SILMem2Reg: Don't add dead values as phi arguments

A dealloc_stack ends the lifetime of an alloc_stack on a path. We don't
have to pass RunningVal beyond the dealloc_stack as phi argument to the
post dominating block.

Author: meg-gupta

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -468,8 +468,12 @@ StackAllocationPromoter::promoteAllocationInBlock(SILBasicBlock *BB) {
 
     // Stop on deallocation.
     if (auto *DSI = dyn_cast<DeallocStackInst>(Inst)) {
-      if (DSI->getOperand() == ASI)
+      if (DSI->getOperand() == ASI) {
+        // Reset LastStore.
+        // So that we don't pass RunningVal as a phi arg beyond dealloc_stack
+        LastStore = nullptr;
         break;
+      }
     }
   }
   if (LastStore) {

test/SILOptimizer/mem2reg.sil

@@ -465,3 +465,30 @@ bb0(%0 : $Optional<Klass>):
   %4 = tuple()
   return %4 : $()
 }
+
+// CHECK-LABEL: sil @multi_basic_block_use_on_one_path :
+// CHECK-NOT: alloc_stack
+// CHECK:bb2:
+// CHECK: br bb3(undef : $Klass)
+// CHECK-LABEL: } // end sil function 'multi_basic_block_use_on_one_path'
+sil @multi_basic_block_use_on_one_path : $@convention(thin) (@owned Klass) -> () {
+bb0(%0 : $Klass):
+  %1 = alloc_stack $Klass
+  cond_br undef, bb1, bb2
+
+bb1:
+  dealloc_stack %1 : $*Klass
+  strong_release %0 : $Klass
+  br bb3
+
+bb2:
+  store %0 to %1 : $*Klass
+  %7 = load %1 : $*Klass
+  dealloc_stack %1 : $*Klass
+  strong_release %7 : $Klass
+  br bb3
+
+bb3:
+  %11 = tuple ()
+  return %11 : $()
+}