[LVA] Record store as read of source for reference types.

lib/SILOptimizer/Analysis/MemoryBehavior.cpp

@@ -250,21 +250,33 @@ MemBehavior MemoryBehaviorVisitor::visitLoadInst(LoadInst *LI) {
 }
 
 MemBehavior MemoryBehaviorVisitor::visitStoreInst(StoreInst *SI) {
+  MemBehavior srcBehabior = MemBehavior::None;
+  if (SI->getSrc()->getType().hasReferenceSemantics())
+    srcBehabior = MemBehavior::MayRead;
+
+  // TODO: Is there any other possible type that has reference/pointer semantics
+  // but isn't a reference or pointer?
+  if (SI->getSrc()->getType().getASTType() ==
+      SI->getSrc()->getType().getASTContext().TheRawPointerType)
+    srcBehabior = MemBehavior::MayRead;
+
   // No store besides the initialization of a "let"-variable
   // can have any effect on the value of this "let" variable.
   if (isLetValue()
       && (getAccessedAddress(SI->getDest()) != getValueAddress())) {
-    return MemBehavior::None;
+    return srcBehabior;
   }
   // If the store dest cannot alias the pointer in question, then the
   // specified value cannot be modified by the store.
   if (!mayAlias(SI->getDest()))
-    return MemBehavior::None;
+    return srcBehabior;
 
   // Otherwise, a store just writes.
   LLVM_DEBUG(llvm::dbgs() << "  Could not prove store does not alias inst. "
                              "Returning MayWrite.\n");
-  return MemBehavior::MayWrite;
+  if (srcBehabior == MemBehavior::None)
+    return MemBehavior::MayWrite;
+  return MemBehavior::MayReadWrite;
 }
 
 MemBehavior MemoryBehaviorVisitor::visitBuiltinInst(BuiltinInst *BI) {

lib/SILOptimizer/Transforms/DeadStoreElimination.cpp

@@ -984,6 +984,11 @@ void DSEContext::processWrite(SILInstruction *I, SILValue Val, SILValue Mem,
     S->DeadStores.push_back(I);
     ++NumDeadStores;
     return;
+  } else {
+    // If the store isn't dead there's a possibility that the store reads memory
+    // because the source is some kind of pointer or reference. Make sure we
+    // process this.
+    processUnknownReadInstForDSE(I);
   }
 
   // Partial dead store - stores to some locations are dead, but not all. This

test/SILOptimizer/dead_store_elim.sil

@@ -231,21 +231,6 @@ bb0:
   return %4 : $()
 }
 
-// CHECK-LABEL: sil @store_after_store
-// CHECK: bb0
-// CHECK: {{ store}}
-// CHECK-NOT: {{ store}}
-// CHECK: return
-sil @store_after_store : $@convention(thin) (@owned B) -> () {
-bb0(%0 : $B):
-  %1 = alloc_box $<τ_0_0> { var τ_0_0 } <B>
-  %1a = project_box %1 : $<τ_0_0> { var τ_0_0 } <B>, 0
-  store %0 to %1a : $*B
-  store %0 to %1a : $*B
-  %4 = tuple()
-  return %4 : $()
-}
-
 // CHECK-LABEL: sil @dead_store_elimination_over_noread_builtins
 // CHECK: bb0
 // CHECK-NEXT: load
@@ -1519,3 +1504,38 @@ bb0:
   dealloc_ref [stack] %1 : $foo
   return %3 : $Int
 }
+
+class F { }
+
+struct S {
+   var f : F
+}
+
+struct U {
+  var s : S
+}
+
+// Test that the second store is recorded as a read of the source.
+// Otherwise the first store will be removed.
+// CHECK-LABEL: @store_is_read_of_src
+// CHECK: store {{%[0-9]+}} to {{%[0-9]+}} : $*S
+// CHECK: store {{%[0-9]+}} to {{%[0-9]+}} : $*Builtin.RawPointer
+// CHECK: store {{%[0-9]+}} to {{%[0-9]+}} : $*F
+// CHECK-LABEL: end sil function 'store_is_read_of_src'
+sil @store_is_read_of_src : $@convention(thin) (S, @inout F) -> () {
+bb0(%0 : $S, %f : $*F):
+  %2 = alloc_stack $S, var, name "vs"
+  store %0 to %2 : $*S
+  %4 = address_to_pointer %2 : $*S to $Builtin.RawPointer
+  %9 = alloc_stack $U
+  %10 = unchecked_addr_cast %9 : $*U to $*Builtin.RawPointer
+  store %4 to %10 : $*Builtin.RawPointer
+  %12 = load %9 : $*U
+  %15 = struct_extract %12 : $U, #U.s
+  %16 = struct_extract %15 : $S, #S.f
+  dealloc_stack %9 : $*U
+  store %16 to %f : $*F
+  dealloc_stack %2 : $*S
+  %19 = tuple ()
+  return %19 : $()
+}