[LV] Generalize conditions for sinking instrs for first order recurrences.

If the recurrence PHI node has a single user, we can sink any
instruction without side effects, given that all users are dominated by
the instruction computing the incoming value of the next iteration
('Previous'). We can sink instructions that may cause traps, because
that only causes the trap to occur later, but not on any new paths.

With the relaxed check, we also have to make sure that we do not have a
direct cycle (meaning PHI user == 'Previous), which indicates a
reduction relation, which potentially gets missed by
ReductionDescriptor.

As follow-ups, we can also sink stores, iff they do not alias with
other instructions we move them across and we could also support sinking
chains of instructions and multiple users of the PHI.

Fixes PR43398.

Reviewers: hsaito, dcaballe, Ayal, rengolin

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D69228

1 files changed, 26 insertions, 14 deletions

diff --git a/llvm/lib/Analysis/IVDescriptors.cpp b/llvm/lib/Analysis/IVDescriptors.cpp
index 6fb600114bc..bdebd71d7f6 100644
--- a/llvm/lib/Analysis/IVDescriptors.cpp
+++ b/llvm/lib/Analysis/IVDescriptors.cpp
@@ -699,25 +699,37 @@ bool RecurrenceDescriptor::isFirstOrderRecurrence(
   // Ensure every user of the phi node is dominated by the previous value.
   // The dominance requirement ensures the loop vectorizer will not need to
   // vectorize the initial value prior to the first iteration of the loop.
-  // TODO: Consider extending this sinking to handle other kinds of instructions
-  // and expressions, beyond sinking a single cast past Previous.
+  // TODO: Consider extending this sinking to handle memory instructions and
+  // phis with multiple users.
+
+  // Returns true, if all users of I are dominated by DominatedBy.
+  auto allUsesDominatedBy = [DT](Instruction *I, Instruction *DominatedBy) {
+    return all_of(I->uses(), [DT, DominatedBy](Use &U) {
+      return DT->dominates(DominatedBy, U);
+    });
+  };
+
   if (Phi->hasOneUse()) {
-    auto *I = Phi->user_back();
-    if (I->isCast() && (I->getParent() == Phi->getParent()) && I->hasOneUse() &&
-        DT->dominates(Previous, I->user_back())) {
-      if (!DT->dominates(Previous, I)) // Otherwise we're good w/o sinking.
-        SinkAfter[I] = Previous;
+    Instruction *I = Phi->user_back();
+
+    // If the user of the PHI is also the incoming value, we potentially have a
+    // reduction and which cannot be handled by sinking.
+    if (Previous == I)
+      return false;
+
+    if (DT->dominates(Previous, I)) // We already are good w/o sinking.
       return true;
-    }
-  }
 
-  for (User *U : Phi->users())
-    if (auto *I = dyn_cast<Instruction>(U)) {
-      if (!DT->dominates(Previous, I))
-        return false;
+    // We can sink any instruction without side effects, as long as all users
+    // are dominated by the instruction we are sinking after.
+    if (I->getParent() == Phi->getParent() && !I->mayHaveSideEffects() &&
+        allUsesDominatedBy(I, Previous)) {
+      SinkAfter[I] = Previous;
+      return true;
     }
+  }
 
-  return true;
+  return allUsesDominatedBy(Phi, Previous);
 }
 
 /// This function returns the identity element (or neutral element) for