[clang][bytecode] Activate primitive fields before initializing them

[tbaederr]

The initializer itself might need the field to be active.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

Changes

The initializer itself might need the field to be active.

---

Full diff: https://github.com/llvm/llvm-project/pull/149963.diff

4 Files Affected:

• (modified) clang/lib/AST/ByteCode/Compiler.cpp (+11-5)
• (modified) clang/lib/AST/ByteCode/Interp.h (+10)
• (modified) clang/lib/AST/ByteCode/Opcodes.td (+1)
• (modified) clang/test/AST/ByteCode/unions.cpp (+67-3)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 952a43a0ecbcf..af2f69bc8573e 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -25,11 +25,18 @@ using APSInt = llvm::APSInt;
 namespace clang {
 namespace interp {
 
+static bool hasTrivialDefaultCtorParent(const FieldDecl *FD) {
+  assert(FD);
+  assert(FD->getParent()->isUnion());
+  const auto *CXXRD = dyn_cast<CXXRecordDecl>(FD->getParent());
+  return !CXXRD || CXXRD->hasTrivialDefaultConstructor();
+}
+
 static bool refersToUnion(const Expr *E) {
   for (;;) {
     if (const auto *ME = dyn_cast<MemberExpr>(E)) {
       if (const auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
-          FD && FD->getParent()->isUnion())
+          FD && FD->getParent()->isUnion() && hasTrivialDefaultCtorParent(FD))
         return true;
       E = ME->getBase();
       continue;
@@ -5934,16 +5941,15 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
       return false;
 
     if (std::optional<PrimType> T = this->classify(InitExpr)) {
+      if (Activate && !this->emitActivateThisField(FieldOffset, InitExpr))
+        return false;
+
       if (!this->visit(InitExpr))
         return false;
 
       bool BitField = F->isBitField();
-      if (BitField && Activate)
-        return this->emitInitThisBitFieldActivate(*T, F, FieldOffset, InitExpr);
       if (BitField)
         return this->emitInitThisBitField(*T, F, FieldOffset, InitExpr);
-      if (Activate)
-        return this->emitInitThisFieldActivate(*T, FieldOffset, InitExpr);
       return this->emitInitThisField(*T, FieldOffset, InitExpr);
     }
     // Non-primitive case. Get a pointer to the field-to-initialize
diff --git a/clang/lib/AST/ByteCode/Interp.h b/clang/lib/AST/ByteCode/Interp.h
index 7f29200f8617f..a234f8ebf0c82 100644
--- a/clang/lib/AST/ByteCode/Interp.h
+++ b/clang/lib/AST/ByteCode/Interp.h
@@ -1983,6 +1983,16 @@ static inline bool Activate(InterpState &S, CodePtr OpPC) {
   return true;
 }
 
+static inline bool ActivateThisField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+
+  const Pointer &Ptr = S.Current->getThis();
+  assert(Ptr.atField(I).canBeInitialized());
+  Ptr.atField(I).activate();
+  return true;
+}
+
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool StoreActivate(InterpState &S, CodePtr OpPC) {
   const T &Value = S.Stk.pop<T>();
diff --git a/clang/lib/AST/ByteCode/Opcodes.td b/clang/lib/AST/ByteCode/Opcodes.td
index 80703ad72d954..abfed77750f87 100644
--- a/clang/lib/AST/ByteCode/Opcodes.td
+++ b/clang/lib/AST/ByteCode/Opcodes.td
@@ -510,6 +510,7 @@ def StoreBitFieldActivate : StoreBitFieldOpcode {}
 def StoreBitFieldActivatePop : StoreBitFieldOpcode {}
 
 def Activate : Opcode {}
+def ActivateThisField : Opcode { let Args = [ArgUint32]; }
 
 // [Pointer, Value] -> []
 def Init : StoreOpcode {}
diff --git a/clang/test/AST/ByteCode/unions.cpp b/clang/test/AST/ByteCode/unions.cpp
index 7cfd0d677a7b3..8fce2e6a738c7 100644
--- a/clang/test/AST/ByteCode/unions.cpp
+++ b/clang/test/AST/ByteCode/unions.cpp
@@ -79,10 +79,9 @@ namespace DefaultInit {
 
   constexpr U1 u1; /// OK.
 
-  constexpr int foo() { // expected-error {{never produces a constant expression}}
+  constexpr int foo() {
     U1 u;
-    return u.a; // both-note {{read of member 'a' of union with active member 'b'}} \
-                // expected-note {{read of member 'a' of union with active member 'b'}}
+    return u.a; // both-note {{read of member 'a' of union with active member 'b'}}
   }
   static_assert(foo() == 42); // both-error {{not an integral constant expression}} \
                               // both-note {{in call to}}
@@ -861,6 +860,71 @@ namespace CopyCtorMutable {
                        // both-note {{in call}}
 }
 
+
+namespace NonTrivialCtor {
+  struct A { int x = 1; constexpr int f() { return 1; } };
+  struct B : A { int y = 1; constexpr int g() { return 2; } };
+  struct C {
+    int x;
+    constexpr virtual int f() = 0;
+  };
+  struct D : C {
+    int y;
+    constexpr virtual int f() override { return 3; }
+  };
+
+  union U {
+    int n;
+    B b;
+    D d;
+  };
+
+  consteval int test(int which) {
+    if (which == 0) {}
+
+    U u{.n = 5};
+    assert_active(u);
+    assert_active(u.n);
+    assert_inactive(u.b);
+
+    switch (which) {
+    case 0:
+      u.b.x = 10; // both-note {{assignment to member 'b' of union with active member 'n'}}
+      return u.b.f();
+    case 1:
+      u.b.y = 10; // both-note {{assignment to member 'b' of union with active member 'n'}}
+      return u.b.g();
+    case 2:
+      u.d.x = 10; // both-note {{assignment to member 'd' of union with active member 'n'}}
+     return u.d.f();
+    case 3:
+    u.d.y = 10; // both-note {{assignment to member 'd' of union with active member 'n'}}
+      return u.d.f();
+    }
+
+    return 1;
+  }
+  static_assert(test(0)); // both-error {{not an integral constant expression}} \
+                          // both-note {{in call}}
+  static_assert(test(1)); // both-error {{not an integral constant expression}} \
+                          // both-note {{in call}}
+  static_assert(test(2)); // both-error {{not an integral constant expression}} \
+                          // both-note {{in call}}
+  static_assert(test(3)); // both-error {{not an integral constant expression}} \
+                          // both-note {{in call}}
+
+}
+
+namespace PrimitiveFieldInitActivates {
+  /// The initializer of a needs the field to be active _before_ it's visited.
+  template<int> struct X {};
+  union V {
+    int a, b;
+    constexpr V(X<0>) : a(a = 1) {} // ok
+  };
+  constinit V v0 = X<0>();
+}
+
 #endif
 
 namespace AddressComparison {