Do single pass recursive propagation

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -4961,55 +4961,68 @@ Instruction *InstCombinerImpl::visitLandingPadInst(LandingPadInst &LI) {
 Value *
 InstCombinerImpl::pushFreezeToPreventPoisonFromPropagating(FreezeInst &OrigFI) {
   // Try to push freeze through instructions that propagate but don't produce
-  // poison as far as possible. If an operand of freeze is one-use and does
-  // not produce poison then push the freeze through to the operands that are
-  // not guaranteed non-poison. The actual transform is as follows.
+  // poison as far as possible. If an operand of freeze does not produce poison
+  // then push the freeze through to the operands that are not guaranteed
+  // non-poison. The actual transform is as follows.
   //   Op1 = ...                        ; Op1 can be posion
-  //   Op0 = Inst(Op1, NonPoisonOps...) ; Op0 has only one use
+  //   Op0 = Inst(Op1, NonPoisonOps...)
   //   ... = Freeze(Op0)
   // =>
   //   Op1 = ...
   //   Op1.fr = Freeze(Op1)
   //   ... = Inst(Op1.fr, NonPoisonOps...)
-  auto *OrigOp = OrigFI.getOperand(0);
-  auto *OrigOpInst = dyn_cast<Instruction>(OrigOp);
 
-  // While we could change the other users of OrigOp to use freeze(OrigOp), that
-  // potentially reduces their optimization potential, so let's only do this iff
-  // the OrigOp is only used by the freeze.
-  if (!OrigOpInst || !OrigOpInst->hasOneUse() || isa<PHINode>(OrigOp))
-    return nullptr;
+  auto CanPushFreeze = [](Value *V) {
+    if (!isa<Instruction>(V) || isa<PHINode>(V))
+      return false;
 
-  // We can't push the freeze through an instruction which can itself create
-  // poison.  If the only source of new poison is flags, we can simply
-  // strip them (since we know the only use is the freeze and nothing can
-  // benefit from them.)
-  if (canCreateUndefOrPoison(cast<Operator>(OrigOp),
-                             /*ConsiderFlagsAndMetadata*/ false))
-    return nullptr;
+    // We can't push the freeze through an instruction which can itself create
+    // poison.  If the only source of new poison is flags, we can simply
+    // strip them (since we know the only use is the freeze and nothing can
+    // benefit from them.)
+    return !canCreateUndefOrPoison(cast<Operator>(V),
+                                   /*ConsiderFlagsAndMetadata*/ false);
+  };
+
+  // Pushing freezes up long instruction chains can be expensive. Instead,
+  // we directly push the freeze all the way to the leaves. However, we leave
+  // deduplication of freezes on the same value for freezeOtherUses().
+  Use *OrigUse = &OrigFI.getOperandUse(0);
+  SmallPtrSet<Instruction *, 8> Visited;
+  SmallVector<Use *, 8> Worklist;
+  Worklist.push_back(OrigUse);
+  while (!Worklist.empty()) {
+    auto *U = Worklist.pop_back_val();
+    Value *V = U->get();
+    if (!CanPushFreeze(V)) {
+      // If we can't push through the original instruction, abort the transform.
+      if (U == OrigUse)
+        return nullptr;
 
-  // If operand is guaranteed not to be poison, there is no need to add freeze
-  // to the operand. So we first find the operand that is not guaranteed to be
-  // poison.
-  SmallSetVector<Value *, 4> MaybePoisonOperands;
-  for (Value *V : OrigOpInst->operands()) {
-    if (isa<MetadataAsValue>(V) || isGuaranteedNotToBeUndefOrPoison(V))
+      auto *UserI = cast<Instruction>(U->getUser());
+      Builder.SetInsertPoint(UserI);
+      Value *Frozen = Builder.CreateFreeze(V, V->getName() + ".fr");
+      U->set(Frozen);
       continue;
-    MaybePoisonOperands.insert(V);
-  }
+    }
 
-  OrigOpInst->dropPoisonGeneratingAnnotations();
+    auto *I = cast<Instruction>(V);
+    if (!Visited.insert(I).second)
+      continue;
 
-  // If all operands are guaranteed to be non-poison, we can drop freeze.
-  if (MaybePoisonOperands.empty())
-    return OrigOp;
+    // reverse() to emit freezes in a more natural order.
+    for (Use &Op : reverse(I->operands())) {
+      Value *OpV = Op.get();
+      if (isa<MetadataAsValue>(OpV) || isGuaranteedNotToBeUndefOrPoison(OpV))
+        continue;
+      Worklist.push_back(&Op);
+    }
 
-  Builder.SetInsertPoint(OrigOpInst);
-  for (Value *V : MaybePoisonOperands) {
-    Value *Frozen = Builder.CreateFreeze(V, V->getName() + ".fr");
-    OrigOpInst->replaceUsesOfWith(V, Frozen);
+    I->dropPoisonGeneratingAnnotations();
+    this->Worklist.add(I);
   }
-  return OrigOp;
+
+  return OrigUse->get();
 }
 
 Instruction *InstCombinerImpl::foldFreezeIntoRecurrence(FreezeInst &FI,