NewBB;
 }
 
+/// Determine the parent loop for \p VPBB by checking if it is an exit block
+/// or if all of its successors lead to exit blocks (following single-successor
+/// chains). Returns the innermost loop containing any of the exits if all
+/// paths lead to exits, or \p DefaultLoop otherwise.
+static Loop *getParentLoopForBlock(VPBlockBase *VPBB, VPlan &Plan, LoopInfo &LI,
+                                   Loop *DefaultLoop) {
+  Loop *ParentLoop = nullptr;
+  for (VPBlockBase *Succ : VPBB->getSuccessors()) {
+    while (Succ && !Plan.isExitBlock(Succ))
+      Succ = Succ->getSingleSuccessor();
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david-arm wrote:

It would be good to explain the logic behind this. Why is it that a block without a single successor (could be a block with > 1 successor or a block with no successors) can use the `DefaultLoop`?

It may be correct, but it relies upon the reader having a very deep understanding of the expected VPlan graph at this phase in the set of transformations. And the VPlan CFG does change quite frequently.

https://github.com/llvm/llvm-project/pull/185618