[clang][bytecode] Implement base casts on integral pointers

clang/lib/AST/ByteCode/Interp.h

@@ -1611,6 +1611,12 @@ inline bool GetPtrDerivedPop(InterpState &S, CodePtr OpPC, uint32_t Off) {
 
 inline bool GetPtrBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
   const Pointer &Ptr = S.Stk.peek<Pointer>();
+
+  if (!Ptr.isBlockPointer()) {
+    S.Stk.push<Pointer>(Ptr.asIntPointer().baseCast(S.getASTContext(), Off));
+    return true;
+  }
+
   if (!CheckNull(S, OpPC, Ptr, CSK_Base))
     return false;
   if (!CheckSubobject(S, OpPC, Ptr, CSK_Base))
@@ -1624,6 +1630,12 @@ inline bool GetPtrBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
 
 inline bool GetPtrBasePop(InterpState &S, CodePtr OpPC, uint32_t Off) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
+
+  if (!Ptr.isBlockPointer()) {
+    S.Stk.push<Pointer>(Ptr.asIntPointer().baseCast(S.getASTContext(), Off));
+    return true;
+  }
+
   if (!CheckNull(S, OpPC, Ptr, CSK_Base))
     return false;
   if (!CheckSubobject(S, OpPC, Ptr, CSK_Base))

clang/lib/AST/ByteCode/Pointer.cpp

@@ -667,3 +667,26 @@ IntPointer IntPointer::atOffset(const ASTContext &ASTCtx,
           .getQuantity();
   return IntPointer{this->Desc, this->Value + FieldOffset};
 }
+
+IntPointer IntPointer::baseCast(const ASTContext &ASTCtx,
+                                unsigned BaseOffset) const {
+  const Record *R = Desc->ElemRecord;
+  const Descriptor *BaseDesc = nullptr;
+
+  // This iterates over bases and checks for the proper offset. That's
+  // potentially slow but this case really shouldn't happen a lot.
+  for (const Record::Base &B : R->bases()) {
+    if (B.Offset == BaseOffset) {
+      BaseDesc = B.Desc;
+      break;
+    }
+  }
+  assert(BaseDesc);
+
+  // Adjust the offset value based on the information from the record layout.
+  const ASTRecordLayout &Layout = ASTCtx.getASTRecordLayout(R->getDecl());
+  CharUnits BaseLayoutOffset =
+      Layout.getBaseClassOffset(cast<CXXRecordDecl>(BaseDesc->asDecl()));
+
+  return {BaseDesc, Value + BaseLayoutOffset.getQuantity()};
+}

clang/lib/AST/ByteCode/Pointer.h

@@ -46,6 +46,7 @@ struct IntPointer {
   uint64_t Value;
 
   IntPointer atOffset(const ASTContext &ASTCtx, unsigned Offset) const;
+  IntPointer baseCast(const ASTContext &ASTCtx, unsigned BaseOffset) const;
 };
 
 enum class Storage { Block, Int, Fn };

clang/test/AST/ByteCode/const-base-cast.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm %s -o - -fexperimental-new-constant-interpreter | FileCheck %s
+
+
+/// Slightly adapted to the version from test/CodeGenCXX/.
+
+struct X { int x[12];};
+struct A : X { char x, y, z; };
+struct B { char y; };
+struct C : A,B {};
+unsigned char x = ((char*)(X*)(C*)0x1000) - (char*)0x1000;
+// CHECK: @x = {{(dso_local )?}}global i8 0
+
+unsigned char y = ((char*)(B*)(C*)0x1000) - (char*)0x1000;
+// CHECK: @y = {{(dso_local )?}}global i8 51
+
+unsigned char z = ((char*)(A*)(C*)0x1000) - (char*)0x1000;
+// CHECK: @z = {{(dso_local )?}}global i8 0
+

clang/test/CodeGenCXX/const-base-cast.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm %s -o - -fexperimental-new-constant-interpreter | FileCheck %s
 
 // Check that the following construct, which is similar to one which occurs
 // in Firefox, is folded correctly.