[llvm] [VPlan] Preserve the branch weights of scalarized selects. (PR #224968)

[Florian Hahn via llvm-commits]

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@@ -1216,6 +1218,23 @@ static VPValue *simplifyLogicalRecipe(VPlan &Plan, VPSingleDefRecipe *Def) {
   return nullptr;
 }
 
+/// Swap the branch weights recorded for \p R, a select whose two selected
+/// operands are being swapped, so that they keep describing the probability of
+/// its condition. Does nothing if there are none, or if they are the marker for
+/// an explicitly unknown profile, which is symmetric.
+static void swapSelectBranchWeights(VPRecipeBase &R, VPlan &Plan) {
+  auto *MD = dyn_cast<VPIRMetadata>(&R);
+  if (!MD)
+    return;
+  SmallVector<uint32_t, 2> Weights;
+  if (!extractBranchWeights(MD->getMetadata(LLVMContext::MD_prof), Weights) ||
+      Weights.size() != 2)
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fhahn wrote:

I don't think we can hit this, turned into an assert, thanks!

https://github.com/llvm/llvm-project/pull/224968