Fix Mem2Reg to avoid creating undef as a replacement for load of an empty tuple

Author: meg-gupta

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -1488,7 +1488,7 @@ static bool isCaptured(AllocStackInst *asi, bool &inSingleBlock) {
   for (auto *use : asi->getUses()) {
     SILInstruction *user = use->getUser();
 
-    if (user->getParent() != singleBlock)
+    if (!isa<DeallocStackInst>(user) && user->getParent() != singleBlock)
       singleBlock = nullptr;
 
     // Loads are okay.
@@ -1763,13 +1763,17 @@ bool MemoryToRegisters::promoteSingleAllocation(AllocStackInst *alloc) {
     if (alloc->use_empty()) {
       deleter.forceDelete(alloc);
     } else {
-      // Handle a corner case where the ASI still has uses:
-      // This can come up if the source contains a withUnsafePointer where
-      // the pointer escapes. It's illegal code but we should not crash.
-      // Re-insert a dealloc_stack so that the verifier is happy.
-      auto *next = alloc->getNextInstruction();
-      SILBuilderWithScope b(next, ctx);
-      b.createDeallocStack(next->getLoc(), alloc);
+      // Delete dealloc uses in other blocks
+      SmallVector<SILInstruction *, 8> deallocs;
+      for (auto *use : alloc->getUses()) {
+        auto *user = use->getUser();
+        assert(isa<DeallocStackInst>(user));
+        deallocs.push_back(user);
+      }
+      for (auto dealloc : deallocs) {
+        dealloc->eraseFromParent();
+      }
+      deleter.forceDelete(alloc);
     }
     return true;
   } else {

test/SILOptimizer/mem2reg_ossa.sil

@@ -489,6 +489,22 @@ bb0:
   return %16 : $((), ())
 }
 
+// CHECK-LABEL: sil [ossa] @load_tuple_of_void2 :
+// CHECK-NOT: alloc_stack
+// CHECK:   [[T:%[0-9]+]] = tuple ()
+// CHECK:   return [[T]] : $()
+// CHECK: } // end sil function 'load_tuple_of_void2'
+sil [ossa] @load_tuple_of_void2 : $@convention(thin) () -> () {
+bb0:
+  %1 = alloc_stack $()
+  %16 = load [trivial] %1 : $*()
+  br bb2
+
+bb2:
+  dealloc_stack %1 : $*()
+  return %16 : $()
+}
+
 // CHECK-LABEL: sil [ossa] @dont_optimize_optional_in_multiple_blocks :
 // CHECK:         alloc_stack
 // CHECK:       } // end sil function 'dont_optimize_optional_in_multiple_blocks'