[clang][bytecode] Fix instance pointer in IndirectFieldDecl inits (llvm#164751)

Points to the first chain link in this case.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -5384,55 +5384,57 @@ bool Compiler<Emitter>::VisitCXXThisExpr(const CXXThisExpr *E) {
   // instance pointer of the current function frame, but e.g. to the declaration
   // currently being initialized. Here we emit the necessary instruction(s) for
   // this scenario.
-  if (!InitStackActive)
+  if (!InitStackActive || InitStack.empty())
     return this->emitThis(E);
 
-  if (!InitStack.empty()) {
-    // If our init stack is, for example:
-    // 0 Stack: 3 (decl)
-    // 1 Stack: 6 (init list)
-    // 2 Stack: 1 (field)
-    // 3 Stack: 6 (init list)
-    // 4 Stack: 1 (field)
-    //
-    // We want to find the LAST element in it that's an init list,
-    // which is marked with the K_InitList marker. The index right
-    // before that points to an init list. We need to find the
-    // elements before the K_InitList element that point to a base
-    // (e.g. a decl or This), optionally followed by field, elem, etc.
-    // In the example above, we want to emit elements [0..2].
-    unsigned StartIndex = 0;
-    unsigned EndIndex = 0;
-    // Find the init list.
-    for (StartIndex = InitStack.size() - 1; StartIndex > 0; --StartIndex) {
-      if (InitStack[StartIndex].Kind == InitLink::K_InitList ||
-          InitStack[StartIndex].Kind == InitLink::K_This) {
-        EndIndex = StartIndex;
-        --StartIndex;
-        break;
-      }
+  // If our init stack is, for example:
+  // 0 Stack: 3 (decl)
+  // 1 Stack: 6 (init list)
+  // 2 Stack: 1 (field)
+  // 3 Stack: 6 (init list)
+  // 4 Stack: 1 (field)
+  //
+  // We want to find the LAST element in it that's an init list,
+  // which is marked with the K_InitList marker. The index right
+  // before that points to an init list. We need to find the
+  // elements before the K_InitList element that point to a base
+  // (e.g. a decl or This), optionally followed by field, elem, etc.
+  // In the example above, we want to emit elements [0..2].
+  unsigned StartIndex = 0;
+  unsigned EndIndex = 0;
+  // Find the init list.
+  for (StartIndex = InitStack.size() - 1; StartIndex > 0; --StartIndex) {
+    if (InitStack[StartIndex].Kind == InitLink::K_InitList ||
+        InitStack[StartIndex].Kind == InitLink::K_This) {
+      EndIndex = StartIndex;
+      --StartIndex;
+      break;
     }
+  }
 
-    // Walk backwards to find the base.
-    for (; StartIndex > 0; --StartIndex) {
-      if (InitStack[StartIndex].Kind == InitLink::K_InitList)
-        continue;
+  // Walk backwards to find the base.
+  for (; StartIndex > 0; --StartIndex) {
+    if (InitStack[StartIndex].Kind == InitLink::K_InitList)
+      continue;
 
-      if (InitStack[StartIndex].Kind != InitLink::K_Field &&
-          InitStack[StartIndex].Kind != InitLink::K_Elem)
-        break;
-    }
+    if (InitStack[StartIndex].Kind != InitLink::K_Field &&
+        InitStack[StartIndex].Kind != InitLink::K_Elem)
+      break;
+  }
 
-    // Emit the instructions.
-    for (unsigned I = StartIndex; I != EndIndex; ++I) {
-      if (InitStack[I].Kind == InitLink::K_InitList)
-        continue;
-      if (!InitStack[I].template emit<Emitter>(this, E))
-        return false;
-    }
-    return true;
+  if (StartIndex == 0 && EndIndex == 0)
+    EndIndex = InitStack.size() - 1;
+
+  assert(StartIndex < EndIndex);
+
+  // Emit the instructions.
+  for (unsigned I = StartIndex; I != (EndIndex + 1); ++I) {
+    if (InitStack[I].Kind == InitLink::K_InitList)
+      continue;
+    if (!InitStack[I].template emit<Emitter>(this, E))
+      return false;
   }
-  return this->emitThis(E);
+  return true;
 }
 
 template <class Emitter> bool Compiler<Emitter>::visitStmt(const Stmt *S) {
@@ -6294,6 +6296,10 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
       }
       assert(NestedField);
 
+      unsigned FirstLinkOffset =
+          R->getField(cast<FieldDecl>(IFD->chain()[0]))->Offset;
+      InitStackScope<Emitter> ISS(this, isa<CXXDefaultInitExpr>(InitExpr));
+      InitLinkScope<Emitter> ILS(this, InitLink::Field(FirstLinkOffset));
       if (!emitFieldInitializer(NestedField, NestedFieldOffset, InitExpr,
                                 IsUnion))
         return false;

clang/test/AST/ByteCode/records.cpp

@@ -1162,6 +1162,19 @@ namespace IndirectFieldInit {
   static_assert(s2.x == 1 && s2.y == 2 && s2.a == 3 && s2.b == 4);
 
 #endif
+
+
+  struct B {
+    struct {
+      union {
+        int x = 3;
+      };
+      int y = this->x;
+    };
+
+    constexpr B() {}
+  };
+  static_assert(B().y == 3, "");
 }
 
 namespace InheritedConstructor {