[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[via llvm-commits]

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@@ -7538,6 +7498,18 @@ LoopVectorizationPlanner::executePlan(
 
   ILV.printDebugTracesAtEnd();
 
+  // Adjust branch weight of the branch in the middle block.
+  auto *MiddleTerm =
+      cast<BranchInst>(State.CFG.VPBB2IRBB[ExitVPBB]->getTerminator());
+  if (MiddleTerm->isConditional() &&
+      hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator())) {
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ayalz wrote:

Thoughts independent of this patch:
- record an indicator that the branch has weights.
- assign new branch weights regardless of existing ones, as the new weights are independent of the old ones.
- assign more accurate weights if trip-count and VFxUF are known to have a (greatest) common divider.

https://github.com/llvm/llvm-project/pull/92651