We can only eliminate dealloc_stack in an unreachable block if we can eliminate all dealloc_stack instructions in the block

Author: aschwaighofer

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -1843,6 +1843,26 @@ bool SimplifyCFG::simplifySwitchValueBlock(SwitchValueInst *SVI) {
   return simplifyTermWithIdenticalDestBlocks(ThisBB);
 }
 
+bool onlyContainsRefcountAndDeallocStackInst(
+    SILBasicBlock::reverse_iterator I, SILBasicBlock::reverse_iterator End) {
+  while (I != End) {
+    auto MaybeDead = I++;
+    switch (MaybeDead->getKind()) {
+      // These technically have side effects, but not ones that matter
+      // in a block that we shouldn't really reach...
+    case SILInstructionKind::StrongRetainInst:
+    case SILInstructionKind::StrongReleaseInst:
+    case SILInstructionKind::RetainValueInst:
+    case SILInstructionKind::ReleaseValueInst:
+    case SILInstructionKind::DeallocStackInst:
+      break;
+
+    default:
+      return false;
+    }
+  }
+  return true;
+}
 /// simplifyUnreachableBlock - Simplify blocks ending with unreachable by
 /// removing instructions that are safe to delete backwards until we
 /// hit an instruction we cannot delete.
@@ -1853,6 +1873,8 @@ bool SimplifyCFG::simplifyUnreachableBlock(UnreachableInst *UI) {
   auto End = BB->rend();
   SmallVector<SILInstruction *, 8> DeadInstrs;
 
+  bool canIgnoreRestOfBlock = false;
+
   // Walk backwards deleting instructions that should be safe to delete
   // in a block that ends with unreachable.
   while (I != End) {
@@ -1865,12 +1887,20 @@ bool SimplifyCFG::simplifyUnreachableBlock(UnreachableInst *UI) {
     case SILInstructionKind::StrongReleaseInst:
     case SILInstructionKind::RetainValueInst:
     case SILInstructionKind::ReleaseValueInst:
-    case SILInstructionKind::DeallocStackInst:
       break;
-
+    // We can only ignore a dealloc_stack instruction if we can ignore all
+    // instructions in the block.
+    case SILInstructionKind::DeallocStackInst: {
+      if (canIgnoreRestOfBlock ||
+          onlyContainsRefcountAndDeallocStackInst(MaybeDead, End)) {
+        canIgnoreRestOfBlock = true;
+        break;
+      }
+      LLVM_FALLTHROUGH;
+    }
     default:
       if (MaybeDead->mayHaveSideEffects()) {
-        if (Changed)
+         if (Changed)
           for (auto Dead : DeadInstrs)
             Dead->eraseFromParent();
         return Changed;