[llvm] [VPlan] Set branch weight metadata on middle term in VPlan (NFC) (PR #143035)

[via llvm-commits]

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@@ -7273,6 +7273,33 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
       BypassBlock, MainResumePhi->getIncomingValueForBlock(BypassBlock));
 }
 
+/// Add branch weight metadata, if the \p Plan's middle block is terminated by a
+/// BranchOnCond recipe.
+static void addBranchWeightToMiddleTerminator(VPlan &Plan, ElementCount VF,
+                                              Loop *OrigLoop) {
+  // 4. Adjust branch weight of the branch in the middle block.
+  Instruction *LatchTerm = OrigLoop->getLoopLatch()->getTerminator();
+  if (!hasBranchWeightMD(*LatchTerm))
+    return;
+
+  VPBasicBlock *MiddleVPBB = Plan.getMiddleBlock();
+  auto *MiddleTerm =
+      dyn_cast_or_null<VPInstruction>(MiddleVPBB->getTerminator());
+  // Only add branch metadata if there is a (conditional) terminator.
+  if (!MiddleTerm)
+    return;
+
+  assert(MiddleTerm->getOpcode() == VPInstruction::BranchOnCond &&
+         "must have a BranchOnCond");
+  // Assume that `Count % VectorTripCount` is equally distributed.
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ayalz wrote:

`Count` is aka original `TripCount`, and `VectorTripCount` should be `VectorStep`: the branch terminating middle block has 1:VectorStep chance of skipping scalar epilog, i.e., when Count or original TripCount is divisible by VectorStep, assuming modulo remainder `Count % VectorStep` is uniformly distributed.

https://github.com/llvm/llvm-project/pull/143035