[llvm] [VPlan] Remove constant branches early. (PR #183397)

[Florian Hahn via llvm-commits]

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@@ -2859,15 +2860,43 @@ void VPlanTransforms::removeBranchOnConst(VPlan &Plan, bool OnlyLatches) {
         cast<VPBasicBlock>(VPBB->getSuccessors()[RemovedIdx]);
     assert(count(RemovedSucc->getPredecessors(), VPBB) == 1 &&
            "There must be a single edge between VPBB and its successor");
-    // Values coming from VPBB into phi recipes of RemoveSucc are removed from
+    // Values coming from VPBB into phi recipes of RemovedSucc are removed from
     // these recipes.
     for (VPRecipeBase &R : RemovedSucc->phis())
       cast<VPPhiAccessors>(&R)->removeIncomingValueFor(VPBB);
 
-    // Disconnect blocks and remove the terminator. RemovedSucc will be deleted
-    // automatically on VPlan destruction if it becomes unreachable.
+    // Disconnect blocks and remove the terminator.
     VPBlockUtils::disconnectBlocks(VPBB, RemovedSucc);
     VPBB->back().eraseFromParent();
+    DeadSuccs.push_back(RemovedSucc);
+  }
+
+  // Transitively disconnect all blocks that became unreachable, removing
+  // incoming values from phi recipes and replacing remaining uses with poison.
+  VPTypeAnalysis TypeInfo(Plan);
+  while (!DeadSuccs.empty()) {
+    VPBlockBase *DeadBlock = DeadSuccs.pop_back_val();
+    if (!DeadBlock->getPredecessors().empty())
+      continue;
+    for (VPBlockBase *Succ :
+         SmallVector<VPBlockBase *>(DeadBlock->successors())) {
+      if (auto *SuccBB = dyn_cast<VPBasicBlock>(Succ))
+        for (VPRecipeBase &R : SuccBB->phis())
+          cast<VPPhiAccessors>(&R)->removeIncomingValueFor(DeadBlock);
+      VPBlockUtils::disconnectBlocks(DeadBlock, Succ);
+      if (Succ->getPredecessors().empty())
+        DeadSuccs.push_back(Succ);
+    }
+    auto *DeadBB = dyn_cast<VPBasicBlock>(DeadBlock);
+    if (!DeadBB)
+      continue;
+    for (VPRecipeBase &R : make_early_inc_range(*DeadBB)) {
+      for (VPValue *Def : R.definedValues())
+        if (Def->getNumUsers() > 0)
+          Def->replaceAllUsesWith(Plan.getOrAddLiveIn(
+              PoisonValue::get(TypeInfo.inferScalarType(Def))));
+      R.eraseFromParent();
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fhahn wrote:

The issue is that dead blocks are only completely deleted when the plan gets destroyed; so this leaves users around, which are now in unreachable blocks, which cna cause some issues (e.g. dominance queries, extra users pessmizing optimizations, hence the replacement here.

https://github.com/llvm/llvm-project/pull/183397