[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;
+  }
+
+  auto NeedsHoisting = [HoistPoint, &VPDT,
+                        &Seen](VPValue *HoistCandidateV) -> VPRecipeBase * {
+    VPRecipeBase *HoistCandidate = HoistCandidateV->getDefiningRecipe();
+    if (!HoistCandidate)
+      return nullptr;
+    assert((!HoistCandidate->getParent()->getParent() ||
+            HoistCandidate->getParent()->getParent() ==
+                HoistCandidate->getParent()->getEnclosingLoopRegion()) &&
+           "CFG in VPlan should still be flattened, without replicate regions");
+    // Hoist candidate has already beend visited, no need to hoist.
+    if (!Seen.insert(HoistCandidate).second)
+      return nullptr;
+
+    // Candidate is outside loop region or a header phi, dominates FOR users w/o
+    // hoisting.
+    if (!HoistCandidate->getParent()->getEnclosingLoopRegion() ||
+        isa<VPHeaderPHIRecipe>(HoistCandidate))
+      return nullptr;
+
+    // If we reached a recipe that dominates HoistPoint, we don't need to
+    // hoist the recipe.
+    if (VPDT.properlyDominates(HoistCandidate, HoistPoint))
+      return nullptr;
+    return HoistCandidate;
+  };
+  auto CanHoist = [&](VPRecipeBase *HoistCandidate) {
+    // Avoid hoisting candidates with side-effects, as we do not yet analyze
+    // associated dependencies.
+    return !HoistCandidate->mayHaveSideEffects();
+  };
+
+  // Recursively try to hoist Previous and its operands before all users of FOR.
+  if (NeedsHoisting(Previous->getVPSingleValue()))
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ayalz wrote:

nit: can early-exit if Previous need not be hoisted.

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