[llvm] [VPlan] Try to hoist Previous (and operands), if sinking fails for FORs. (PR #108945)

[via llvm-commits]

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@@ -772,6 +772,97 @@ sinkRecurrenceUsersAfterPrevious(VPFirstOrderRecurrencePHIRecipe *FOR,
   return true;
 }
 
+/// Try to hoist \p Previous and its operands before all users of \p FOR.
+static bool hoistPreviousBeforeFORUsers(VPFirstOrderRecurrencePHIRecipe *FOR,
+                                        VPRecipeBase *Previous,
+                                        VPDominatorTree &VPDT) {
+  using namespace llvm::VPlanPatternMatch;
+  if (Previous->mayHaveSideEffects() || Previous->mayReadFromMemory())
+    return false;
+
+  // Collect recipes that need hoisting.
+  SmallVector<VPRecipeBase *> HoistCandidates;
+  SmallPtrSet<VPRecipeBase *, 8> Seen;
+  VPRecipeBase *HoistPoint = nullptr;
+  // Find the closest hoist point by looking at all users of FOR and selecting
+  // the recipe dominating all other users.
+  for (VPUser *U : FOR->users()) {
+    auto *R = dyn_cast<VPRecipeBase>(U);
+    if (!R)
+      continue;
+    if (!HoistPoint || VPDT.properlyDominates(R, HoistPoint))
+      HoistPoint = R;
+  }
+
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ayalz wrote:

nit: may be worth asserting HoistPoint (is non-null and) dominates all FOR->users().

https://github.com/llvm/llvm-project/pull/108945